Climate Forcing by Aerosol Particles:
Report of the NOAA Panel Meeting,
October 13-14, 1994

Executive Summary

Table of Contents

1. Introduction

2. Process and Closure Studies

3. Continuous Observations and Monitoring

4. Modeling

Appendix A: Panel Members and Observers

Appendix B: Summary of current programs

Appendix C: Budget summary

Appendix D: List of Acronyms


Atmospheric aerosols affect the Earth's radiation budget directly through the scattering and absorption of solar and thermal radiation, and indirectly through their effects on the microphysical and radiative properties of clouds and through their effects on atmospheric trace gases. Present estimates of the direct radiative forcing for only one component of the aerosol, sulfate compounds, yield a forcing over industrialized regions (eastern U.S.A., Europe, eastern Asia) that is greater in magnitude, but of the opposite sign, than the radiative forcing from anthropogenic greenhouse gases. This sulfate forcing, along with the forcing from other components of the atmospheric aerosol, may have masked detection of the effects of greenhouse warming in the long-term temperature records. Clearly, a forcing component of this magnitude must be properly represented in global climate models in order to provide reliable guidance for policy-makers, who are responding to concerns about anthropogenic effects on climate.

This report responds to a request from the Director of NOAA's Environmental Research Laboratories for a summary of current research programs funded by NOAA that are directed towards climate forcing by aerosol particles, and for recommendations for an integrated program that draws on NOAA's unique strengths and that could define NOAA's contribution to a national, multi-agency research program on climate forcing by aerosol particles. The NOAA research will be part of the inter-agency planning charged to the Global Change Research Program. In fiscal year 1994, research within NOAA related to climate forcing by anthropogenic aerosols was conducted by nine laboratories with a total expenditure of $2.7M, including over 22 full- time equivalent salaries.

Specific recommendations are presented for an integrated set of process, closure, monitoring, and modeling studies that focus on reducing uncertainties in estimates of the climate forcing from anthropogenic aerosols. Since climate forcing is defined as the anthropogenic perturbation to the background atmosphere, field studies are needed in regions of both natural and perturbed air masses in order to quantify the non-linear changes to the background atmosphere. Knowledge of the chemical composition of the particles is crucial for assigning them specific radiative properties and chemical reactivities. The ultimate goal of this integrated set of studies is to incorporate aerosols in three-dimensional chemical transport models that can be used to calculate the direct and indirect radiative forcing by the particles. Global climate models that explicitly incorporate the aerosol forcings would subsequently be used to calculate the response of the climate to the combined anthropogenic forcings, particularly from aerosols and greenhouse gases, in order to assist policy-makers in formulating measures to minimize the effects of the changing climate on human society.

The recommendations can be grouped into four categories:


1. Introduction

2. Process and Closure Studies

2.1. Laboratory Studies

2.2. Field Process Studies

2.3. Closure Studies

2.4. Specific Recommendations for Process Studies

3. Continuous Observations and Monitoring

3.1. Surface Networks

4. Modeling

4.1. Modeling of aerosol radiative properties

4.2. Modeling the column radiative transfer

4.3. Aerosol microphysics and chemistry modeling

4.4. Mesoscale modeling

4.5. Global General Circulation Models (GCMs)

4.6. Specific Modeling Recommendations

Appendix A: Panel Members and Observers

Appendix B: Summary of current programs

B.1 Aeronomy Laboratory (AL)

B.2 Air Resources Laboratory (ARL)

B.3 Atlantic Oceanographic and Meteorological Laboratory (AOML)

B.4 Climate Monitoring and Diagnostics Laboratory (CMDL)

B.5 Environmental Technology Laboratory (ETL)

B.6 Geophysical Fluid Dynamics Laboratory (GFDL)

B.7 National Severe Storms Laboratory (NSSL)

B.8 Pacific Marine Environmental Laboratory (PMEL)


Appendix C: Budget summary

C.1 Aeronomy Laboratory

C.2 Air Resources Laboratory

C.3 Atlantic Oceanographic and Meteorological Laboratory

C.4 Climate Monitoring and Diagnostics Laboratory

C.5 Environmental Technology Laboratory

C.6 Geophysical Fluid Dynamics Laboratory

C.7 National Severe Storms Laboratory

C.8 Pacific Marine Environmental Laboratory

C.9 NESDIS/ORA (Office of Research and Applications)

Appendix D: List of Acronyms

1. Introduction

1.1. Charge to panel

This report is in response to a memorandum from the director of NOAA's Environmental Research Laboratories to D. Hofmann of NOAA/CMDL, stating the following:

A recent canvassing of ERL and NESDIS indicated that over $3M/yr is being spent on aerosol research at NOAA, with OGP providing about 25% of that sum. In light of the current interest in climatic effects of atmospheric aerosols, and in response to the NAS Panel on Aerosol Radiative Forcing and Climate, I request that you convene and chair a panel to:

a) summarize current research programs funded by NOAA (including external grants) directed towards climate forcing by aerosol particles, and

b) recommend an integrated program that draws on NOAA's unique strengths and that could define NOAA's contribution to a national, multi-agency research program on climate forcing by aerosol particles.

Your panel should have representatives from the NOAA laboratories that have active programs in these areas. ERL is conducting aerosol research not directly related to climate. These activities should be kept informed of your panel's deliberations and representatives of these activities should be included as observers in your initial meeting. It would also be desirable to have one or two external representatives who are widely respected as leaders in atmospheric aerosol research. The study should be completed in time to provide input to the NAS Aerosol Panel. ERL will assist by providing a breakdown of ERL and NESDIS aerosol research programs, including both actual dollars spent and FTEs (with names of people), from laboratory base budgets, OGP, and external sources.

The panel met on October 13-14, 1994. The first day was devoted to presentations from the panel members and observers (Appendix A) of current research in their laboratories on aerosol forcing of climate. Summaries of these presentations are included in Appendix B. Each participating laboratory was also responsible for submitting a summary of their expenditures in FY1994 on research related to the role of aerosols in climate variability, stratospheric ozone reduction and air quality (Appendix C). On the second day, the panel discussed and formulated recommendations for an integrated research program on climate forcing by aerosols. Those recommendations comprise the body of this report.

1.2. Invited Presentation on "Aerosol Forcing of Climate"

Following the recommendation in the charge to the panel, two external representatives who are widely respected as leaders in atmospheric aerosol research were invited to join the panel: Prof R. Charlson (Univ. of Washington) and Dr. S. Schwartz (Brookhaven National Laboratory). Although both accepted the invitation, only Prof. Charlson was able to attend the meeting. The main points of Prof. Charlson's presentation are summarized below:

1.3 Scientific Questions

There are two primary questions that need to be answered:

A more specific set of questions defines the core components of the recommended NOAA aerosol climate research program. These questions involve understanding the processes controlling aerosol properties and distributions, characterizing the spatial and temporal distributions of these aerosol properties, assessing the climate forcings resulting from these aerosols, and evaluating the climate responses to these forcings.

Understand the processes:

Characterize the atmospheric aerosol:

Assess the forcings:

Evaluate the responses:

There are additional questions regarding the climate response to anthropogenic aerosol forcing; and while it is important to state them, it is necessary to recognize that they cannot be fully addressed until the above questions are resolved.

1.4 Summary of Recommendations

This report provides specific recommendations for an integrated set of process, closure, monitoring, and modeling studies focussed on reducing uncertainties in estimates of the climate forcing from anthropogenic aerosols. The ultimate goal of this integrated set of studies is to incorporate aerosols in three-dimensional chemical transport models that can be used to calculate the direct and indirect radiative forcing by the particles. Global climate models that explicitly incorporate the aerosol forcings would subsequently be used to calculate the response of the climate to the combined anthropogenic forcings, particularly from aerosols and greenhouse gases, in order to assist policy-makers in formulating measures to minimize the effects of the changing climate on human society.

The recommendations can be grouped into four categories:

2. Process and Closure Studies

Prognostic aerosol climate models must be able to parameterize realistically the processes that control the formation, transformation and fate of atmospheric aerosols in order to calculate direct and indirect radiative forcing accurately. An understanding of these processes will require both laboratory and field studies. Laboratory studies are needed to determine reaction mechanisms and radiative properties and to quantify reaction rates and efficiencies under controlled conditions. Field process studies extend these laboratory experiments to in-situ conditions.

Closure experiments are a useful approach for integrating models and measurements and can be applied to studies of aerosol properties and processes. In such an experiment, an over-determined set of observations is obtained, where the measured value of a dependent variable is compared with the value that is calculated from the measured values of the independent variables, using an appropriate model.

2.1. Laboratory Studies

Studies Of Chemical Reactions and Physical Processes

Sulfate aerosol formation: SO3 is the major product of the atmospheric gas phase oxidation of SO2. The formation of gas phase H2SO4 from this species is believed to be a key requirement for the formation of new aerosol particles. The atmospheric conversion of SO3 into gas phase H2SO4 is determined by competition between gas-phase reaction (with H2O, NH3, etc.) and its deposition on aerosol particles. Rate coefficients for SO3+H2O and SO3+ NH3 reactions and uptake of SO3 and H2SO4 by aerosol particles are needed to evaluate the potential of SO3 to produce new particles.

Nucleation of new particles: Elucidation of the requirements for particle formation is essential for predicting the loading, distribution and light scattering efficiency of atmospheric aerosols. Condensation of H2SO4 and H2O is believed to be the major source of new aerosol particles. However, the rates of and conditions for this condensation and the role of species such as NH3 in the particle formation are uncertain and need to be better quantified.

DMS oxidation mechanism: DMS oxidation is expected to be the largest natural source of sulfate aerosols. The yields of sulfur containing products (e.g., SO2 , SO3, and MSA) from the atmospheric oxidation of DMS are needed to understand its potential to produce new particles in the atmosphere. Even though the initial elementary steps of the mechanism have been characterized, the key intermediate steps that determine the branching in the formation of various other products are unclear; these steps need to be established to calculate the end-product yields over the range of appropriate atmospheric conditions.

Studies Of Radiative Properties Of Aerosols

Refractive index measurements: The UV and visible refractive indices of liquids containing H2SO4, NH3, HNO3, and H2O are needed to quantify accurately the radiative forcing of aerosols in the troposphere and to characterize/measure aerosols using light scattering techniques. [The refractive indices of mixtures of sulfuric acid and nitric acid and water have been measured between 200 and 400 nm.]

Radiative properties of non-spherical particles: The angular distribution of light scattered from solid particles present in the troposphere is uncertain (morphology and composition are not well characterized) and introduces large uncertainties in the calculated radiative forcing due to aerosols. Characterization of these parameters under controlled laboratory conditions are needed.

2.2. Field Process Studies

To represent aerosol particles in climate models, the processes governing nucleation, growth, distribution and removal of particles must be accurately known. Model parameterizations need to start with an emissions inventory and calculate what fraction of the gas phase precursors is oxidized to aerosol particles, how the size distribution of these particles evolves, and how long these particles remain in the atmosphere. Although some of the key processes can initially be studied in controlled laboratory experiments (section 2.1), the results from these experiments must be tested under in-situ conditions.

Field process studies are a major component of the IGAC- Aerosol Characterization Experiments (ACE). These studies generally require a large number of investigators in order to measure simultaneously all the necessary gas and aerosol parameters. NOAA can make a major contribution to these studies by participating in multi-agency aerosol field programs. The major processes that will be studied as part of ACE include:

1. Pathways, rates and yields of sulfur gas oxidation: The oxidation rates and conversion efficiencies of the various sulfur species are critical input parameters for calculating aerosol column burdens in aerosol/climate models. Both Eulerian and Lagrangian observations can be used to study the processes controlling the oxidation of DMS and SO2 under different oxidant concentrations.

2. Aerosol formation: The partitioning of gas-phase species between new particle production and particle growth will affect the sub-micrometer aerosol size distribution and, in turn, the effect of these particles on climate. Parameters that determine whether gaseous species condense to form new particles or condense onto existing particles include the saturation vapor pressure of the condensing species (H2SO4, MSA, and NH3), RH, temperature, and the existing particle number concentration and surface area. Observations in the boundary layer and free troposphere can be used to identify conditions under which and regions where new particles are formed.

3. Aerosol growth and processing: Particle growth occurs through condensation of gas phase species and is strongly dependent on the lifetime of the aerosol in the lower boundary layer. Cloud processing also transforms the aerosol size distribution. Observed size distributions and condensable vapor concentrations can be used as inputs for aerosol models to derive information about aerosol growth and processing (Eulerian observations). In addition, the Lagrangian studies can be used to observe changing aerosol number and mass size-distributions as a function of time.

4. Aerosol removal: Aerosol lifetime is a key parameter in aerosol/climate models and currently has a very high uncertainty (Penner et al., 1994). Wet and dry deposition measurements can be compared with calculated deposition fluxes and aerosol formation rates to provide additional constraints on aerosol lifetimes.

2.3. Closure Studies

A simultaneous chemical, physical and radiative characterization of the atmospheric aerosol is needed in a variety of different air masses to develop and test model calculations of aerosol radiative effects and to refine and validate the algorithms used to interpret satellite observations of the aerosol (Penner et al., 1994). Since the aerosol chemical, physical and radiative properties are not independent of one another, it is possible to over-determine aerosol properties or related processes using a variety of measurement and modeling techniques in order to examine the internal consistency of these different measurement and modeling strategies. These over- determined data sets are referred to as closure experiments and have four specific goals that are essential to reducing the uncertainties in the calculated radiative forcing due to human- influenced aerosols:

1. to estimate measurement uncertainties from the internal consistencies of the data set,

2. to test the validity of models to predict:

a) aerosol radiative properties based on Mie theory and measurements of chemical and physical aerosol properties and

b) cloud nucleating properties based on Köhler theory and measurements of chemical and physical aerosol and cloud properties,

3. to test and calibrate remotely sensed and indirectly determined aerosol properties for use in climate models,

4. to identify areas where improvements in instrumentation or modeling are needed.

Local closure experiments are ones in which all measurements are made at a single location and time. Several local closure experiments are planned for ACE to address uncertainties in the estimated climate forcing by anthropogenic aerosols. These experiments are one of NOAA’s main emphases and include the following intercomparisons:

1. Mass closure: Aerosol mass as a function of particle size derived from the number size distribution, chemical analysis of aerosol species and gravimetric analysis.

An accurate knowledge of the mass of both the total aerosol and individual aerosol components is needed to determine the contribution of different aerosol species to aerosol scattering.

2. Mass response to hygroscopic growth: Measured hygroscopic response of the aerosol size to changes in relative humidity (RH) and the hygroscopic response calculated from the measured aerosol number and chemical mass size distributions, RH and published functional relationships between chemical composition and water uptake.

This intercomparison addresses the ability to parameterize the response of ambient aerosol to changes in RH.

3. Scattering closure: Measured aerosol scattering and calculated aerosol scattering derived from Mie theory applied to measured number and chemical mass size distributions.

This intercomparison directly addresses the uncertainty in determining the aerosol/climate parameters of aerosol mass scattering efficiency and aerosol hemispheric backscatter fraction.

4. Scattering response to hygroscopic growth: Measured increase in aerosol scattering due to hygroscopic growth and the calculated increase based on measured number and chemical mass size distributions, RH, and published functional relationships between particle chemical composition and water uptake.

This intercomparison addresses the aerosol/climate parameter of fractional increase in aerosol scattering efficiency due to hygroscopic growth.

5. CCN closure: Measured CCN supersaturation spectrum, the CCN supersaturation spectrum derived from Köhler theory applied to the measured aerosol number and chemical mass size distributions, the CCN supersaturation spectrum derived from TDMA growth factors, and the CCN supersaturation spectrum derived from cloud droplet number concentration.

This intercomparison addresses questions of aerosol activation that effect the indirect radiative influence of aerosols on climate.

Column closure experiments extend the local (zero-dimensional) closure experiments listed above to multiple altitudes (one-dimensional closure) in order to compare and calibrate satellite and surface based column-integrated radiation measurements with in-situ (aircraft) aerosol chemical, physical and radiative measurements. The ACE and TARFOX projects both plan to conduct these experiments in a variety of air masses.

The concept of closure, in principle, can be extended to process studies: measured changes in gas phase species and aerosol physical and chemical characteristics during transport can be compared with changes predicted by process models. Of particular interest is the interaction of the gas and aerosol species. Does the evolution of the gas-phase chemical composition of the atmosphere influence the evolution of the chemical composition and radiative properties of aerosols? From the opposite perspective, how do aerosol particles affect the oxidizing capacity of the atmosphere and the concentration of greenhouse gases? Several IGAC projects (ACE-1, ACE-2, NARE) have experiments planned to address these questions.

2.4. Specific Recommendations for Process Studies

2.4.1 Laboratory Studies

1. Conduct laboratory process studies of chemical reactions and physical processes involved in DMS oxidation mechanisms, nucleation of new particles and sulfate aerosol formation.

Elucidation of the mechanism for the atmospheric oxidation of DMS (and CH3SH, H2S, and CH3SSCH3) has been ongoing in AL. Individual elementary reactions and photochemical processes are directly studied in the laboratory to construct the mechanism. This mechanism aims at predicting the variations in the yields of SO2, MSA, etc. from the gas-phase oxidation processes as function of composition and temperature. A basic mechanism is already constructed and is being tested to see how it fits with atmospheric observations.

Reactions of SO3 radical are being studied to understand the first steps in the formation of sulfate aerosols.

2. Conduct laboratory studies of aerosol radiative properties to determine the refractive index of key species and the effect of non-sphericity on aerosol scattering.

Measurements of refractive indices (real and imaginary) of aerosol materials are measured in the laboratory in AL. Thus far, the refractive indices of sulfuric acid have been measured as functions of composition and temperature.

2.4.2 Field Process Studies

1. Continue the development of instrumentation to better characterize aerosol chemical, physical and radiative properties for both process and closure studies. Knowledge of the chemical composition of aerosols is crucial for assigning them specific radiative properties and chemical reactivities. Faster response chemical instrumentation is needed to integrate gas-phase and aerosol chemical processes and properties with meteorological and aerosol physical processes and aerosol physical, radiative, and cloud nucleating properties. Techniques for obtaining physical size distributions need to be improved in order to cover accurately the size range from 3 to 10,000 nm.

Instrument development is already a major component of NOAA’s atmospheric chemistry and aerosol programs. AL scientists have developed a portable instrument, Particle Analysis by Laser Mass Spectrometry (PALMS), to make in situ measurements of the composition of individual particles. AL scientists have been leaders in the development of techniques for studying gas-phase oxidant chemistry. PMEL scientists have developed a 10 minute time-resolution ammonia instrument to study effects of gas- phase ammonia on aerosol nucleation. PMEL/JISAO scientists have worked with European colleagues to develop ultra-fine particle sizing instruments. CMDL and PMEL/JISAO scientists have actively participated in the development of a new nephelometer to quantify aerosol total and back scattering. ETL scientists have developed a robust aerosol lidar suitable for both process and monitoring studies.

2. Conduct process studies to determine the rates and efficiencies of the factors controlling the nucleation, growth, distribution, transport and removal of particles in the atmosphere.

AL, CMDL, and PMEL/JISAO scientists have participated in several national and international atmospheric chemistry field studies and are planning to participate in both ACE-1 and ACE-2. NOAA can bring to these experiments expertise in gas phase oxidant chemistry (AL & PMEL), gas phase ammonia measurements (PMEL), individual particle chemical analysis (AL), size-resolved bulk chemical analysis (PMEL), and aerosol physical and radiative measurements (CMDL & PMEL/JISAO).

3. Actively support the aerosol activities of IGAC by taking a leadership role in coordinating field projects and conducting field measurements. This type of international cooperation will help to ensure that the data sets collected by individual groups throughout the world are directly comparable and will provide the critical mass of investigators and instruments necessary for column closure and process studies.

PMEL and CMDL scientists are members of the IGAC aerosol activities. PMEL scientists have been instrumental in organizing both ACE-1 and ACE-2 and will continue to take a leading role in these field campaigns. PMEL/JISAO scientists are leading workshops on aerosol impactor and aerosol sizing techniques to test methodologies and develop standard sampling techniques.

2.4.3 Closure Studies

1. Conduct local closure experiments in a variety of different air masses that represent mixtures of the key aerosol types (water-soluble inorganic, organic, elemental carbon, and mineral dust). These closure studies need to be conducted in both natural and anthropogenically perturbed air masses in order to be able to separate the forcing due to anthropogenic aerosol from the total (natural and anthropogenic) aerosol system.

PMEL scientists have been conducting local closure studies on transit cruises throughout the Pacific Ocean and during intensive field experiments (PSI, IGAC-MAGE). The transit cruise and intensive study of ACE-1 and ACE-2 will provide an opportunity to conduct these local closure experiments in a variety of different air-masses.

2. Conduct clear-sky radiation closure experiments in a variety of different air masses, including both natural and anthropogenically-perturbed, to compare and calibrate satellite radiation measurements and surface-based, column-integrated radiation measurements. While NOAA has the expertise to make the surface and satellite measurements needed for these column closure experiments, the expertise in aircraft measurements of aerosol chemical, physical, and radiative properties will need to be provided by PI’s from other agencies. Plans are currently underway to conduct these interagency experiments as part of ACE and TARFOX.

PMEL and NESDIS scientists will conduct both the ground based and satellite measurements during the column closure experiments of ACE and TARFOX.

3. Continuous Observations and Monitoring

A primary hypothesis of the NOAA aerosol/climate research program is that the climate forcing by anthropogenic sulfate will change in response to future changes in sulfur emissions. The forcing is expected to decrease in and downwind of the U.S. as a result of emission controls mandated by the Clean Air Act, while continued economic development in China and other developing countries is expected to lead to an increased forcing in and downwind of those areas. Testing this hypothesis will require a coordinated research program involving modeling, monitoring, process, and closure studies. This section describes the continuous observations and monitoring component of the program.

NOAA has major strengths in aerosol measurements that can be brought to bear on the question of climate forcing by aerosols. No single approach to observing the atmospheric aerosol will provide the necessary data for monitoring all the relevant dimensions and spatial/temporal scales. In-situ observations from fixed surface sites, ships, balloons, and aircraft can provide very detailed characterizations of the atmospheric aerosol, but on limited spatial scales. Remote sensing methods, from satellites, aircraft, or even from the surface, can determine a limited set of aerosol properties from local to global spatial scales, but cannot provide the chemical information needed for closure with global chemical models. Fixed ground stations are suitable for continuous observations over extended time periods, but lack vertical resolution. Aircraft and balloons can provide the vertical dimension, but not continuously. Only when systematically combined, can these various types of observations produce a data set where point measurements can be extrapolated with models to large geographical scales, where satellite measurements can be compared with the results of large-scale models, and where process studies have a context for drawing general conclusions from experiments conducted under specific conditions.

The aerosol properties needed from the continuous monitoring component of an aerosol/climate program can be divided into extensive and intensive categories. Extensive properties depend on the aerosol concentration, while intensive properties are independent of the amount of aerosol present. For calculations of direct forcing, chemical mass concentrations or the aerosol optical depth are examples of extensive properties needed, while the mass scattering efficiency, single- scatter albedo, and asymmetry factor are examples of intensive properties used in the calculations. In general, extensive properties describe a single property of the aerosol, while intensive properties describe relationships among various properties of the particles. These two categories have different applications and require different observational strategies. Model parameterizations and algorithms used to interpret some remote-sensing observations require values of various intensive properties of the aerosol. These values are needed for the various aerosol types that are included in the models (e.g., sulfate, organic carbon). Verification of model predictions and remote-sensing algorithms is based on the spatial and temporal distribution of extensive properties; these properties are also used directly to evaluate trends, effects, and responses to changes in emissions. Explicitly recognizing the different uses of these two categories, the objectives of NOAA's aerosol/climate continuous observations and monitoring program are:

3.1. Surface Networks

The two different strategies for continuous measurements of intensive and extensive aerosol properties are reflected in the required measurement platforms. A limited number of comprehensively-instrumented sites or platforms, located in areas dominated by different aerosol types, is appropriate for monitoring intensive aerosol properties. Monitoring of extensive properties requires either satellite-based instruments or simple surface-based instruments at many locations. A comprehensive set of measurements is needed for characterization of the important intensive properties, much more than is needed for characterizing extensive properties, and so only a limited number of sites can be supported. Table 1 lists the categories of monitoring sites that are needed.

Table 1: Categories of sites needed for comprehensive monitoring of intensive and extensive aerosol properties.

polluted continental Industrial and other anthropogenic aerosols
polluted marine Anthropogenic aerosols, sampled after one or more days of transport and transformation after leaving the source regions
clean continental characterize continental aerosols under conditions of minimal anthropogenic influence, for comparison with polluted conditions. Given the wide variety of continental conditions (forest, desert, plains), it will be necessary to sample at a number of sites, although perhaps not all simultaneously.
clean marine characterize marine aerosols under conditions of minimal anthropogenic influence, for comparison with polluted conditions.
biomass-combustion Aerosols formed by combustion of biomass
mineral dust Wind-blown dust, sampled after one or more days of transport and transformation after leaving the source regions
free troposphere characterize aerosols above the boundary layer under conditions of minimal anthropogenic influence, for comparison with polluted conditions.
upper troposphere and lower stratosphere Surface-based monitoring of intensive properties is impossible. Primary monitoring platform for intensive properties should be balloons. Satellites and surface-based lidars should be used for extensive radiative properties.

The set of parameters that should be measured at the comprehensive monitoring sites is summarized in Table 2. The recommended measurements will provide a continuous time-series of the intensive aerosol properties needed for calculating aerosol radiative forcing. It may become appropriate to add measurements of CCN number concentration at a later date, but considerable process research and instrument development are necessary before a suitable monitoring strategy can be derived.

Table 2: Recommended Measurements at Comprehensive Monitoring Sites

Description Recommended approach
Mass concentration of important chemical species including major ions, organic carbon, elemental carbon, and trace elements. The total mass concentration should also be determined. Impactor/filter sampler. Ion chromatography, combustion, PIXE. Measurements should be obtained in two size fractions (submicrometer, supermicrometer).
Aerosol optical depth at ca. 5 wavelengths in the range 0.35-0.90 µm Tracking sun photometer or shadowband radiometer
Sattering, hemispheric backscattering, and absorption components of the aerosol light extinction coefficient, at wavelengths of 450, 550, and 700 nm. Integrating nephelometer, continuous light absorption photometer
Total number concentration Condensation particle counter
Hygroscopic growth factor for scattering and hemispheric backscattering components of the aerosol light extinction coefficient. Humidity-controlled integrating nephelometer
Vertical profile of aerosol backscatter LIDAR
Number size distribution, 0.05-5 µm diameter Differential mobility analyzer, optical particle counter, aerodynamic particle spectrometer

Measurements of the surface radiation budget (upward and downward, solar and terrestrial radiation) should also be made at the comprehensive aerosol monitoring sites. Changes in the aerosol, e.g. as a result of mandated reductions of sulfur emissions in the U.S., are likely to be major contributors to changes in the radiation budget, and dedicated networks exist for monitoring changes in solar radiation reaching the surface. However, the existing radiation networks are not measuring aerosol properties, and thus interpretation of the effects of aerosols on the results from these networks is severely limited. By complementing the comprehensive aerosol monitoring sites with radiation budget measurements, it will at least be possible to evaluate the effects of aerosol on the surface radiation budget at a few locations.

Another reason for measuring the surface radiation budget together with aerosol properties is to allow evaluation of the effects of changes in aerosol on cloud radiative properties. By combining satellite observations of cloud albedo and the radiation budget at the top of the atmosphere with surface-based aerosol observations and the surface radiation budget, it should be possible to make an initial estimate of the sensitivity of cloud radiative properties to the below-cloud aerosol.

The most important extensive aerosol properties to monitor are aerosol optical depth and chemical mass concentration. These parameters are primary outputs from aerosol chemical transport and radiative transfer models, and model verification needs data from a sufficiently large number of sites to characterize the major gradients across the U.S. The now-defunct NOAA network for monitoring aerosol optical depth across the U.S. should be revived, and the measurements should be closely coordinated with optical depth measuring networks being supported by NASA and DOE.

The strategy for monitoring aerosol mass concentrations should be to utilize existing monitoring networks to the largest extent possible. Although many local, state, national, and international monitoring networks include aerosol measurements, much of the available data are of limited utility for climate forcing calculations because they lack an appropriate upper particle size limit. Measurements of total suspended particulate mass, and measurements made according to the U.S. PM-10 standard, are largely unsuitable because they include particles that often dominate the total mass but contribute little to the radiative or cloud-nucleating properties. This problem is greatly reduced, but not eliminated, with measurements made according to the U.S. PM-2.5 standard, which excludes particles larger than 2.5 µm diameter. Extensive aerosol properties determined with this size cut are suitable for comparisons with model predictions of aerosol mass concentrations. These data are also useful for determining the aerosol mass balance by chemical species, which can be used to estimate the relative contribution of different species to light extinction. However, measurements of mass scattering or absorption efficiencies of aerosol particles made with a size cut larger than about 1 µm are very sensitive to local variations.

3.2. Mobile Platforms

The utility of surface-based measurements is greatly enhanced with an understanding of the vertical distribution of the aerosols. Airborne measurements of intensive and extensive aerosol properties can be used to evaluate the representativeness of the surface observations, both through the vertical column and over larger horizontal scales. A limited-duration program, using a single aircraft to support the surface observations in a nearly continuous circuit of the geographically distributed sites is recommended. Such an airplane could operate essentially continually, shuttling to sites across the U.S. with a roughly weekly schedule. Instrumentation on the aircraft should be kept fairly simple: a suite of spectral radiometers, in-situ measurements of aerosol scattering, hemispheric backscattering and absorption coefficients, and size-resolved chemical samplers. This suite of measurements covers the most important extensive and intensive aerosol properties. By shuttling between the surface monitoring sites (where cooperative vertical soundings would be made), not only would the surface program be routinely augmented with soundings, but observation over a unique geographical scale component could be added to the program.

At present, the only long-term record of the vertical distribution of tropospheric and stratospheric aerosols is the 24-year record of vertical profiles of aerosol size distribution obtained by the Univ. of Wyoming at Laramie, Wyoming. These measurements are useful for identifying the sources and sinks of aerosols in the upper atmosphere, for observing long-term changes in aerosol radiative properties, and for testing the assumptions in satellite data retrieval algorithms. Continuation of this long-term record is clearly warranted.

In a similar fashion to airborne monitoring, a routine monitoring program from ships would add a valuable component for developing and testing chemical transport models and satellite data retrieval algorithms. This approach is an integral part of flask-sampling networks for monitoring atmospheric greenhouse gases from ships carrying freight along regular routes. The suite of measurements would need to be quite limited, for logistical reasons. As a minimum, size-resolved samples of the aerosol should be collected in fixed, pre-determined regions along the track, with subsequent analyses for total mass, major ions, and light absorption. For satellite applications, continuous measurements of aerosol optical depth are also needed.

3.3. Satellite Platforms

The only feasible approach for global-scale aerosol monitoring is through satellite platforms. The current NOAA operational product of aerosol optical depth has made significant contributions to understanding qualitatively the global-scale distribution of aerosol optical depth over the oceans, its seasonality, and the frequency of occurrence of events with elevated optical depths. However, the retrieval of aerosol optical depth from upwelling radiance is an ill-posed problem that requires assumptions about aerosol properties that are not measured from the satellite. Furthermore, the observations are limited to oceanic regions only, whereas the bulk of the direct radiative forcing by anthropogenic sulfate is calculated to occur over the continents. In order to surpass these limitations, NOAA should work with other agencies to address the question of how best to quantify the measurement of aerosol optical depth from satellite, and then implement the resulting recommendations. Another question to be addressed, although of lower priority, is the measurement of other aerosol properties, such as effective particle size, from satellites. Because of the limited ability of satellites to observe the intensive properties of aerosols that are needed by the satellite data retrieval algorithms, data from fixed and mobile platforms must be an integral part of the approach for satellite-based aerosol monitoring.

3.4. Supplemental Research

In addition to obtaining long-term measurements, the continuous monitoring program must also include a research component into the way monitoring is done. Use of standardized sampling protocols is a necessity for obtaining comparable results, but no single protocol is optimal for all conditions and chemical species. For example, humidity-controlled sampling is necessary to ensure that the results are not controlled by variations in atmospheric relative humidity. However, the process of lowering the relative humidity also can affect the chemical composition of the particles, possibly introducing systematic biases into the measurements. The magnitude of such effects must be evaluated over the broad range of conditions encountered at the sampling sites, and revised or supplemental sampling protocols developed to remove any such biases. Another recommended activity is to review the various aerosol monitoring programs presently or formerly in operation, and determine which measurements are suitable for climate studies. Efforts should also be made to guide the choice of sampling protocols by future monitoring programs, so that their results are useful for climate studies.

Some of the needed measurements are difficult and expensive to perform on a continuous basis. Consequently, the measurement strategy should include intermittent, short-term observational periods for such parameters, perhaps in conjunction with closure experiments. The measurement strategy also needs a periodic evaluation of the results of the monitoring program, to address the questions of how long to continue the measurements and what changes in experimental approach are necessary. Meteorological variability from year-to-year suggests that an initial observational period of 5-10 years is needed, and may need to continue even longer if significant trends or large variability in the results are observed.

3.5. Specific Monitoring Recommendations

The specific recommendations for the monitoring component of NOAA's aerosol/climate research program are:

1. Monitor intensive and extensive properties of key aerosol types at the surface for an initial period of ten years, with an evaluation at the end of the period to determine if the results warrant continuation of the observations. Supplement the aerosol observations with measurements of the radiation budget at each site.

CMDL and PMEL have established four sites in North America and are currently building up the measurement capabilities there. ARL is measuring the radiation budget at one of these sites, and CMDL plans to do this at the other three sites.

2. Monitor the aerosol optical depth from a network of surface sites spanning the U.S.

CMDL and ARL both have on-going programs that could be brought to bear.

3. Co-ordinate with other national and international aerosol monitoring programs to obtain data that can supplement the NOAA aerosol/climate monitoring program.

CMDL scientists participate actively on the WMO/GAW aerosol steering group, and serve as contact persons on three of the five main aerosol parameter areas in GAW.

4. Monitor the vertical distribution of aerosol size distribution in the troposphere and lower stratosphere from balloon-borne platforms.

The on-going collaboration between CMDL and University of Wyoming could be formalized to ensure continuation of the existing 24-year record.

5. Routinely measure from airborne platforms the vertical and horizontal distributions of intensive and extensive aerosol properties in the troposphere across North America for a period of three years, with an evaluation at the end of the period to determine if the results warrant continuation of the observations.

CMDL will conduct a pilot study as part of the AL SOS program in the summer of 1995.

6. Routinely measure intensive and extensive aerosol properties from ships along transects crossing the Pacific and Atlantic Oceans for a period of three years, with an evaluation at the end of the period to determine if the results warrant continuation of the observations.

PMEL has proposed a pilot study in the Pacific.

7. Work with other agencies to address the question of how best to quantify the measurement of aerosol optical depth from satellite, and implement the resulting recommendations. Measurement of other aerosol properties, such as effective particle size, should also be addressed, albeit at a lower priority.

The NESDIS operational product of aerosol optical depth based on AVHRR measurements has limitations that must be surpassed.

4. Modeling

The central objective in this component of the Aerosol/Climate project is to simulate quantitatively, using numerical models, the distributions of the aerosols in the atmosphere, their spatial and temporal characteristics, their radiative properties and their effects on the climate in a robust, self-consistent manner. In order to attain this goal, research on the following elements is necessary: (a) theoretical developments combined with observational studies to improve our basic knowledge of the relevant physical and chemical processes that govern the concentrations of aerosols in the atmosphere; (b) accurate representations and improved modeling of the relevant processes in three-dimensional models (such as limited domain and mesoscale models, and global general circulation models or GCMs); and (c) gain a comprehensive understanding of the quantitative nature of the interactions amongst the relevant physical and chemical processes associated with aerosols, including meteorological processes, thereby improving the scientific assessment of the aerosol-climate problem. Because of the critical significance of this problem in the context of global climate change, attention has to be focussed especially on the anthropogenic aerosols.

The success of the modeling component requires a firm understanding of the mechanisms governing the sources, transport and transformation, and sinks of aerosols. It is important to recognize that, at present, a major uncertainty in the aerosol-climate problem is the incomplete understanding of some of the individual physical and chemical processes. This precludes their accurate representation in climate models, and thus leads to uncertainties in the three- dimensional modeling of the interactions that occur between the different processes. Therefore, the modeling component necessarily has to include research that would elucidate the quantitative aspects of the different processes and facilitate parameterizations for use in climate models. Theoretical analysis and laboratory measurements could contribute to developments in this regard. However, some of the needed improvements may come only via field observations and process/closure experiments. It is further emphasized that both the process/closure study and the monitoring components (as described elsewhere in this report) are indispensable for ascertaining the fidelity of the results from the modeling exercises, be it a verification of aerosol radiative effects, aerosol chemistry, aerosol microphysics, the transport aspects or the response of the climate system to anthropogenic aerosol forcing. In fact, only through simultaneous research and joint progress in the modeling, laboratory/observational process/closure study, and the monitoring components can we hope to make substantive advancements in our knowledge of atmospheric aerosols and their climate effects.

4.1. Modeling of aerosol radiative properties

The parameters of concern include the wavelength-dependence of the extinction coefficient, single-scattering albedo and the asymmetry factor of the particles, their dependence on the shape, size distribution, chemical composition and relative humidity. The usual practice in dealing with the optics of aerosols is to assume spherically-shaped particles of a homogeneous composition. Clearly, the actual atmospheric aerosol could be nonspherical and heterogenous. Radiative properties of such particles need to be understood - this implies better theoretical methods as well as laboratory and field campaigns.

4.2. Modeling the column radiative transfer

The factors determining the spectrally-dependent radiances and radiative fluxes in atmospheres containing aerosols need to be investigated thoroughly. It is necessary to carry out a systematic series of sensitivity tests that establish the quantitative dependence of the radiances and fluxes to relative humidity, vertical profile of particles, geographical and temporal variations of surface albedo and clouds, and overlap of aerosol radiative effects with those of water vapor and carbon dioxide. Results are required for the transfer in both the solar and the infrared spectrum. Computations using rigorous radiative transfer algorithms, as applicable to scattering-absorbing atmospheres, should provide 'benchmarks' that are useful for calibrating and verifying the parameterizations employed in GCMs. These sensitivity tests would also aid in the comparative analyses of the computed and measured radiative quantities e.g., in the closure experiment campaigns. Such comparisons ought to provide a stringent test of the ability to model the radiative effects of aerosols in clear and overcast skies.

Radiative transfer computations, coupled with long-term monitoring of aerosol properties (e.g., optical depth) and routine transmission measurements, should be undertaken to provide a measure of the temporally evolving radiative effects due to aerosols. For a global perspective of the effect of aerosols on the net energy absorbed in the climate system, a comparison of the computed and satellite-derived fluxes at the top-of-the-atmosphere is highly desirable; currently, however, there are practical limitations in the remote sensing techniques that prevent rigorous comparisons.

4.3. Aerosol microphysics and chemistry modeling

The microphysical and chemical aspects of aerosols are important factors determining the aerosol concentrations and radiative properties. It is necessary to model the distribution of aerosols in the atmosphere to a high degree of accuracy given the source strengths and locations. For example, in the case of sulfate aerosols, it is necessary to incorporate appropriate chemical reactions, including heterogeneous ones, and relevant microphysical transformation equations. Another important aspect is the model's meteorological processes (e.g., winds, convection and precipitation) which governs the sinks, the residence time of the aerosols, and their dispersion and fall-out away from the source regions. One way to determine the aerosol distributions is to specify the meteorological fields from some source (these could be analyzed fields, say, from a weather prediction or an assimilation model or the outputs from a climate GCM) and use them to 'drive' a microphysics/chemistry model. Radiative parameterizations can be added to such models in order to determine the resulting forcing.

It is also important to be able to carry out modeling studies that determine the effects due to the alteration of cloud properties by aerosols, the so-called 'Twomey' (indirect) effect. However, this is not a well-posed problem as yet and there are several facets that are not presently understood at a fundamental level. More observations are needed to understand the important factors governing this mechanism. As far as modeling efforts are concerned, what is feasible at present is to perform a series of sensitivity computations involving the potential microphysical changes in clouds and obtain an approximate estimate of the radiative effects.

4.4. Mesoscale modeling

The synthesis of the radiative-meteorological-chemical-microphysical processes and the consequences of the interactions can be studied in the context of limited domain or global general circulation models. In contrast to the more elaborate and complex global GCMs, mesoscale models are potentially more useful in understanding the aerosol effects over small spatial domains (continental scales or less). Such models can afford to have more elaborate representations of the physical and chemical processes than GCMs. These models also provide a useful framework to examine in detail the effects due to parameterizations of the processes operating on small spatial scales, such as those associated with microphysics and chemistry. Also, transport of aerosols on the mesoscale can be examined with the help of these models. A further usefulness of these models is the ability to study in detail the role of the planetary boundary layer, particularly the surface heat and moisture balance, in the response to aerosol radiative perturbations. A specific goal of such models could be the investigation of the effects of anthropogenic sulfate aerosol (produced by the sulfur gas emissions) on the decreases in solar radiation and temperature, particularly the diurnal variations of the surface temperature. Another goal of the limited area models, particularly those with coupled 3-D dynamics, radiation and explicit microphysics, could be the assessment of the indirect climatic effects of aerosols, as introduced by changes in cloud-scale processes.

4.5. Global General Circulation Models (GCMs)

Of central importance in the global climate change problem is the accurate simulation of the aerosol distributions as a function of space and time and their radiative and climatic effects. While GCMs have begun to attack the microphysics-chemistry-transport-radiation-climate interaction problem, there remain several elements of uncertainty concerned with various aspects of the physical and chemical processes. Considerable model development work needs to occur to resolve the uncertainties, some of them at fundamental levels of technical details. A key aspect of such modeling efforts is to be able to verify the results of simulations which, in essence, is a test of the representation of the processes and their interactions in the model. While a test of the radiative transfer parameterizations has already been discussed above and should be extended to whatever reliable space-time observations become available, it is equally important to be able to verify the chemistry and transport aspects on large scales. Opportunities to do the latter could come about during the closure experiments.

GCM sensitivity experiments need to be performed with aerosols, and their responses need to be systematically analyzed vis-a-vis well-mixed greenhouse gas simulations. A complete understanding is needed of the global and hemispheric surface temperature responses, the geographical and seasonal distributions of the changes in meteorological parameters, the role of the aerosols in determining the transient temperature changes, and the impact of aerosols on the change in the diurnal temperature variations. An all important objective is to compare and interpret objectively the meteorological responses of the model with the available observations.

Although the modeling of both the direct and indirect forcing are of importance, the current lack of adequate knowledge on the latter suggests that attention should be directed first at the direct forcing problem. A common factor in both cases is the diagnoses of the feedback processes, especially cloud feedbacks, due to the aerosol perturbations. An array of carefully designed GCM experiments are required to delineate the different scientific aspects of the aerosol-climate problem.

4.6. Specific Modeling Recommendations

The specific thrusts of the project are aimed at the basic understanding of the relevant processes, their accurate representation in models, the simulation of the space-time aerosol properties and their radiative and climatic effects. Specific areas of action are:

Appendix A: Panel Members and Observers


D. Hofmann (chair), R/E/CG

T. Bates, R/E/PM R. Charlson, U. Washington

Y. Kogan, U. Oklahoma J. Ogren, R/E/CG1

V. Ramaswamy, R/E/GF A. Ravishankara, R/E/AL2
S. Schwartz, DOE/BNL L. Stowe, E/RA11


D. Gillette, R/E/AR2 M. Hardesty, R/E/ET2
S. Piotrowicz. R/PDC M. Coughlan, R/OGP
D. Albritton, R/AL S. Liu, R/E/AL4
S. Solomon, R/AL8

Appendix B: Summary of current programs

B.1 Aeronomy Laboratory (AL)

Aerosol research in the Aeronomy Laboratory includes three major components: laboratory studies, field campaigns, and model studies. Because of strong interest in the stratospheric ozone depletion and acid precipitation problems in the 1980's, emphasis of aerosol research has been on the effects of heterogeneous reactions on/in aerosols on trace gas concentrations. Recently, there has been a significant shift in emphasis to include assessing the radiative effect of aerosols on the regional climate, understanding the chemical and physical properties of aerosols, and evaluating the effects of heterogeneous reactions on greenhouse gas concentrations. It is important to recognize that substantial advances have been made in the understanding of the fundamental processes of polar stratospheric clouds (PSC) and the heterogeneous reactions between trace gases and aerosols. These advances have enhanced and will continue to increase our understanding of key processes in the aerosol/climate system.

B.1.1. Laboratory Studies

DMS oxidation: This study examines the key gas phase photochemical processes in the atmospheric oxidation of dimethylsulfide (DMS) that ultimately lead to aerosol formation. The major objective is to test the CLAW hypothesis, which states that biogenic emissions of reduced sulfur compounds affect the albedo of clouds over the ocean, and thereby the climate. The key questions being addressed include: (1) what are the atmospheric end products of DMS oxidation? (2) what fraction of the DMS leads to formation of new particles? and (3) Where in the atmosphere would these particles be created?

Reactions on/in Aerosols: The research includes the determination of rates and products of heterogeneous reactions and the effect of these reactions on the atmospheric composition, particularly on the concentrations of greenhouse gases.

Radiative properties of Aerosol Constituents: The major objective is to measure the refractive indices, both real and imaginary parts, of aerosol constituents. These radiative properties are fundamental for calculating radiative forcing due to aerosols. They are also critical for characterizing aerosols in field and laboratory experiments.

B.1.2. Field Experiments

Instrument Development: A crucial component of field experiments is to develop advanced instruments needed for characterizing the chemical and physical properties of aerosols. Two major efforts in this area include an instrument for analyzing the composition of individual aerosols and a fast response, high sensitivity HNO3 detector. Elemental composition measurements utilize the photoionization and mass-spectrometry (PALMS), a new experimental tool pioneered in AL, to measure composition of individual aerosol particles. It has already been tested in a field campaign in the summer of 1994. The instrument to measure HNO3 is still under development and is expected to be field-tested in the spring of 1995.

Tropospheric Chemistry Campaigns: Since early 1980's, a large number of ground- and aircraft- based campaigns have been carried out to study tropospheric chemistry. Although the major objective of these campaigns is to understand the budgets of oxidants and their precursors, a significant effort has been directed towards investigating the sulfur budget and aerosol composition. For example, four field experiments involved with the Southern Oxidants Study (SOS) have been completed over the southeastern United States in the last five years. In addition, two multiple-nation and multiple-agency field campaigns over the western Pacific (PEM-West A & B) were conducted in 1991 and 1994, respectively.

Stratospheric O3 Depletion: In collaboration with many national and international agencies and universities, several ground- and aircraft-based campaigns over the Antarctic and the Arctic (e.g., AAOE and AASE) have been successfully conducted since the mid 1980's to study the PSCs and related heterogeneous reactions. Additional campaigns are now being carried out and are expected to continue in the future.

B.1.3. Modeling Studies:

Tropospheric Aerosols: The study utilizes the three-dimensional regional dynamical- photochemical model developed at the AL. The foci include modeling the aerosol composition and distribution, comparison with field observations, and evaluating the radiative effects of aerosols on the regional climate. For example, a recent study investigated the transient radiative effect of anthropogenic aerosols on the climate/weather over the eastern United States.

Stratospheric O3 Depletion: In the last decade, substantial advances have been made in understanding the role of polar stratospheric clouds (PSC), sulfuric acid aerosols and the heterogeneous reactions on such particles in the depletion of stratospheric ozone.

B.2 Air Resources Laboratory (ARL)

Two activities of the Air Resources Laboratory are directed towards climate forcing by atmospheric aerosols: the study of soil derived aerosols and the investigation of solar radiation changes using a rotating shadow-band sunphotometer network.

Studies of the radiative properties of soil derived aerosols show relatively unchanging size distributions and real and imaginary parts of the index of refraction for soil-derived aerosols ranging from the Sahara to Texas to Central Asia. Concentrations of soil-derived aerosols vary greatly in time and location, however. It is proposed that these rather uniform aerosol characteristics and the extremely variable concentration of soil-derived aerosols make the most cost-effective studies on the forcing of climate by soil aerosols to be quantification of the change of soil aerosols with time. Present studies directed towards this goal are mechanistic studies of aerodynamics of soil particle emissions, of sandblasting mechanisms that produce aerosols finer than 10 micrometers, and verification of models for soil aerosol production. The two main study areas used in this activity are the Owens (dry) Lake in California and the Jornada del Muerto experimental range in New Mexico.

The second activity relates to the development and optimal use of various kinds of rotating shadow band radiometers. There are three variations now in use, each with a specific purpose but each also employing an identical measurement philosophy - as much as possible to depend on the use of a single sensor to measure different irradiance components, to minimize difficulties caused by sensor deterioration and to minimize the consequences of different sensor performance characteristics when operating in a spatial array.

Some practical applications of atmospheric extinction data do not require exceedingly high accuracy but rather continuing long-term precision. Relative measurements that encapsulate changes in scattering conditions can be made at small cost but with lasting benefit. Such measurements include monitoring the relative magnitude of the direct and diffuse irradiance components in an optically active portion of the solar spectrum, in order to derive the aerosol optical depth. A single sensor that is periodically shaded by a rotating shadow band provides adequate data for many purposes of this kind. The simplest form of rotating shadow band radiometer is of this type. A single silicon cell quantum sensor is mounted horizontally at the center of the sphere described by a rotating band, thick enough to provide a short period of shade but narrow enough to minimize errors in the measurement of total irradiance. Broad-band data are collected automatically, usually at intervals short enough to ensure that a representative fully-shaded value is reported for each brief pass of the shadow-band.

Instruments at various levels of complexity are now available, and some of these are potentially suitable for routine use in areas lacking expert technical capabilities. At this time, there has been only limited side-by-side comparison of these various sensors; there has not yet been an objective and independent assessment of their relative benefits. Some field tests have already been started; plans are in place to build upon the experience obtained so far in a series of additional tests conducted in collaboration with the WMO.

B.3 Atlantic Oceanographic and Meteorological Laboratory (AOML)

The AOML aerosol/climate research program began in 1990 in collaboration with PMEL/JISAO and funding from the NOAA Office of Global Programs. This funding permitted AOML scientists under the direction of A. Pszenny take a lead role in the IGAC/MAGE/ASTEX field experiment and to conduct numerous oceanic latitudinal transects. The results from these field programs have been reported in 9 refereed journal articles (1991-1994). The major emphasis of this work was on aerosol chemical mass size distributions and the effect of these particles on the cycling of sulfur and chlorine in the marine boundary layer.

The current AOML aerosol program is funded from laboratory base and includes:

B.4 Climate Monitoring and Diagnostics Laboratory (CMDL)

Aerosol research has been an important activity at CMDL since the inception of the Geophysical Monitoring for Climate Change program in 1974. The GMCC activity involved establishing baseline stations in regions far removed from anthropogenic pollution sources, in an effort to detect trends in global-scale pollution. In response to reports that anthropogenic aerosols might be responsible for a climate forcing that is comparable in magnitude, but opposite in sign, to the forcing by anthropogenic greenhouse gases, CMDL began in 1991 to establish a regional-scale aerosol network.

B.4.1. Climate Forcing by Anthropogenic Aerosols.

The regional-scale network consists of four stations across North America, with sites in marine and continental, clean and polluted areas. There is an additional, polluted continental station located in central Hungary. These sites are operated cooperatively with local academic or governmental organizations. Measurements are focussed on determining means, variability, and trends of aerosol chemical, radiative, and microphysical properties relevant to climate forcing, for different aerosol types. These properties include the aerosol light scattering and absorption coefficients, optical depth, total number concentration, total mass concentration, and mass concentration of major ionic species. A key aspect of the experimental approach is linkage between the chemical and physical measurements, so that the observations can be used to develop and validate the treatment of aerosols in both chemical transport models and global climate models. This linkage is enhanced by the use of size-resolved and humidity-controlled sampling protocols that specifically address the systematic dependence of particle composition on size, as well as the strong dependence of particle size on relative humidity.

B.4.2. Other Aerosol-related Research

Measurements of aerosol light scattering coefficient and total number concentration have been obtained at the four CMDL baseline stations for 15-20 years. During recent years, measurements of aerosol light absorption have been obtained at three of the stations. The results can be used to guide the choice of aerosol parameters, such as Ångström exponent and single- scatter albedo, used in aerosol climate forcing calculations for remote, free tropospheric aerosols. Measurements at the Mauna Loa, HI observatory also include aerosol optical depth and vertical profiles of aerosol backscattering, both of which can be used in studies of climate forcing by volcanic aerosols. CMDL's network of eight sites for studying the surface radiation budget provides a time series of high-quality observations that could be combined with aerosol measurements to study aerosol-related perturbations to the surface radiation budget.

Over the years, CMDL has contributed to a number of special aerosol research projects. A notable example is the Front Range Lidar, Aircraft, and Balloon experiment of 1989-90, which was a closure experiment involving vertical profiles of aerosol physical and radiative properties. Such special projects continue, with contributions planned for the IGAC/ACE experiment at Cape Grim, Tasmania (light scattering and hemispheric-backscattering coefficients at three wavelengths, in two particle size ranges) and the NOAA/SOS aircraft measurement program in the eastern United States (light scattering and hemispheric-backscattering coefficients at three wavelengths, light absorption coefficient, total number concentration, chemical composition).

B.5 Environmental Technology Laboratory (ETL)

B.5.1. Long term aerosol backscatter climatology

From the period 1981-1993, ETL measured weekly profiles of aerosol backscatter in the troposphere and lower stratosphere. Measurements were obtained at both 10.6 µm and 694 nm (the 694 nm measurements were discontinued in the late 1980's, then begun again on an intermittent basis following the Pinatubo eruption). The data include measurements obtained during a one-month period at Mauna Loa observatory. Primary goal of the effort was characterization of 10.6 µm backscatter climatology to determine feasibility of a space-based Doppler lidar system for wind measurements. Because the measurements encompassed two significant volcano events (El Chichon and Pinatubo), the data were used to compute and compare the settling times for large particles, as well as to investigate stratospheric-tropospheric exchange during tropopause folding events.

The aerosol backscatter climatology work was stopped after support for a space-based Doppler lidar operating in the 9-11 µm region was significantly cut back. Currently, because of renewed interest in the possibility of a space-based lidar operating at 2 µm, we have initiated discussions with NASA to measure lower tropospheric aerosol backscatter at both 2 and 10 µm wavelengths.

B.5.2. Development of an unattended lidar for cloud and aerosol monitoring.

A new instrument designed specifically for long-term, unattended monitoring of cloud and aerosol structure is nearly complete. The system incorporates a high pulse rate, low energy, doubled Nd:YLF laser transmitter operating at 523 nm to obtain continuous profiles of aerosol structure and cloud base height. It will be used to identify cloud presence and cloud base height for surface radiation studies, to measure aerosol structure for space-based sensor calibration and correction, and to determine the radiative effects of small boundary layer ice crystals (so-called "diamond dust") in the Arctic. System design incorporates a weather-proof housing and remote data transmission capability to enable unattended operation.

B.5.3. Lidar Remote Sensing of Aerosol: Distinction between hygroscopic and non-hygroscopic particles.

We have completed a theoretical study which examines the differential backscattering from multiwavelength lidar systems and the feasibility of differentiating between hygroscopic and non-hygroscopic aerosol. Results show that infrared or near-infrared wavelengths in conjunction with a UV wavelength provide a clear measure of mean particle size. This information can then be used together with the ambient relative humidity to obtain a measure of the ratio of hygroscopic to non-hygroscopic aerosol concentration, provided giant aerosol particles are absent. This work needs to be followed up by simultaneous in-situ and remote measurements to determine just how robust the technique is. Airborne measurements with horizontally pointing lidar systems should provide the best chance of success. If successful, the significance for climate monitoring, cloud and visibility studies will be far reaching.

B.5.4. Modeling of Cloud Processing of Aerosol

A number of recent studies have shown that not only is the microphysical structure of clouds (and hence their radiative properties) a function of the cloud condensation nucleus (CCN) size distribution, but also that cloud processes themselves affect the size distribution of CCN. Two factors are at work; the first is related to gas and aqueous phase chemistry within the cloud, while the second is due to drop collision-coalescence, and has been termed "coalescence scavenging".

We have undertaken a study to quantify cloud processing of aerosol in a large eddy simulation of marine stratocumulus cloud. The model includes explicit treatment of cloud droplet and interstitial aerosol spectra and has recently been modified to keep track of solute content within the cloud droplets. As such, it forms a framework for evaluating the process of coalescence scavenging as well as a future coupling with a gas and aqueous phase chemistry.

B.6 Geophysical Fluid Dynamics Laboratory (GFDL)

Aerosol research at the Geophysical Fluid Dynamics Laboratory is focussed on understanding the effects due to aerosols on radiative transfer and their impacts on the climate of the surface- atmosphere system. The research makes extensive use of the `benchmark' radiative transfer algorithms and the General Circulation Models developed at the Laboratory, along with available and relevant satellite data sets. Topics of current research are described below:

B.6.1. Top-of-the-atmosphere clear-sky shortwave flux

Using a high-spectral resolution radiative transfer model, the computed clear-sky shortwave reflected fluxes at the top-of-the-atmosphere have been compared with the Earth Radiation Budget satellite observations over the Tropical Western and Central Pacific. The comparisons reveal that, in the presence of only the molecular constituents, the computed and observed fluxes do not agree anywhere over the domain considered. The underestimate in the computed fluxes strongly suggests the presence of aerosols and also indicates their significant role in determining the albedo of clear skies and, thus, the clear-sky solar energy absorbed by the surface- atmosphere system. This result emphasizes that even small aerosol optical depths, as estimated for the present-day column and also as estimated for the anthropogenic perturbation over the past century, are capable of resulting in significant radiative flux changes within the atmosphere- surface system.

B.6.2. Albedo perturbations and the global climate sensitivity

Numerical experiments have been undertaken using the GFDL Climate GCM. These experiments have been designed to investigate the response of the atmosphere, coupled to a mixed layer static ocean, to global and regional imposed albedo perturbations. The regional albedo perturbations mimic the effects due to the anthropogenic aerosols in the midlatitude northern hemisphere. These responses are compared with the responses to greenhouse gas increases. The results show that i) the ratio of global surface temperature response to forcing (i.e., climate sensitivity) is essentially invariant, whether the radiative perturbation imposed is global or regional; further, the climate sensitivity for albedo perturbations is virtually similar to that for CO2 increases; ii) the albedo perturbations in northern midlatitudes yield a pronounced regional signature in the surface temperature response that is absent when the perturbations are globally distributed (e.g., CO2); and iii) the differential responses in the two hemispheres due to a northern midlatitude albedo perturbation lead to a distinct radiative-dynamical interaction near the equator; these, in turn, alter the meridional gradient of the diabatic heating, accompanied by changes of opposite signs in the precipitation field on either side of the equator; such a feature is not present in the context of a globally uniform radiative perturbation.

B.6.3. Modeling the transport of trace substances in the atmosphere

Work is in progress on developing a version of the GFDL 'SKYHI' GCM as a tracer model, suitable for performing a global simulation of the distribution of radiatively active species in the atmosphere. The physical schemes necessary for a successful simulation of the atmospheric concentrations of particles include a reasonable radiative transfer parameterization of the aerosol radiative properties and parameterization of the aerosol chemical and microphysical processes (e.g., gas-to-particle conversion, coagulation, wet and dry transformation processes). A computationally efficient radiative transfer parameterization has been developed and its accuracy is being examined using the results obtained from the `benchmark' algorithm.

B.7 National Severe Storms Laboratory (NSSL)

Research at the Cooperative Institute for Mesoscale Meteorological Studies (CIMMS) at the University of Oklahoma is focused primarily on aerosol indirect climatic effects introduced by changes in cloud microstructure. The CIMMS group has accumulated substantial experience in the modeling of both dynamics and microphysics of convective and stratocumulus clouds. The modeling part of the proposed research is based on the CIMMS stratocumulus cloud model which includes coupled 3-D large-eddy simulation dynamics, explicit microphysics, and radiative processes. The dynamical framework of the model is based on a 3-D LES spectral code developed at NCAR. Cloud physics processes of nucleation, condensation, evaporation, and coalescence are treated explicitly based on the prediction equations for cloud condensation nuclei and cloud drop spectra. The formulation of explicit warm rain microphysics has been recently expanded by inclusion of ice-phase processes based on the distribution function describing ice crystal spectra with 28 size categories. Microphysical processes considered include generation and growth of ice crystals and phase transition processes of freezing and melting. The processes of ice crystal generation include freezing of water drops due to immersion-freezing nuclei or due to collision with contact-freezing nuclei, heterogeneous water vapor deposition on deposition or condensation-freezing nuclei, homogeneous freezing of water drops when temperature falls below -40 C, as well as secondary ice particle production due to splintering during rime growth. The ice crystal spectrum evolves as a result of the processes of diffusional growth, evaporation, melting, coagulation among crystals and with water drops. CIMMS' model also includes a long wave radiation code, a 24-band solar radiation package, and a computationally efficient broadband solar radiation package.

In summary, the CIMMS LES explicit microphysical model provides a very powerful tool to examine cloud responses to various forcing mechanisms, including interaction between boundary layer dynamics, radiation, aerosols, cloud and drizzle particles. The main objectives of the research at CIMMS are:

B.8 Pacific Marine Environmental Laboratory (PMEL)

The PMEL/JISAO aerosol/climate program consists of process, closure, and monitoring studies designed to quantify the chemical and physical processes controlling the evolution and properties of the atmospheric aerosol that are relevant to radiative forcing and climate. The major focus of the program has been on the marine atmosphere which provides an opportunity to establish the chemical, physical and radiative properties of the natural aerosol and to compare and quantify the anthropogenic perturbations to this background aerosol. The process and closure studies conducted during these field-intensive campaigns are designed to provide the necessary data to incorporate aerosols into global climate models and to reduce the overall uncertainty in the calculation of climate forcing by aerosols.

The PMEL/JISAO aerosol/climate program began in 1990 with funding from the NOAA Office of Global Programs and some supplementary funding from NASA. This funding has enabled the PMEL/JISAO group to take a lead role in several national (PSI) and international (SAGA, IGAC/MAGE) aerosol field experiments and to conduct numerous oceanic latitudinal transects. The results from these field programs have been reported in special sessions of 3 AGU meetings and in 19 refereed journal articles (1991-1994).

The current PMEL/JISAO research emphases include:


Aerosol research at NESDIS involves remote sensing studies with NOAA/AVHRR (Advanced Very High Resolution Radiometer) satellite data. The objectives of the work are: 1) to measure aerosol radiative properties from reflected sunlight measurements; and 2) to correct for aerosol effects on sea surface temperature retrievals from AVHRR.

Aerosol optical thickness at 0.5 mm wavelength has been an operational product at NESDIS since January 1990. Daily, weekly, and monthly orbital and mapped products are available in chart form and in digital record format archived at the National Climate Data Center in Asheville, NC. The digital record began in July 1989, and has been interrupted by the failure of the NOAA/11 AVHRR on 13 September, 1994. It will be reinstated with the launch of NOAA/J early in 1995. The first two years of data have been analyzed to show, for the first time, temporal and spatial distributions of tropospheric aerosols over the oceans. Subsequently, the data were used to describe the evolution of the volcanic haze layer in the stratosphere resulting from the eruption of Mt. Pinatubo in June 1991. These latter results have been reported in the peer reviewed literature.

As the data are being subjected to more quantitative analyses, validation studies using very accurate simultaneous sun-photometer estimates of aerosol optical thickness have been conducted. The results of this validation have led to changing the ocean reflectance model and the aerosol model used in the retrieval. The latter was changed from a Junge power law to a log normal size distribution with a mean radius of 0.1 mm, geometric standard deviation of 2.03, and a complex index of refraction of 1.4 -0.0i. Further validation studies will be conducted as part of TARFOX (Tropospheric Aerosol Radiative Forcing Observational Experiment), a field experiment being planned for the summer of 1996 off the east coast of the U.S. The principal objective of this experiment is to measure parameters used in model simulations of the radiative forcing by anthropogenic aerosols. Additional validation and improvements of the retrieval model will be made using data from the ACE-1 and ACE-2 field programs, as well as from surface sites that monitor aerosol optical depth.

The information content of channel 2 (0.85 mm) and of a future generation AVHRR channel 3A (1.6 mm) is currently under investigation as part of our work within the NASA/EOS Cloud and Earth's Radiant Energy System (CERES) instrument science team. Conceptually the ratio of reflectances in either of these channels with channel 1 is sensitive to effective columnar averaged particle size.

Finally, infrared radiative transfer simulations have been used to study the effects of tropospheric and stratospheric aerosol layers on the derivation of sea surface temperature with AVHRR data.

Appendix C: Budget summary

A summary of ERL and NESDIS research (FY 1994) addressing the role of aerosols in climate variability, stratospheric ozone reduction and air quality is presented in the tables below.

C.1 Aeronomy Laboratory

a. Resources    
  Budget NOAA/OAR/ERL/AL Base Funds $633K
  Cost allocation 60% salaries, 40% other costs  
  FTEs 4 (Federal and Joint Institute)  
b. Environmental Issues    
  Stratospheric Ozone Depletion   $315K
  Radiation Balance Perturbation   $255K
  Air Quality   $63K
c. Research Components Program (PI)  
  Theory/Modeling Heterogeneous chemistry of stratospheric ozone depletion (Solomon) $50K
      Regional weather/climate perturbations due to aerosol radiative effects (Liu) $30K
  Laboratory   Heterogeneous chemical reactions involved in stratospheric ozone depletion (Ravishankara) $150K
      Radiative characteristics of aerosols and chemistry in/on aerosols (Ravishankara) $75K
  Field In situ chemical characterization of single aerosol composition with application to stratospheric ozone depletion (Murphy) $115K
    In situ chemical characterization of single aerosol composition with application to the tropospheric radiative balance and cloud-condensation microphysics (Murphy) $150K
    Filter sampling of airborne aerosols to better characterize the chemical composition of aerosols and their role in the chemistry of regional air quality (Norton) $63K

C.2 Air Resources Laboratory

a. Resources    
  Budget NOAA/OAR/ERL/ARL Base Funds $610K
    NOAA/OGP $14K
      Total: $624K
  Cost allocation 70% salaries, 30% other costs  
  FTEs 5.5 (Federal and Student)  
b. Environmental Issues    
  Geochemical Cycling   $290K
  Radiation Balance Perturbation   $334K
c. Research Components Program (PI)  
  Theory/Modeling Meso- to global-scale transport and dispersion modeling (Draxler) $100K
    Effects of clouds on wet deposition (Artz) $ 50K
  Laboratory Emission and resuspension of soil (Gillette)
    Rotating shadow band development (Hicks) $220K
  Field Development of inferential deposition monitoring techniques (Meyers) $ 90K
    Transport of marine aerosols (Artz) $ 14K

C.3 Atlantic Oceanographic and Meteorological Laboratory

a. Resources    
  Budget NOAA/OAR/ERL/AOML Base Funds $225K
  Cost allocation 70% salaries, 30% other costs  
  FTEs 5 (Federal)  
b. Environmental Issues    
  Radiation Balance Perturbation   $225K
c. Research Components Program (PI)  
  Field Ammonia-aerosol interactions in the marine boundary layer (Whung) $175K
    Characterization of aerosols in the marine boundary layer within continental plumes (Carsey) $50K

C.4 Climate Monitoring and Diagnostics Laboratory

a. Resources    
  Budget NOAA/OAR/ERL/CMDL Base Funds $533K
    NOAA/OGP $155K
      Total: $688K
  Cost allocation 50% salaries, 50% other costs  
  FTEs 5 (Federal)  
b. Environmental Issues    
  Radiation Balance Perturbation   $518K
  Stratospheric Ozone Depletion   $170K
c. Research Components Program (PI)  
  Field Climate Forcing by anthropogenic aerosols (Ogren) $375K
    NDSC lidar monitoring of stratospheric aerosols at Mauna Loa (Barnes) $93K
    Ozone perturbations associated with volcanic aerosols (Hofmann) $90K
    Stratospheric aerosol optical depth at baseline stations (Dutton) $60K
    Surface aerosol monitoring at baseline stations (Bodhaine) $70K

C.5 Environmental Technology Laboratory

a. Resources    
  Budget NOAA/OAR/ERL/ETL $250K
    NOAA/OGP through ARL $50K
      Total: $300K
  Cost allocation 50% salaries, 50% other costs  
  FTEs 2.5 (Federal and Joint Institute)  
b. Environmental Issues    
  Radiation Balance Perturbation   $300K
c. Research Components Program (PI)  
  Field Instrument development for and field studies of cloud/aerosol/radiation

feedbacks (Hardesty, Grund)

    Cloud processing of aerosols (Feingold) $ 50K

C.6 Geophysical Fluid Dynamics Laboratory

a. Resources    
  Budget NOAA/OAR/ERL/GFDL Base Funds $200K
  Cost allocation 20% salaries, 80% other costs  
  FTEs 1 (Federal Equivalent)  
b. Environmental Issues    
  Radiation Balance Perturbation   $200K
c. Research Components Program (PI)  
  Theory/Modeling Radiative transfer theory and modeling (Ramaswamy, Freidenreich) $40K
    Modeling of climate response (Ramaswamy, Chen) $80K
    Atmospheric chemistry and transport modeling (Levy, Kasibhatla, Perliski) $80K

C.7 National Severe Storms Laboratory

a. Resources    
  Budget NOAA/OGP/CIMMS $100K
  Cost allocation 60% salaries, 40% other costs  
  FTEs 1 (Joint Institute)  
b. Environmental Issues    
  Radiation Balance Perturbation   $100K
c. Research Components Program (PI)  
  Theory/Modeling Theoretical and observational study of aerosol effects on marine stratiform clouds (Kogan) $100K

C.8 Pacific Marine Environmental Laboratory

a. Resources    
  Budget NOAA/OGP $421K
    NASA $100K
    Total $661K
  Cost allocation 70% salaries, 30% other costs  
  FTEs 5.5 (Federal and Joint Institute)  
b. Environmental Issues    
  Radiation Balance Perturbation   $661K
c. Research Components Program (PI)  
  Field Effect of ammonia and particle chemical composition on the radiative properties of marine sulfate aerosols (Quinn) $ 90K
    Atmospheric aerosols and climate change: Process and closure studies (Bates) $276K
    Physical, chemical and radiative properties of aerosols at a remote, midlatitude marine site (Covert) $140K
    Radiative properties of aerosols (Bates) $100K
    Atmospheric aerosols and climate change monitoring (Quinn) $55K

C.9 NESDIS/ORA (Office of Research and Applications)

a. Resources    
  Budget NOAA Base Funds $70K
    NOAA (other than base funds) $40K
    NASA $65K
    Total $175K
  Cost allocation 100% salaries  
  FTEs 2.7 (Federal, Contractor, NRC Associate)  
b. Environmental Issues    
  Radiation Balance Perturbation   $175K
c. Research Components Project (PI)  
  Theory Remote Sensing Algorithms (Stowe, Ignatov, Singh) $95K
    Atmospheric Corrections (Rao) $15K
  Field Sun-photometer validation (Colon, Ignatov, Singh, Stowe) $65K

Appendix D: List of Acronyms

AAOE Airborne Antarctic Ozone Experiment

AASE Airborne Arctic Stratospheric Expedition

ACE aerosol characterization experiments

AGU American Geophysical Union

AL Aeronomy Laboratory

AOML Atlantic Oceanographic and Meteorological Laboratory

ARL Air Resources Laboratory

ASTEX Atlantic stratocumulus transition experiment

AVHRR Advanced Very High Resolution Radiometer

BNL Brookhaven National Laboratory

CCN cloud condensation nucleus

CERES Cloud and Earth's Radiant Energy System

CIMMS Cooperative Institute for Mesoscale Meteorological Studies

CLAW Charlson, Lovelock, Andreae, Warren

CMDL Climate Monitoring and Diagnostics Laboratory

DMS dimethylsulfide

DOE Department of Energy

EOS Earth Observing System

ERL Environmental Research Laboratories

ETL Environmental Technology Laboratory

FTE full-time equivalent

FY1994 fiscal year 1994 (Oct. 1, 1993 - Sept. 30, 1994)

GAW Global Atmosphere Watch

GCM global climate model, general circulation model

GFDL Geophysical Fluid Dynamics Laboratory

GMCC Geophysical Monitoring for Climate Change

IGAC Internation Global Atmospheric Chemistry

JISAO Joint Institute for the Study of the Atmosphere and Ocean

LES large eddy simulation

LIDAR light detection and ranging

MAGE Marine Aerosol and Gas Experiment

MSA methane sulfonic acid

NAS National Academy of Sciences

NASA National Aeronautics and Space Administration

NCAR National Center for Atmospheric Research

NDSC Network for the Detection of Stratospheric Change

NESDIS National Environmental Satellite, Data, and Information Service

NOAA National Oceanic and Atmospheric Administration

NRC National Research Council

NSF National Science Foundation

NSSL National Severe Storms Laboratory

Nd:YLF neodymium:yttrium lithium fluoride

OAR Oceanic and Atmospheric Research

OGP Office of Global Programs

ORA Office of Research and Applications

PALMS particle analysis by laser mass spectroscopy

PEM Pacific Exploratory Mission

PI principal investigator

PIXE particle-induced X-ray emission; proton-induced X-ray emission

PM particulate matter

PMEL Pacific Marine Environmental Laboratory

PSC polar stratospheric cloud

PSI Pacific Sulfur/Stratus Investigation

RH relative humidity

SAGA Soviet-American Gas-Aerosol Experiment

SOS Southern Oxidants Study

TARFOX Tropospheric Aerosol Radiative Forcing Observational Experiment

TDMA tandem differential mobility analyzer

UV ultraviolet

WMO World Meteorological Organization

Written copies of this report can be obtained by sending e-mail to the panel chairman, Dave Hofmann, at