3.1. Aerosol Monitoring

D. Delene (Editor), E. Andrews, D. Jackson, A. Jefferson, J. Ogren, P. Sheridan, and J. Wendell

3.1.1. Scientific Background

Aerosol particles affect the radiative balance of Earth both directly, by scattering and absorbing solar and terrestrial radiation, and indirectly, through their action as cloud condensation nuclei (CCN) with subsequent effects on the microphysical and optical properties of clouds. Evaluation of the climate forcing by aerosols, defined here as the perturbation of the Earth's radiation budget induced by the presence of airborne particles, requires knowledge of the spatial distribution of the particles, their optical and cloud-nucleating properties, and suitable models of radiative transfer and cloud physics. Obtaining a predictive relationship between the aerosol forcing and the physical and chemical sources of the particles additionally requires knowledge of regional and global-scale chemical processes, physical transformation, and transport models for calculating the spatial distributions of the major chemical species that control the optical and cloud-nucleating properties of the particles. Developing and validating these various models requires a diverse suite of in situ and remote observations of the aerosol particles on a wide range of spatial and temporal scales.

Aerosol measurements began at the CMDL baseline observatories in the mid-1970s as part of the Geophysical Monitoring for Climatic Change (GMCC) program. The objective of these "baseline" measurements was to detect a response, or lack of response, of atmospheric aerosols to changing conditions on a global scale. Since the inception of the program, scientific understanding of the behavior of atmospheric aerosols has improved considerably. One lesson learned is that residence times of tropospheric aerosols are generally less than 1 week, and that human activities primarily influence aerosols on regional/continental scales rather than global scales. In response to this increased understanding, and to more recent findings that anthropogenic aerosols create a significant perturbation in the Earth's radiative balance on regional scales [Charlson et al., 1992; National Research Council, 1996], CMDL expanded its aerosol research program to include regional aerosol monitoring stations. The goals of this regional-scale monitoring program are: (1) to characterize means, variabilities, and trends of climate-forcing properties of different types of aerosols, and (2) to understand the factors that control these properties.

A primary hypothesis to be tested by NOAA's aerosol 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 United States 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 report describes the observations that CMDL is conducting towards this goal.

No single approach to observing the atmospheric aerosol can provide the necessary data for monitoring all the relevant dimensions and spatial/temporal scales necessary to evaluate climate forcing by anthropogenic aerosols. 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 from the surface can determine a limited set of aerosol properties from local to global spatial scales, but they cannot provide the chemical information needed for linkage 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 to the results of large-scale models, and where process studies have a context for drawing general conclusions from experiments conducted under specific conditions.

Measurements of atmospheric aerosols are used in three fundamentally different ways for aerosol/climate research: algorithm development for models and remote-sensing retrievals, parameter characterization, and model validation. Laboratory and field process studies guide the development of parameterization schemes and the choice of parameter values for chemical transport models that describe the relationship between emissions and concentration fields of aerosol species. Systematic surveys and monitoring programs provide characteristic values of aerosol properties that are used in radiative transfer models for calculating the radiative effects of the aerosols, and for retrieving aerosol properties from satellites and other remote sensing platforms. And finally, monitoring programs provide spatial and temporal distributions of aerosol properties that are compared to model results to validate the models. Each of these three modes of interaction between applications and measurements require different types of data and entail different measurement strategies. Ogren [1995] applied the thermodynamic concept of “intensive” and “extensive” properties of a system to emphasize the relationship between measurement approach and applications of aerosol observations.

Intensive properties do not depend on the amount of aerosol present and are used as parameters in chemical transport and radiative transfer models (e.g., atmospheric residence time, single-scattering albedo). Extensive properties vary strongly in response to mixing and removal processes and are most commonly used for model validation (e.g., mass concentration, optical depth). Intensive properties are more difficult and expensive to measure than extensive properties because they generally are defined as the ratio of two extensive properties. As a result, different measurement strategies are needed for meeting the data needs of the various applications. Measurements of a few carefully chosen extensive properties, of which aerosol optical depth and species mass concentrations are prime candidates, are needed in many locations to test the ability of the models to predict spatial and temporal variations on regional to global scales and to detect changes in aerosol concentrations resulting from changes in aerosol sources. The higher cost of determining intensive properties suggests a strategy of using a limited number of highly-instrumented sites to characterize means and variabilities of intensive properties for different regions or aerosol types, supplemented with surveys by aircraft and ships to characterize the spatial variability of these parameters. CMDL's regional aerosol monitoring program is primarily focused on characterizing intensive properties.

CMDL's measurements provide ground truth for satellite measurements and global models, as well as key aerosol parameters for global-scale models (e.g., scattering efficiency of sulfate particles and hemispheric backscattering fraction). An important aspect of this strategy is that the chemical measurements are linked to the physical measurements through simultaneous, size-selective sampling that allows the observed aerosol properties to be connected to the atmospheric cycles of specific chemical species.

3.1.2. Experimental Methods

Extensive aerosol properties monitored by CMDL include condensation nucleus (CN) concentration, aerosol optical depth (d), and components of the aerosol extinction coefficient at one or more wavelengths (total scattering s_sp, backwards hemispheric scattering s_bsp, and absorption s_ap). At the regional sites, size-resolved impactor and filter samples (submicrometer and supermicrometer size fractions) are obtained for gravimetric and chemical (ion chromatograph) analyses. All size-selective sampling, as well as the measurements of the components of the aerosol extinction coefficient at the regional stations, is performed at a low, controlled relative humidity (<40%) to eliminate confounding effects due to changes in ambient relative humidity. Data from the continuous sensors are screened to eliminate contamination from local pollution sources. At the regional stations, the screening algorithms use measured wind speed, direction, and total particle number concentration in real-time to prevent contamination of the chemical samples. Algorithms for the baseline stations use measured wind speed and direction to exclude data that are likely to have been locally contaminated.

Prior to 1995, data from the baseline stations were manually edited to remove spikes from local contamination. Since 1995 an automatic editing algorithm has been applied to the baseline data in addition to manual editing of local contamination spikes. For the baseline stations (BRW, Mauna Loa, Hawaii (MLO), American Samoa (SMO), and South Pole, Antarctica (SPO), as well as Sable Island (WSA)), data are automatically removed when the wind direction is from local sources of pollution (such as generators and buildings) as well as when the wind speed is less than a threshold value (0.5-1 m s^-1). In addition, at MLO data for upslope conditions (1800-1000 UTC) are excluded since the airmasses do not represent “background” free tropospheric air for this case. A summary of the data editing criteria is given in Table 3.1.

Integrating nephelometers are used to determine the light scattering coefficient of the aerosol. These instruments operate by illuminating a fixed sample volume from the side and observing the amount of light that is scattered by particles and gas molecules in the direction of a photomultiplier tube. The instrument integrates over scattering angles of 7-170°. Depending on the station, measurements are performed at three or four wavelengths in the visible and near-infrared. Newer instruments allow determination of the hemispheric backscattering coefficient by using a shutter to prevent illumination of the portion of the instrument that yields scattering angles less than 90°. A particle filter is inserted periodically into the sample stream to measure the light scattered by gas molecules; which is subtracted from the total scattered signal to determine the contribution from the particles alone. The instruments are calibrated by filling the sample volume with CO₂ gas which has a known scattering coefficient.

TABLE 3.1. Data Editing Summary for NOAA
Baseline and Regional Stations

Station	Editing	Clean Sector
South Pole	a,b,c	0° < WD < 110°, 330°<WD < 360°
Samoa	a,b,c	0° < WD < 165°, 285°<WD < 360°
Mauna Loa	a,b,c,d	90° < WD < 270°
Barrow	a,b,c	0° < WD < 130°
Sable Island	a,b,c	0° < WD < 35°, 85° < WD < 360°
Southern Great Plains	a
Bondville	a

a: Manual removal of local contamination spikes;

b: Automatic removal of data not in clean sector;

c: Automatic removal of data for low wind speeds;

d: Removal of data for upslope wind conditions;

WD: Wind direction.

The aerosol light absorption coefficient is determined with a continuous light absorption photometer. This instrument continuously measures the amount of light transmitted through a quartz filter, while particles are being deposited on the filter. The rate of decrease of transmissivity, divided by the sample flow rate, is directly proportional to the light absorption coefficient of the particles. Newer instruments were calibrated in terms of the difference of light extinction and scattering in a long-path extinction cell, for laboratory test aerosols. Instruments at the baseline stations (aethalometers, Magee Scientific, Berkley, California) were calibrated by the manufacturer in terms of the equivalent amount of black carbon (BC) from which the light absorption coefficient is calculated assuming a mass absorption efficiency of the calibration aerosols of 10 m² g^-1.

Particle number concentration is determined with a CN counter that exposes the particles to a high supersaturation of butanol vapor. This causes the particles to grow to a size where they can be optically detected and counted. The instruments in use have lower particle-size detection limits of 10-20 nm diameter.

Summaries of the extensive measurements obtained at each site are given in Tables 3.2 and 3.3. Table 3.4 lists the intensive aerosol properties that can be determined from the directly-measured extensive properties. These properties are used in chemical transport models to determine the radiative effects of the aerosol concentrations calculated by the models. Inversely, these properties are used by algorithms for interpreting satellite remote-sensing data to determine aerosol amounts based on measurements of the radiative effects of the aerosol.

TABLE 3.2. CMDL Baseline Aerosol Monitoring Stations (Status as of December 1999)

Category	Baseline Arctic	Baseline Free Troposphere	Baseline Marine	Baseline Antarctic
Location	Point Barrow	Mauna Loa	American Samoa	South Pole
Designator	BRW	MLO	SMO	SPO
Latitude	71.323ºN	19.539ºN	14.232ºS	89.997ºS
Longitude	156.609ºW	155.578ºW	170.563ºW	102.0ºE
Elevation (m)	8	3397	77	2838
Responsible Institute	CMDL	CMDL	CMDL	CMDL
Status	Operational 1976. Major upgrade 1997.	Operational 1974	Operational, 1977	Operational, 1974
Sample RH	RH <40%	Uncontrolled	Uncontrolled	Uncontrolled
Sample Size Fractions	D<1 µm D<10 µm	Uncontrolled	Uncontrolled	Uncontrolled
Optical measurements	s_sp(3l), s_bsp(3l), s_ap(1l)	s_sp(3l), s_ap(1l), d(6l)	none	s_sp(4l)
Microphysical measurements	CN concentration	CN concentration	CN concentration	CN concentration
Chemical measurements	Major ions, mass	None	None	None

TABLE 3.3. CMDL Regional Aerosol Monitoring Sites (Status as of December 1999)

Category	Perturbed Marine	Perturbed Continental	Perturbed Continental
Location	Sable Island, Nova Scotia, Canada	Bondville, Illinois	Lamont, Oklahoma
Designator	WSA	BND	SGP
Latitude	43.933ºN	40.053ºN	36.605ºN
Longitude	60.007ºW	88.372ºW	97.489ºW
Elevation (m)	5	230	315
Responsible Institute	CMDL	CMDL	CMDL
Collaborating Institute	AES Canada, NOAA/PMEL	University of Illinois, Illinois State Water Survey	DOE/ARM
Status	Operational, August 1992	Operational, July 1994	Operational, July 1996
Sample RH	RH <40%	RH <40%	RH <40%
Sample Size Fractions	D<1 µm, D<10 µm	D<1 µm, D<10 µm	D<1 µm, D<10 µm
Optical measurements	s_sp(3l), s_bsp(3l) s_ap(1l)	s_sp(3l), s_bsp(3l), s_ap(1l)	s_sp(3l),s_bsp(3l), s_ap(1l), d(7l)
Microphysical measurements	CN concentration	CN concentration	CN, n(D) concentration
Chemical measurements	Major ions, mass	Major ions, mass	None

TABLE 3.4. Intensive Aerosol Properties Derived From CMDL Network

Properties	Description
å	The Ångström exponent, defined by the power-law s_spµl^-å, describes the wavelength-dependence of scattered light. In the figures below, å is calculated from measurements at 550 and 700 nm wavelength. Situations where the scattering is dominated by submicrometer particles typically have values around 2, while values close to 0 occur when the scattering is dominated by particles larger than a few microns in diameter.
w_o	The aerosol single-scattering albedo, defined as s_sp/(s_ap + s_sp), describes the relative contributions of scattering and absorption to the total light extinction. Purely scattering aerosols (e.g., sulfuric acid) have values of 1, while very strong absorbers (e.g., elemental carbon) have values around 0.3.
g, b	Radiative transfer models commonly require one of two integral properties of the angular distribution of scattered light (phase function): the asymmetry factor g or the hemispheric backscatter fraction b. The asymmetry factor is the cosine-weighted average of the phase function, ranging from a value of -1 for entirely backscattered light to +1 for entirely forward-scattered light. The hemispheric backscatter fraction b is defined as s_bsp/s_sp.
a_i	The mass scattering efficiency for species i, defined as the slope of the linear regression line relating s_sp and themass concentration of the chemical species, is used in chemical transport models to evaluate the radiative effects of each chemical species prognosed by the model. This parameter has typical units of m² g^-1.

3. Aerosols and Radiation

3.1.1. Scientific Background

3.1.2. Experimental Methods

3.1.3. Annual Cycles