Aerosols: Climate and Air Quality
Predicting regional and global climate requires an understanding of the radiative effects of aerosol particles of natural and human origin, their cloud nucleating properties, and their effects on precipitation. These same particles affect the quality of the air that we breathe. NOAA is charged with providing the atmospheric measurements and analyses required to characterize the physical and chemical properties of these particles, and their effects on climate and human health.
Aerosol particles are emitted from Earth's surface both naturally (e.g., dust, sea-salt, biogenic emissions), and as a result of human activities. They also are created and modified by chemical processes in the atmosphere and they are constantly cycled among the Earth's oceans, atmosphere, and biosphere. They have an effect on the energy balance of the atmosphere either by directly scattering and absorbing radiation, by serving as condensation nuclei during cloud formation, and by influencing precipitation. Their presence also affects photosynthesis and agricultural production. These same particles that affect climate also impact the quality of the air that we breathe and the health of all living organisms.
On balance, aerosol particles tend to scatter energy back to space and cool the planet. This tends to offset the warming associated with greenhouse gases. Aerosols also can adversely affect human health. While greenhouse gases in the atmosphere act like a blanket, trapping longwave radiation and warming the planet, most aerosol particles have the opposite effect since they reflect some of the incoming solar radiation. Some types of particles, however, absorb solar radiation and can contribute to local warming of the atmosphere. An example of light absorbing particles is carbon soot generated from combustion processes or forest fires. Because aerosols have much shorter lifetimes and more varied distributions than greenhouse gases, the net effect on global climate is hard to predict.
The scattering of energy associated with aerosols is amplified by the fact that some aerosols act as nuclei for cloud droplets and can thereby increase the reflectance of clouds. They also may modify the lifetime of clouds by affecting precipitation. The extent of these mutual effects of aerosols on clouds and of clouds on aerosols are perhaps the single largest unknown in climate change prediction.
Common examples of the adverse health effects of aerosols among humans are the higher incidence of bronchial infections during periods of high aerosol loading, and short- and long-term adverse reactions to the toxicity of pollution particles.
For decades, the concentration of aerosols in the lower atmosphere has increased steadily primarily owing to industrial activity and biomass burning, but also to secondary aerosol formation, changes in land-use, and volcanic activity. There is clear evidence of the effect of aerosols on radiation as measured at Earth's surface and at the top of the atmosphere and its link to aerosol size distribution and composition. Aerosol size and composition are largely variable, which has important implications for climate as shown in figure 1. For example, some particles have a higher affinity for water vapor, grow larger in humid conditions, and scatter more light as shown in figure 2.
Fig. 2 Schematic depicting the effect of relative humidity on the size of aerosol particles. Inorganic particles such as sea salt tend to take up more water vapor, grow larger, and therefore scatter more radiation than organic particles.
There is also strong evidence of aerosol modification of cloud microphysical properties as manifested in increased cloud reflectance associated with effluent from ship stacks and other pollution sources. Aerosol particles appear to suppress precipitation, although multiple feedbacks make it difficult to detect this effect. By reducing the amount of radiation reaching the surface, absorbing aerosol particles tend to reduce cloudiness (Figure 3).
Fig. 3 Schematic of the role of partially absorbing smoke particles on surface radiation and implications for cloud formation. Partially absorbing particles absorb and scatter radiation (1) and diminish the net radiation at the surface (2). This results in a reduction in surface latent and sensible heat flows from the surface (3), which reduces the strength of convection and cloud formation.
Increased aerosol loading in high density population centers has resulted in health problems that are a nuisance at best and lead to serious health problems or even mortality at worst. Understanding and predicting these events requires knowledge of emissions, chemical processing, and meteorology.
- Needed Developments
- Improved modeling of the aerosol-cloud-climate system at a range of scales
- Methods for integrating observations and measurements at a range of scales
- Improved air-quality forecast models
- Improved global coverage of aerosol radiative forcing
- Improved aerosol and cloud measurements (in-situ and remote)
- Global radiative effects of aerosols on climate are still largely uncertain, requiring that we extend the geographic scope and frequency of our measurements;
- Aerosol effects on radiation at the top of the atmosphere and at the surface must be supplemented by radiative heating profiles. NOAA is addressing this problem with its small aircraft profiling program (Figures 4, 5), which, in turn, will be valuable for validation of satellite-derived aerosol products;
- There is evidence of local effects of aerosol on clouds, e.g., as manifested in ship tracks (Figure 6), but we do not have a firm understanding of the global implications of these effects, or a clear understanding of the effects on precipitation and the hydrological cycle;
- It is crucial that we improve our understanding of the various processes and feedbacks in the aerosol-cloud-climate system at the full range of scales (Fig. 7).
Fig. 4 The Cessna Turbo 206 aircraft used for the In-Situ Aerosol Profiling (IAP) program. This aircraft and its predecessor (Cessna 172XP) have made over 650 successful vertical profile flights over an instrumented surface site in Oklahoma since early 2000. Measurements include aerosol optical properties and trace gases that affect the solar radiation budget.
Fig. 5. Cumulative plot of radiative forcing efficiency (W m-2 per unit optical depth) versus altitude over north-central Oklahoma for the period of March 2000 through April 2006 (616 flights). These data provide important information on the vertical profile of the climate effects of aerosols.
The vertical lines that form the box ends represent the 25th and 75th percentiles of the population, the vertical line through the middle of the box shows the median (50th percentile), while the ends of the horizontal lines show the 5th and 95th percentiles. For comparison, the surface measurements made with identical instrumentation is shown in yellow.
Fig. 6 Ship tracks: Bright, linear features that are manifestations of enhanced cloud reflectance associated with pollution particles emitted by ships. (Near IR image courtesy of Naval Postgraduate School)
Fig. 7 Schematic showing the importance of understanding aerosol-cloud-climate interactions at a range of spatial scales. Knowledge acquired at small scales needs to be transferred to larger scales in order to improve our representation of the climate system.
- Contributions to Climate-Relevant Efforts
NOAA measures aerosol particles at a number of monitoring sites, and works with partners to improve the accuracy and reliability of these measurements in order to enhance our understanding of the sources, sinks, and trends in aerosol properties (Figure 8).
Specific to air quality, NOAA provides scientific information and tools for air quality decision-makers and timely and accurate air quality forecast guidance (Figure 9). Particulate matter is one of the most important foci of this effort.
NOAA also engages in intensive field studies with aircraft and ships that enhance our understanding of the processes that affect aerosols (Figure 10). These, together with the long-term monitoring networks, provide a continuous record that is critical to improving our understanding of regional and global air quality and climate, as well as aiding or verifying international management strategies.
Fig. 8 NOAA aerosol monitoring network: This figure displays the surface stations where NOAA has aerosol measurements being made, data available at http://www.cmdl.noaa.gov/aero/net/.
Fig. 9. Schematic showing NOAA's role in air quality forecasting
|Figure 10: NOAA platforms used for intensive field operations|
|NOAA R/V Ronald H. Brown||NOAA WP3 Research Aircraft||NOAA Gulfstream GIV-SP N-49RF|
|In situ: Trace gases, aerosols (mass, composition, size distribution,
light scattering, light absorption)
Vertical Profiling: radar wind profiler, ozone/aerosol lidar, microwave radiometer, rawinsondes
|A flying laboratory, reaching altitudes up to 25,000 ft, that can make comprehensive measurements of the chemical and radiative properties of the atmosphere||Measurement capabilities being developed to measure ozone, aerosols, gas phase chemical precursors and the radiative properties of the atmosphere. Meteorological observation. Altitudes up to 45,000 ft.|
Quantifying natural and human-induced (anthropogenic) emissions, transport, atmospheric processing, and aerosol sinks is crucial. Quantifying the radiative effects associated with aerosol-cloud interactions is a priority. Key questions include:
- How can we improve our in-situ and remote sensing monitoring of aerosols, clouds and their effect on the Earth's radiation?
- How can we improve representation of aerosols and their effects on clouds in climate models?
- How can we improve our air-quality forecasting capabilities at the local and regional scales?
- How can we gain enough confidence in the models for them to aid in decision making?
- Which features can be validated?
The key to predicting the effects of aerosols on climate change and air quality lies in improving our understanding of key processes, in long-term monitoring, and in improved predictive capabilities through models.
NOAA's aerosol research enhances its ability to predict the effects of aerosols on climate change and air quality. State of the art measurements of radiatively important aerosol properties both reduce the uncertainties in the effects of these aerosols and provide valuable validation for satellite measurements, which can in turn be used to refine models for improved predictive capabilities. Accurate predictions of climate change are critical tools for policymakers to make informed decisions on how best to respond to a changing climate. Measurements and modeling of local to regional air quality will improve air quality forecasting, with the goal of minimizing the health risk to citizens and the adverse effects to US agricultural production.
- Aerosol Introduction
- Aerosol Sources, Sinks, Distributions, and Processes
- ESRL Research on Aerosol Direct Radiative Forcing of Climate
- Aerosol - Cloud Interactions
- Coupled Modeling Systems that allow for Aerosol, Air Quality, Weather, and Climate Interactions
- Aerosol Summary and Outlook