MISR's Study of Atmospheric Aerosols
By Ralph Kahn, MISR Science Team
What are aerosols?
Aerosols are tiny particles suspended in the air. They occur naturally, originating from volcanoes, dust storms, forest and grassland fires, emissions from living vegetation, and sea spray. Human activities, such as the burning of fossil fuels and the alteration of natural surface cover, also generate aerosols. Averaged over the globe, anthropogenic aerosols (aerosols made by human activities) currently account for about 10% of the total amount, but most of this is concentrated in the northern hemisphere, especially downwind of industrial sites, slash-and-burn agricultural regions, and overgrazed grassland.
Aerosol particles may be solid or liquid; they range in size from 0.01 microns to several tens of microns. [Cigarette smoke particles are in the middle of this size range; typical cloud drops are 10 or more microns in diameter.] Under normal circumstances, the majority of aerosols reside in the troposphere (lower atmosphere), where they are washed out of the air in about a week by rain. Aerosols are also found in the stratosphere (the atmosphere just above the troposphere). A severe volcanic event, such as the Pinatubo eruption in the Philippines in 1991, can put large amounts of aerosol into the stratosphere. Since it does not rain in the stratosphere, these aerosols can last for many months, producing beautiful sunsets around the globe, and possibly causing summer temperatures to be cooler than normal.
Why do we care about aerosols?
Aerosols tend to cause cooling of the surface below them, since most aerosols are bright particles that reflect sunlight back to space, reducing the amount of solar radiation that can be absorbed at the surface. The magnitude of this effect depends on the size and composition of the aerosols, and on the reflecting properties of the underlying surface. Aerosol cooling may partly offset the expected warming due to increases in the amount of carbon dioxide from human activities.
Aerosols are also believed to have an "indirect" effect on climate, by changing the properties of clouds. Adding aerosols to a cloudy area may create smaller cloud particles. This could produce brighter clouds, and ones that last longer, since smaller cloud drops are less likely to fall out of the atmosphere as rain.
Over the past 30 years, major aerosol types have been identified, and general ideas about the amount of aerosol to be found in different seasons and locations have been developed. But key details about aerosol amounts and properties are needed to calculate even their current effect on surface temperatures, and there are few observations that reveal the trends in any of these quantities. Particle abundances, variations in aerosol size and composition, and the magnitudes of aerosol sources and sinks have never been systematically measured on a global basis. In addition, the size of the indirect aerosol effects on climate remains unknown.
How will MISR help?
Currently, satellite instruments provide our best hope of making, at a reasonable cost, routine, global observations of aerosols needed to measure their climatic effects. But such instruments must rely on remote sensing -- interpreting the light collected at a distance from the aerosols themselves -- to figure out aerosol amount (called optical depth) and particle properties. This science is termed remote sensing aerosol retrieval. Since the scattered light contains only circumstantial evidence about aerosols, we must exploit our understanding of the physics of light scattering to deduce aerosol properties from such data; we must also make some assumptions.
Remote sensing aerosol retrieval has been an area of active research for more than 20 years. To date, the only routine, global-scale aerosol product available uses single-channel (i.e., single spectral band or color), single view-angle data to derive aerosol amount over oceans, based on an assumed particle size distribution and composition.
MISR will collect multi-angle as well as multi-spectral data never before obtained by satellite instruments. The additional information contained in these data will make it possible to set limits on particle size and composition, as well as aerosol amount, measured over ocean. The new data will also be used to derive aerosol properties in the atmosphere over heterogeneous land and dense dark vegetation. We use different methods to derive aerosol properties over different types of surface. Click on the image to see a summary diagram of the methods used to retrieve measurements of atmospheric aerosols from MISR data -- 850 x 599 pixels, 47 kilobytes.
Preliminary studies indicate that aerosol abundance, as described by optical depth, can be retrieved by MISR over calm ocean to about ±0.05 or 20%, whichever is larger. We expect this accuracy over a wide range of clear-sky conditions, even when the detailed particle properties are imprecisely known. With the help of an aerosol climatology to remove some of the indeterminacy, the MISR aerosol retrieval will also be able to distinguish among many common particle "types" (sea salt, soot, sulfates, etc.,) which represent constraints on a combination of particle shape, size distribution, and composition.
In order to study the climatic and environmental impacts of atmospheric aerosols, the MISR team is planning a systematic, global monitoring program to collect data about particle type and amount. These will be used in studies of the planetary energy balance, and as inputs to computer programs that model the regional and global trends in Earth's climate.