|
|
MISR Science Goals and Objectives
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.
Related article:
The Signature of Global Warming
Return to top
Next: MISR's Study of Clouds
|
