Research Satellites for Atmospheric Sciences, 1978-Present

Dust in the Wind
Aerosols are tiny particles suspended in the air (mostly in the troposphere). Some aerosols come from natural sources, such as volcanic eruptions, dust storms, forest and grassland fires, living vegetation, and sea spray. About 11 percent of the total emitted aerosols in our atmosphere come from human activities, such as the burning of vegetation and fossil fuels and changing the natural land surface cover, which again leads to windblown dust. Yet human-produced aerosols account for about half of the total effect of all aerosols on incoming sunlight. From a satellite’s perspective, aerosols raise the Earth’s albedo, or make it appear brighter by scattering and reflecting sunlight back to space. The overall effect of these tiny particles is to cool the surface by absorbing and reflecting incoming solar radiation. They also serve as cloud condensation nuclei, or “seeds” for cloud formation, which again helps to cool the surface. In terms of their net influence on global climate, aerosols represent scientists' greatest subject of uncertainty. Yet computer climate models estimate that over the last century human-produced aerosols have offset global warming due to greenhouse gases by about 40 percent.

Aerosol optical depth
The MODIS sensor, aboard NASA’s Terra satellite, measures aerosol optical thickness over the entire globe every day. Basically, aerosol optical thickness is a measure of how much sunlight is prevented from traveling down through a column of atmosphere by the particles (dust, smoke, and sea salt, etc.) suspended in the air. This image shows a composite for April 2001. Reds and yellows indicate high values while blues and purples show where the air is relatively clear; no data are available for the regions colored black.

Through the 1980s and most of the 1990s the NOAA AVHRR was the most frequently used satellite sensor for measuring aerosol optical thickness. (Aerosol optical thickness is a measure of how much sunlight airborne particles prevent from traveling through a column of atmosphere.) However, AVHRR can only make such measurements over the ocean, as the sensor requires a relatively uniform and dark-colored background. Because TOMS is particularly sensitive to absorbing aerosols, over both land and ocean, this sensor has also been widely used to measure aerosol optical thickness. In April 1991, the European Space Agency launched a new type of multi-angle sensor, called the Along Track Scanning Radiometer (ATSR), aboard their first European Remote Sensing Satellite (ERS-1). The ATSR makes aerosol optical thickness measurements by remotely sensing visible and near-infrared wavelengths at nadir and oblique forward scan angles (both within a two-minute interval). A modified version of the sensor, called the Advanced Along Track Scanning Radiometer (AATSR), was launched in 1995 aboard ERS-2. While data from neither of these missions have yet been used to produce global-scale aerosol measurements, this should be possible.

In 1996, Japan launched the first in their series of Advanced Earth Observation Satellites (ADEOS) satellites, which carried a payload of two sensors—the Polarization and Directionality of the Earth’s Reflectances (POLDER) sensor, contributed by the French Space Agency, and the Ocean Color and Temperature Scanner (OCTS), provided by NASDA. Both sensors can retrieve aerosol measurements, but POLDER was the first satellite sensor specifically designed to measure aerosols and it can make its measurements over both land and ocean. The sensor observes Earth targets from 12 directions that enable measurements of the bidirectionality and polarization of solar radiation reflected from within the atmosphere. Unfortunately, due to its solar panel failing, the ADEOS mission ended prematurely after only eight months in orbit.

The MISR instrument, aboard NASA’s Terra satellite, is sensitive to four different wavelengths—red, green, blue, and one channel in the near-infrared. MISR “sees” the Earth simultaneously at nine different angles, so it is particularly well designed for measuring how much sunlight aerosols reflect back to space. (Image courtesy Shigeru Suzuki and Eric M. De Jong, NASA JPL)

Three sensors aboard NASA’s Terra satellite are particularly well suited for studying the effects of aerosols on climate: CERES, MISR, and MODIS. The Global Imager (GLI) planned for launch aboard ADEOS II offers aerosol measurement capabilities similar to those of MODIS. Both these sensors have the capacity to measure both aerosol optical thickness as well as the sizes of aerosol particles over both ocean and land. Particle size is an indicator of the source of the aerosol particles and helps scientists distinguish aerosols of natural origin from those that are man-made. Moreover, with its nine different look angles, MISR is ideally designed to quantify the reflective properties. Again, CERES complements MODIS and MISR by providing measurements of the shortwave radiation that aerosols reflect back into space. Together, these sensors are providing new insights into the roles of clouds and aerosols in Earth’s total energy budget.

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Remote Sensing
Balancing Earth’s Radiant Energy Budget
Dust in the Wind
Abstract Art or Arbiters of Energy?
Serendipity and Stratospheric Ozone
The Chemistry of Earth’s Atmosphere
Where Storm Clouds Gather

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