Dust in the Wind
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 sensorsthe Polarization and Directionality of the Earths 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.
Three sensors aboard NASAs 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 Earths total energy budget.