MODIS operandi for mapping haze.

Particles that are put into the air in one spot on the globe can wind up being inhaled thousands of miles away. Desert dust has traveled from Asia to the west coast of North America in sufficient amounts to influence air quality there, and clouds of carbon monoxide originating from grassland and forest fires in Africa and South America have reached Australia. These traveling particles, or aerosols, can have a potentially major impact not only on human health but also on climate and regional rainfall. A unique observational instrument called MODIS (Moderate-Resolution Imaging Spectroradiometer) now allows climatologists to study how aerosols move across the planet.

"MODIS was designed to track, fine aerosols through the atmosphere," says Yoram Kaufman, the principal investigator for MODIS aerosol research at the National Aeronautics and Space Administration (NASA). In 1999, when NASA launched Terra, the first of three Earth-observing satellites to study atmospheric pollution (among other things), MODIS technology was onboard. MODIS also is on the Aqua satellite launched in 2002.

MODIS measures aerosol optical thickness (AOT), which indicates how much sunlight is prevented from traveling through the atmosphere. An AOT of 0.2 means that 20% of the overhead sunlight will be blocked by the aerosol layer--roughly equivalent to a mildly hazy day. By comparison, during the summer in Washington, D.C., the AOT typically ranges from 0.4 to 0.8.

MODIS lets researchers determine the concentrations and dimensions of atmospheric aerosols. This, combined with knowledge of wind patterns and surface population distributions, can reveal sources of these pollutants. That will, in turn, tell how pollutants move across the Earth's surface, their residence time in certain areas, and the impact of other climatological factors, and will reveal more about how all of these different particulate forms interact.

MODIS yields information on the ratio between finer particles (with a radius roughly less than 0.5 micrometer) and coarser particles (1 micrometer and larger). This is important, says Didier Tanre, director of the Laboratoire d'Optique Atmospherique at France's Universite des Sciences et Technologies de Lille and one of the developers of the algorithms that power MODIS, because most of the aerosols that are produced by human activity are finer. MODIS therefore helps to distinguish between naturally occurring and anthropogenic aerosols, Tanre says.

But MODIS can paint only part of the atmospheric portrait, and so works best in conjunction with other, similar tools. For example, the system has a hard time detecting dust over the deserts of Asia because these regions reflect so much light, says Steven Massie, an atmospheric chemist with the National Center for Atmospheric Research. On the other hand, a tool known as the Total Ozone Mapping Spectrometer, or TOMS, can detect desert dust because it uses a different system to measure different wavelengths of light. Other systems provide complementary data on other pollutants. Measurements of Pollution in the Troposphere (MOPITT) is a system developed by the Canadian Space Agency to measure carbon monoxide and methane.

When MODIS data are combined with data from other satellite-based systems, such as TOMS, MOPITT, and Global Ozone Monitoring Experiment (a European-built and -launched instrument measuring ozone and nitrogen dioxide), it yields a more comprehensive atmospheric portrait. For example, says John Gille, head of the Global Observations, Modeling, and Optical Techniques Section of the National Center for Atmospheric Research and the principal U.S. investigator for MOPITT, scientists can use data from these different sources to study relationships between pollutant sources and their relative proportions and concentrations. They can also see how pollutants' sources impact their formation and the quantity of each formed relative to the other. Combining those data with wind data presents a clearer picture of how pollutants are transported through the atmosphere.

To understand the interaction of aerosols with water vapor in the atmosphere, one ideally would know the vertical distribution of aerosols and moisture. Kaufman says that MODIS measures only the total column concentration of aerosols and water vapor. However, other instruments aboard the Aqua satellite compile moisture profiles, and other missions will use further technologies to compile the aerosol vertical profile. This will allow scientists to study the effect of moisture on aerosol particles.

"The dust particles are also an issue," says Tanre. "They are not spherical, when our algorithm assumes they are. Aerosol models that are assumed in the algorithm may be improved."

All that being said, Kaufman says, "I think MODIS is a very capable part of a valuable [observational satellite] program that's going to provide a greater understanding of our atmospheric dynamics over the coming years."