Image: Courtesy JACQUES DESCLOITRES, MODIS Land Rapid Response Team, NASA GSFC
Through its travels, this cloud did more than obscure a few views: it dramatically demonstrated the fact that pollution can no longer be considered an urban or regional problem. And because it contained high quantities of aerosols¿microscopic particles such as dust and sea salt, as well as man-made sulfates, nitrates, soot and organic compounds¿it highlighted a need to better understand how these grains affect our planet's weather and climate.
Indeed, although climatologists and other scientists have long focused on gases in the atmosphere, they have not closely examined the role played by so-called condensed phase particles. "Ours is really a younger field in many ways than the study of gas-phase chemistry," says aerosol researcher Barry Huebert of the University of Hawaii. "There was a feeling that if you understood the gas phase, you understood everything that was interesting for a long time." Even when scientists did consider the condensed phase, most thought that because the particles are short-lived, their effects would be temporary. Although the particles might have a regional effect on visibility, they would be rained out of the atmosphere before they had time to really affect climate.
Some crude calculations in the early 1990s (based mainly on sulfates, because the only sufficient data available came from studies of acid rain) showed that aerosols could cool the atmosphere by back-scattering incoming solar radiation. But later studies suggested that aerosols could also warm the atmosphere through their effects on cloud cover and the behavior of less well studied components of pollution, such as soot.
In search of a clearer picture, large teams of scientists from around the world have recently pooled their resources to perform major field experiments involving aircraft, ships, surface stations and balloons. The Asian-Pacific Regional Aerosol Characterization Experiment (ACE-Asia), which took place during last year's dust storms, is the third in a series of experiments designed to investigate different aspects of aerosol behavior in different locales around the world. Another massive undertaking, the Indian Ocean Experiment (INDOEX), meanwhile, was specifically designed to see if climate forcing on the part of aerosol particles could be directly measured.
Conducted in 1999, INDOEX exploited a unique occurrence during the winter monsoon in which air currents carry pollutants from India and Southeast Asia in a southern direction out over the open ocean. South of the equator, however, the air currents are predominantly from the south, keeping the sky relatively free of man-made pollutants. INDOEX planes that flew through the cloud and south over the equator were thus able to sample both the aerosols and a cleaner sky to serve as a reference.
Studying aerosols has not been easy. Because they are short-lived, they do not mix homogeneously around the planet, and so concentrations may differ by many orders of magnitude¿making accurate descriptions of their effects hard to come by. What's more, according to Tim Bates of the National Oceanic and Atmospheric Administration (NOAA), "there's a very wide range of sizes [for aerosol particles], and the effect that the particle is going to have on climate is going to be very dependent on its size, which makes it trickier." Finally, the composition of the particles covers a wide range of chemical species that can combine and interact in numerous ways, all leading to different, and sometimes opposing, radiative effects.