In recent years, scientists have detected very high levels of aerosol pollution in the air over India. Some of it is the result of industrial and agricultural activity, and some of it is nature at work.

These maps were built from data from the Multi-angle Imaging Spectroradiometer (MISR) instrument on NASA’s Terra spacecraft. The top image shows aerosol optical depth, a measure of the amount of light that the aerosols scatter and absorb in the atmosphere, and a proxy for how many particles are in the air. The lower map shows the likely source—natural or human-made (anthropogenic)—based on the size of the particles and other factors. Data depicted are averages for the pre-monsoon season (March through May), the monsoon (June to September), post-monsoon (October, November), and winter (Dec to Feb) for the years 2000 to 2008.

Aerosols are tiny solid and liquid particles suspended in the air, and they come from many natural sources, including volcano emissions, sand and dust storms, and salt from sea spray. Nearly 90 percent of all aerosols (by mass) arise naturally, and most tend to be relatively large particles. The rest of the aerosol load in the air comes from man: sulfates, black and brown carbon, and other pollutants associated with the burning of fossil fuels and of agricultural land. Aerosols produced by human activity tend to be smaller and more damaging to human lungs.

Researchers Sagnik Dey and Larry Di Girolamo of the University of Illinois at Urbana-Champaign assembled and analyzed nine years worth of measurements and found that the level of aerosol pollution was, depending on the season and location, two to five times higher than World Health Organization guidelines. More significantly, the dominant sources of the pollution shift with seasonal weather patterns.

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