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Human activity is a major source of sulfate aerosols, but smokestacks don't emit sulfate particles directly. Rather, coal power production and other industrial processes release sulfur dioxide -- the same gas that billows from volcanoes -- that later reacts with atmospheric molecules called hydroxyl radicals to produce sulfates as a byproduct. Hydroxyl is so reactive scientists consider it an atmospheric "detergent" or "scrubber" because it cleanses the atmosphere of many types of pollution.

In the chemical soup of the lower atmosphere, however, sulfur dioxide isn't the only substance interacting with hydroxyl. Similar reactions influence the creation of nitrate aerosols. And hydroxyls drive long chains of reactions involving other common gases, including ozone.

Methane and carbon monoxide use up hydroxyl that would otherwise produce sulfate, thereby reducing the concentration of sulfate aerosols. It's a seemingly minor change, but it makes a difference to the climate. "More methane means less hydroxyl, less sulfate, and more warming," Shindell explained.

His team's modeling experiment, one of the first to rigorously quantify the impact of gas-aerosol interactions on both climate and air quality, showed that increases in global methane emissions have caused a 26 percent decrease in hydroxyl and an 11 percent decrease in the number concentration of sulfate particles. Reducing sulfate unmasks methane's warming by 20 to 40 percent over current estimates, but also helps reduce negative health effects from sulfate aerosols.

In comparison, the model calculated that global carbon monoxide emissions have caused a 13 percent reduction in hydroxyl and 9 percent reduction in sulfate aerosols.

Nitrogen oxides -- pollutants produced largely by power plants, trucks, and cars
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