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Researchers at Georgia Tech have analyzed the seasonal differences of sulfate aerosols—a major pollutant in the United States—to examine the long-term impact from sulfur dioxide (SO₂) emission reductions since the enactment of the Clean Air Act amendments in 1990.

School of Earth and Atmospheric Sciences Professor Yuhang Wang and his team studied the factors affecting SO₂ and sulfate concentrations during winter and summer in the "Rust Belt"—from New York through the Midwest—and the Southeast regions of the U.S. over two decades (2004 to 2023). The team also developed an ensemble machine learning approach to project seasonal patterns until 2050.

"Power plants, particularly those burning coal and oil, are a major source of SOâ‚‚ emissions in these regions," says Wang, who co-authored, with Ph.D. students Fanghe Zhao and Shengjun Xi, the study recently in Environmental Science & Technology Letters.

Seasonal differences in atmospheric chemistry

In the U.S., the chemistry in the atmosphere varies among the seasons. During summer, solar radiation from ample sunlight activates oxidant reactions that produce hydrogen peroxide (Hâ‚‚Oâ‚‚) in the atmosphere. The supply of Hâ‚‚Oâ‚‚ is determined by the amount of emitted air pollution, and once in the atmosphere, Hâ‚‚Oâ‚‚ can oxidize SOâ‚‚ quickly into sulfate aerosols in the aqueous phase.

Sulfate aerosols from the oxidation of SOâ‚‚ contribute to the formation of particulate matter less than 2.5 micrometers in diameter (PM_2.5). Particulate sulfate poses significant environmental and public health risks, including air pollution, acid rain, and circulatory and respiratory issues.

"The supply of H₂O₂ in summer is eight times greater than in winter—a huge difference—which means sulfate concentrations are generally higher in summer and a reduction in SO₂ emissions leads to a proportional decrease in sulfate concentrations," explains Wang. "When SO₂ emissions exceed the available supply of H₂O₂ in winter, the reduction in sulfate concentrations can be much smaller because of a 'chemical damping' effect that causes sulfate levels to decline more slowly than SO₂ emissions."

Narrowing the disparities between seasonal sulfate levels

The study's two-decade observations revealed distinct patterns in the reduction of SOâ‚‚ emissions and sulfate concentrations during winter and summer.

While SO₂ emissions significantly decreased in both seasons over time—primarily from the Clean Air Act and more power plants transitioning from coal to natural gas—the reduction of sulfate concentrations initially showed large seasonal differences. However, over the past decade, the disparity between winter and summer sulfate levels narrowed as SO₂ emissions decreased.

According to Wang, the seasonal disparity of sulfate was caused by changing chemical regimes in winter over time. Although the lower supply of H₂O₂ remained stable in winter, SO₂ wintertime emissions were higher from 2004 to 2013, then dropped below the level of H₂O₂ after 2013—reaching parity with the levels of reduced SO₂ emissions in the summer.

"When you have this complexity of atmospheric chemistry, there is a non-linear effect in winter—as SO₂ emissions decreased, sulfate aerosol production efficiency increased until 2013, then flattened as of today. The reduction in sulfate aerosols initially lagged behind the decrease in SO₂ emissions but eventually caught up as a result of sustained air quality control efforts," says Wang. "Conversely, there is a simple, linear effect in summer—the more SO₂ emissions, the more sulfate aerosols in the atmosphere—and if you reduce one, the other is reduced by the same proportion."

Decades-long full impact

From now until 2050, the researchers' machine learning projections indicate a continuing decrease of winter and summer sulfate levels, which are currently about 20%, as SOâ‚‚ emission controls achieve comparable efficacy across the seasons.

"We're now seeing the full impact from the Clean Air Act," concludes Wang, "and the nation's sustained effort in pollution reduction is key to improving air quality and health outcomes."