Researchers at the Georgia Institute of Technology have conducted a comprehensive analysis of how reductions in sulfur dioxide (SO₂) emissions have impacted air quality across different seasons in the United States. The study, led by Professor Yuhang Wang from the School of Earth and Atmospheric Sciences, focused on sulfate aerosols—a significant pollutant—over two decades, examining regions stretching from New York through the Midwest and into the Southeast.
The research team, which included Ph.D. students Fanghe Zhao and Shengjun Xi, analyzed data from 2004 to 2023. Their work was supported by the National Science Foundation and Georgia Tech’s Brook Byers Institute for Sustainable Systems. They also developed a machine learning model to project seasonal patterns in sulfate levels up to 2050.
“Power plants, particularly those burning coal and oil, are a major source of SO₂ emissions in these regions,” said Wang. The findings were published recently in Environmental Science & Technology Letters.
The study highlights that atmospheric chemistry differs significantly between summer and winter. In summer, increased sunlight leads to higher production of hydrogen peroxide (H₂O₂), which reacts with SO₂ to form sulfate aerosols more rapidly. Sulfate aerosols contribute to fine particulate matter (PM2.5), posing environmental and public health risks such as air pollution, acid rain, 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,” Wang explained. “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.”
Over time, both winter and summer saw significant decreases in SO₂ emissions due largely to regulatory measures like the Clean Air Act amendments and power plant transitions from coal to natural gas. However, initial reductions in sulfate concentrations showed notable seasonal differences; this gap has narrowed over the past decade as emission controls became more effective.
Wang noted that changes observed after 2013 were especially important: although wintertime H₂O₂ supplies remained low, SO₂ emissions dropped below this threshold after 2013—aligning with reduced summertime levels.
“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,” said 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.”
Looking ahead to 2050 using their machine learning projections, researchers expect continued declines in both winter and summer sulfate levels—currently around 20 percent—as emission controls maintain similar effectiveness throughout all seasons.
“We’re now seeing the full impact from the Clean Air Act,” concluded Wang. “and the nation’s sustained effort in pollution reduction is key to improving air quality and health outcomes.”
