Researchers at the Georgia Institute of Technology have conducted a study analyzing the long-term effects of sulfur dioxide (SO₂) emission reductions on sulfate aerosols in the United States. The research focused on seasonal differences in sulfate concentrations across regions such as the “Rust Belt” and the Southeast from 2004 to 2023, examining how air quality has changed since amendments to the Clean Air Act were enacted in 1990.
The team, led by Professor Yuhang Wang from the School of Earth and Atmospheric Sciences, investigated factors influencing SO₂ and sulfate levels during both winter and summer. The project was supported by the National Science Foundation and Georgia Tech’s Brook Byers Institute for Sustainable Systems. The researchers also developed an ensemble machine learning model to predict future trends through 2050.
“Power plants, particularly those burning coal and oil, are a major source of SO₂ emissions in these regions,” said Wang. He co-authored the study with Ph.D. students Fanghe Zhao and Shengjun Xi; their findings were published in Environmental Science & Technology Letters.
Atmospheric chemistry differs between seasons in the U.S., with summer sunlight driving reactions that produce hydrogen peroxide (H₂O₂), which oxidizes SO₂ into sulfate aerosols more rapidly than in winter. Sulfate aerosols contribute to particulate matter less than 2.5 micrometers in diameter (PM2.5), which is linked to environmental hazards such as acid rain and health issues including respiratory problems.
“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,” explained 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.”
Over two decades, observations showed significant declines in SO₂ emissions during both seasons, largely due to regulatory changes under the Clean Air Act and shifts from coal-fired power plants to natural gas. However, reductions in sulfate concentrations initially differed between seasons but became more similar over time as overall emissions fell.
Wang noted that this change was due to evolving chemical regimes during winter: “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,” 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.”
Machine learning projections suggest that both winter and summer sulfate levels will continue declining through 2050 as emission controls remain effective 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.”
