Sulfate aerosols play an important role in climate and air quality, but understanding how they form in the atmosphere is complex. Different chemical reactions, weather conditions, and pollution sources all influence their formation. A new method using a special oxygen isotope signature (Δ¹⁷O) helps scientists trace how sulfate forms through different chemical pathways.

In this study, we used an advanced air quality model (CMAQ) to simulate these oxygen signatures across the U.S. for the years 2006, 2010, 2014, and 2019. We also fixed a known issue in the model that had affected previous results. Our findings show that sulfate mainly forms through reactions in water droplets with hydrogen peroxide (H₂O₂) and ozone (O₃), and that ozone’s role has grown over time due to decreasing sulfur dioxide emissions. The model results agree well with measurements in the southeastern U.S., but may overestimate sulfate formation in some western regions with high ammonia levels from agriculture. Overall, this research improves our understanding of how sulfate forms and changes with air pollution controls.