Plants emit molecules into the air during growth, reproduction, and defense processes. These compounds undergo chemical reactions in the atmosphere, forming fine particles known as biogenic secondary organic aerosols (SOA) worldwide. These aerosols are minute and inhalable, impacting air quality, human health and Earth’s climate system. Human-made emissions, such as nitrogen and sulfur oxides, also result in a significant amount of SOA. While the SOA from plant sources in the atmosphere is relatively well understood, we do not yet have a full understanding of how human activities contribute to the total amount of biogenic SOA in the atmosphere and how they interact chemically with plant emissions. To address this gap, the Climate Program Office’s Atmospheric Chemistry, Carbon Cycle and Climate (AC4) Program partially funded a new study that investigates the role of anthropogenic sources in biogenic SOA production in the southeastern United States using a combination of observational datasets and the GFDL Global Atmospheric Chemistry-Climate Model. The results, published in Atmospheric Chemistry and Physics, revealed a strong link between SOA production from plants and human-caused emissions that is currently absent from climate models. An international group of scientists, including AC4-supported researcher Jingqiu Mao of the University of Alaska, Fairbanks, identified specific trends, like the overall decreasing organic aerosol concentration in the summertime. The authors presented evidence that a decline in nitrogen and sulfur oxide emissions, mainly from human sources, drives this trend. This project was funded by AC4 as part of an initiative to understand the role of nitrogen in chemical transformations in the atmosphere that lead to aerosol formation.