Changing Emissions and Atmospheric Chemistry: Ongoing Impacts on Air Quality and Climate

H uman activity has profoundly altered atmospheric composition, leaving an unmistakable fingerprint on the Earth's system, with investigations into atmospheric chemistry a core research topic at ES&T over many decades. 1 An overarching theme in air quality research has been oxidation chemistry of primary emissions.For example, ozone formation occurs via oxidation of volatile organic carbon (VOC) species in the presence of nitrogen oxides (NO x ).Secondary aerosol formation occurs through oxidation when sulfate forms from sulfur dioxide, nitrate from NO x , and highly oxidized organic molecules form from both biogenic and anthropogenic hydrocarbon emissions.Understanding this oxidation chemistry is the foundation for regulatory actions, which have led to remarkable improvements in air quality in many cities around the world.In the US alone, 2 trillion US dollars of economic value arise each year from improvements in air quality via the Clean Air Act. 2 ES&T has been an important partner in this research area by supporting impactful and direction-changing research which ultimately has resulted in policy change and global environmental improvements.However, major changes in atmospheric chemistry are currently underway.Greenhouse gas emissions continue to rise globally, and aerosols and their precursor vapors have declined in many regions as a result of air-quality regulation.Advances in exhaust after-treatment are dramatically reducing primary emissions of regulated species, particle mass, and black