The atmosphere contains thousand of chemical species that originate from natural and anthropogenic sources. Atmospheric chemists study their emission, transport, reactions and deposition throughout the environment. Everyone is affected in numerous ways by seemingly small amounts of gases and aerosols (i.e., minute droplets and particles) in the air around us. Agricultural production may be impaired by ground-level ozone at sub-part-per-million levels. Microgram-per-cubic-meter concentrations of aerosols formed by atmospheric reactions of ammonia with sulfuric acid strongly influence the formation of clouds and regional haze. Inhalation of complex mixtures of ambient aerosols in urban areas may adversely affect human health. Research at the Climate and Atmospheric Science section focuses on identifying, measuring and understanding such chemical species and their effects.