The Atmospheric Chemistry Division of the National Center for Atmospheric Research identifies and quantifies the natural and anthropogenic processes that regulate the chemical composition of the troposphere and middle atmosphere, and assesses future changes caused by human activities. ACD studies the photochemistry of the troposphere, biosphere/atmosphere interactions, middle atmospheric dynamics and chemistry, and climate/chemistry couplings. Laboratory projects, field campaigns, data interpretation and modeling studies are developed to address societally important questions related to global and regional environmental changes.