The Smith group studies the chemistry of aerosol particles that are important in the atmosphere. In particular, we are learning about how organic aerosols react with oxidants such as OH, O3, NO3 and Cl. By characterizing the chemical composition of reacted particles using mass spectrometry we can determine how fast a gas-particle reaction occurs, which products are formed and their yields. This information helps us to understand how organic aerosols are transformed chemically during their lifetimes in the atmosphere and helps us to predict how they will grow, take up water and scatter sunlight. To obtain the detailed chemical information necessary for these experiments, we use an Aerosol CIMS (chemical ionization mass spectrometer) developed in this lab.