Air pollution and global warming are critical environmental issues facing humans and other beautiful creatures on the Earth. Our group studies tropospheric oxidative chemistry and aerosol formation through a combination of laboratory experiments, field measurements, and numerical modeling, to improve our understanding and prediction of air quality and climate. In particular, we are interested in (1) how sulfur species are oxidized in the marine boundary layer, (2) the abundances of reactive halogens in the troposphere and their formation mechanisms, (3) snow photochemistry at mid-latitudes, and (4) aerosol sources in the warming Arctic. A range of research tools are used, including the GEOS-Chem global chemical transport model, chemical ionization mass spectrometry (CIMS), ambient ion monitor – ion chromatography (AIM-IC), and isotope-ratio mass spectrometry (IRMS, collaboration with other groups).