With rapid sea ice loss and warming, there is an urgent need to understand the unique chemistry involving multiphase reactions of atmospheric aerosols and the snow-covered sea ice surface in the Arctic. Yet, the harsh environment and low analyte concentrations pose analytical challenges. The Pratt Lab develops and applies novel mass spectrometry methods to measure the complex chemistry of trace gases, aerosols, and snow in the Arctic. Using chemical ionization mass spectrometry, we are advancing understanding of Arctic halogen photochemistry through measurements of trace gas species at ppt to ppq levels, including observations of several trace gases species for the first time in the ambient atmosphere. Bromine, chlorine, and iodine chemistry, and the coupling of the cycles involving these halogen species, have significant impacts on the fate of greenhouse gases and pollutants, including ozone, methane, and mercury. Sunlit and artificial light experiments conducted in the Alaskan Arctic, combined with atmospheric measurements and numerical modeling, were utilized to elucidate chemical mechanisms driving the unique multiphase processes. The new chemical insights obtained are providing crucial scientific detail needed to understand and predict changing atmospheric composition in the Arctic.