The UNHMERC instrument provides detailed information on atmospheric mercury. Measurements of total gaseous mercury (TGM) and gaseous elemental mercury (Hg°) are performed simultaneously with one minute time resolution using a custom four-channel atomic fluorescence spectrometer. The relative amount of reactive gaseous mercury (RGM = HgCl2 + HgBr2+ HgOBr + …) will be assessed through careful examination of the difference between TGM and Hg°. TGM is defined as the sum Hg° + RGM. Targeted aerosol sampling will also be conducted for particulate-phase mercury (HgP).