Atmospheric Chemistry of Gaseous Oxidized Mercury at a Coastal Site in Atlantic Canada

Abstract

A chemistry box model containing a comprehensive suite of mercury (Hg) oxidation mechanisms involving O₃, OH, H₂O₂, Br, BrO, NO₂, HO₂, and other oxidants was used to simulate the formation of gaseous oxidized mercury (GOM) and understand the chemical processes driving the observed trends in GOM at Kejimkujik, Nova Scotia, Canada. Simulations were conducted using chemical schemes with and without oxidation by O₃ and OH. The major oxidants of Hg are O₃ and OH (79%), H₂O₂ (10%), Br with second-stage HgBr oxidation by NO₂ (7%), and BrO (3%) in simulations where all GEM oxidation reactions were considered simultaneously. In an alternative chemical scheme without gas-phase oxidation by O₃ and OH, the dominant GOM species were HgBrNO₂ (58%) and HgBrO (23.5%). Using this chemical scheme, the model reproduced the observed GOM at sub-ppqv Br₂ mixing ratios. In the scheme with O₃ and OH, the variability in GOM between seasons and between continental and marine air masses was mainly due to the variability in gaseous elemental Hg, O₃, OH, and aerosol liquid water content (LWC). LWC governs the partitioning of GOM to the aerosol aqueous phase in the model. In the scheme without O₃ and OH, the variability in GOM by season and airmass origin strongly depends on Br and BrO, suggesting that rigorous validation of modeled Br and BrO data are essential for improving the model predictions of GOM in coastal environments.