PNNL

Abstract

Gas phase halogens influence lower atmospheric oxidation cycles and ozone (O3) but are generally low in abundance and typically arise from chemical reactions involving natural sources such as sea salt. Here we report aircraft observations of very large quantities of HCl and the dihalogens Cl2, Br2, and BrCl in plumes emitted from an industrial plant near the Great Salt Lake, Utah. Complete depletion of O3 was observed concurrently with halogen emissions as a direct result of photochemically produced halogen radicals. Estimated emission fluxes for Cl2, HCl, and NOx agreed with facility-reported emissions inventories. Bromine emissions are not required to be reported in the inventory, but are estimated here as 173 Mg year-1 Br2 and 949 Mg year-1 BrCl, representing a major uncounted oxidant source. A zero-dimensional photochemical box model was used to reproduce the observed O3 depletions, and the model indicates that bromine radicals are the dominant species responsible for the rapid O3 depletion. Furthermore, the inclusion of these halogen emissions in both the box model and a chemical transport model is associated with significant increases in HO2, RO2, and particulate nitrate in the populated regions downwind of the plant. This measurement and modeling of halogen emissions and O3 depletion provides an important test for current understanding of tropospheric halogen chemistry and demonstrates the potential impact of underreported industrial bromine emissions on oxidation sources and air quality within a major urban area of the western US.