Midlatitude Ozone Depletion and Air Quality Impacts from Industrial Halogen Emissions in the Great Salt Lake Basin.

We report aircraft observations of extreme levels of HCl and the dihalogens Cl2, Br2, and BrCl in an industrial plume near the Great Salt Lake, Utah. Complete depletion of O3 was observed concurrently with halogen enhancements as a direct result of photochemically produced halogen radicals. Observed 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 as 173 Mg year-1 Br2 and 949 Mg year-1 BrCl, representing a major uncounted oxidant source. A zero-dimensional photochemical box model reproduced the observed O3 depletions and demonstrated that bromine radical cycling was principally responsible for the rapid O3 depletion. Inclusion of observed halogen emissions in both the box model and a 3D chemical model showed significant increases in oxidants and particulate matter (PM2.5) in the populated regions of the Great Salt Lake Basin, where winter PM2.5 is among the most severe air quality issues in the U.S. The model shows regional PM2.5 increases of 10%-25% attributable to this single industrial halogen source, demonstrating the impact of underreported industrial bromine emissions on oxidation sources and air quality within a major urban area of the western U.S.

Production of N2O5 and ClNO2 through Nocturnal Processing of Biomass-Burning Aerosol.

Biomass burning is a source of both particulate chloride and nitrogen oxides, two important precursors for the formation of nitryl chloride (ClNO2), a source of atmospheric oxidants that is poorly prescribed in atmospheric models. We investigated the ability of biomass burning to produce N2O5(g) and ClNO2(g) through nocturnal chemistry using authentic biomass-burning emissions in a smog chamber. There was a positive relationship between the amount of ClNO2 formed and the total amount of particulate chloride emitted and with the chloride fraction of nonrefractory particle mass. In every fuel tested, dinitrogen pentoxide (N2O5) formed quickly, following the addition of ozone to the smoke aerosol, and ClNO2(g) production promptly followed. At atmospherically relevant relative humidities, the particulate chloride in the biomass-burning aerosol was rapidly but incompletely displaced, likely by the nitric acid produced largely by the heterogeneous uptake of N2O5(g). Despite this chloride acid displacement, the biomass-burning aerosol still converted on the order of 10% of reacted N2O5(g) into ClNO2(g). These experiments directly confirm that biomass burning is a potentially significant source of atmospheric N2O5 and ClNO2 to the atmosphere.

N2O5 reactive uptake kinetics and chlorine activation on authentic biomass-burning aerosol.

We examined the reactive uptake of dinitrogen pentoxide (N2O5) to authentic biomass-burning aerosol (BBA) using a small chamber reservoir in combination with an entrained aerosol flow tube. BBA was generated from four different fuel types and the reactivity of N2O5 was probed from 30 to 70% relative humidity (RH). The N2O5 reactive uptake coefficient, γ(N2O5), depended upon RH, fuel type, and to a lesser degree on aerosol chloride mass fractions. The γ(N2O5) ranged from 2.0 (±0.4) ×10-3 on black needlerush derived BBA at 30% RH to 6.0 (±0.6) ×10-3 on wiregrass derived BBA at 65% RH. Major N2O5 reaction products were observed including gaseous ClNO2 and HNO3 and particulate nitrate, and used to create a reactive nitrogen budget. Black needlerush BBA had the most particulate chloride, and the only measured ClNO2 yield > 1%. The ClNO2 yield on black needlerush decayed from an initial value of ∼100% to ∼30% over the course of the burn experiment, suggesting a depletion of BBA chloride over time. Black needlerush was also the only fuel for which the reactive nitrogen budget indicated other N-containing products were generated. Generally, the results suggest limited chloride availability for heterogeneous reaction for BBA in the RH range probed here, including BBA with chloride mass fractions on the higher end of previously reported values (∼17-34%). Though less than fresh sea spray aerosol, ∼50%. We use these measured quantities to discuss the implications for nocturnal aerosol nitrate formation, the chemical fate of N2O5(g), and the availability of particulate chloride for activation in biomass burning plumes.