Re-evaluating the reactive uptake of HOBr in the troposphere with implications for the marine boundary layer and volcanic plumes

Abstract

The reactive uptake of HOBr onto halogen-rich aerosols promotes conversion of Br−(aq) into gaseous reactive bromine (incl. BrO) with impacts on tropospheric oxidants and mercury deposition. However, experimental data quantifying HOBr reactive uptake on tropospheric aerosols is limited, and reported values vary in magnitude. This study introduces a new evaluation of HOBr reactive uptake coefficients in the context of the general acid-assisted mechanism. We emphasise that the termolecular kinetic approach assumed in numerical model studies of tropospheric reactive bromine chemistry to date is strictly only valid for a specific pH range and, according to the general acid-assisted mechanism for HOBr, the reaction kinetics becomes bimolecular and independent of pH at high acidity. This study reconciles for the first time the different reactive uptake coefficients reported from laboratory experiments. The re-evaluation confirms HOBr reactive uptake is rapid on moderately acidified sea-salt aerosol (and slow on alkaline aerosol), but predicts very low reactive uptake coefficients on highly acidified submicron particles. This is due to acid-saturated kinetics combined with low halide concentrations induced by both acid-displacement reactions and the dilution effects of H2SO4(aq). A mechanism is thereby proposed for reported Br enhancement (relative to Na) in H2SO4-rich submicron particles in the marine environment. Further, the fact that HOBr reactive uptake on H2SO4-acidified supra-micron particles is driven by HOBr+Br− (rather than HOBr+Cl−) indicates self-limitation via decreasing γHOBr once aerosol Br- is converted into reactive bromine. First predictions of HOBr reactive uptake on sulfate particles in halogen-rich volcanic plumes are also presented. High (accommodation limited) HOBr+Br- uptake coefficient in concentrated (> 1 μmol mol−1 SO2) plume environments supports potential for rapid BrO formation in plumes throughout the troposphere. However, reduced HOBr reactive uptake may reduce the rate of BrO cycling in dilute plumes in the lower troposphere. In summary, our re-evaluation of HOBr kinetics provides a new framework for the interpretation of experimental data and suggests that the reactive uptake of HOBr on H2SO4-acidified particles is substantially overestimated in current numerical models of BrO chemistry in the troposphere.