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The reactive uptake of HOBr onto halogen-rich aerosols promotes conversionof Br−(aq) into gaseous reactive bromine (incl. BrO) with impactson tropospheric oxidants and mercury deposition. However, experimental dataquantifying HOBr reactive uptake on tropospheric aerosols is limited, andreported values vary in magnitude. This study introduces a new evaluation ofHOBr reactive uptake coefficients in the context of the general acid-assisted mechanism. We emphasise that the termolecular kinetic approachassumed in numerical model studies of tropospheric reactive brominechemistry to date is strictly only valid for a specific pH range and,according to the general acid-assisted mechanism for HOBr, the reactionkinetics becomes bimolecular and independent of pH at high acidity. This study reconciles for the first time the different reactive uptakecoefficients reported from laboratory experiments. The re-evaluation confirmsHOBr reactive uptake is rapid on moderately acidified sea-salt aerosol (andslow on alkaline aerosol), but predicts very low reactive uptake coefficientson highly acidified submicron particles. This is due to acid-saturatedkinetics combined with low halide concentrations induced by bothacid-displacement reactions and the dilution effects ofH2SO4(aq). A mechanism is thereby proposed for reportedBr enhancement (relative to Na) in H2SO4-rich submicron particlesin the marine environment. Further, the fact that HOBr reactive uptake onH2SO4-acidified supra-micron particles is driven by HOBr+Br−(rather than HOBr+Cl−) indicates self-limitation viadecreasing γHOBr once aerosol Br- is converted intoreactive bromine. First predictions of HOBr reactive uptake on sulfate particles inhalogen-rich volcanic plumes are also presented. High (accommodationlimited) HOBr+Br- uptake coefficient in concentrated (> 1 μmol mol−1 SO2) plume environments supports potential for rapid BrOformation in plumes throughout the troposphere. However, reduced HOBrreactive uptake may reduce the rate of BrO cycling in dilute plumes in thelower troposphere. In summary, our re-evaluation of HOBr kinetics provides a new framework for theinterpretation of experimental data and suggests that the reactive uptake of HOBron H2SO4-acidified particles is substantially overestimated in currentnumerical models of BrO chemistry in the troposphere

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