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

Tjarda Roberts (1) , Line Jourdain (1) , P. T. Griffiths (2) , Michel Pirre (1)

1 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
2 Centre for Atmospheric Science [Cambridge, UK]

The reactive uptake of HOBr onto halogen-richaerosols promotes conversion of Br−(aq) into gaseous reactivebromine (incl. BrO) with impacts on tropospheric oxidantsand mercury deposition. However, experimental data quantifyingHOBr reactive uptake on tropospheric aerosols is limited,and reported values vary in magnitude. This study introducesa new evaluation of HOBr reactive uptake coefficientsin the context of the general acid-assisted mechanism. Weemphasise that the termolecular kinetic approach assumedin numerical model studies of tropospheric reactive brominechemistry to date is strictly only valid for a specific pH rangeand, according to the general acid-assisted mechanism forHOBr, the reaction kinetics becomes bimolecular and independentof pH at high acidity.This study reconciles for the first time the different reactiveuptake coefficients reported from laboratory experiments.The re-evaluation confirms HOBr reactive uptake israpid on moderately acidified sea-salt aerosol (and slow onalkaline aerosol), but predicts very low reactive uptake coefficientson highly acidified submicron particles. This isdue to acid-saturated kinetics combined with low halide concentrationsinduced by both acid-displacement reactions andthe dilution effects of H2SO4(aq). A mechanism is therebyproposed for reported Br enhancement (relative to Na) inH2SO4-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 decreasingHOBr once aerosol Br− is converted into reactivebromin First predictions of HOBr reactive uptake on sulfate particlesin halogen-rich volcanic plumes are also presented.High (accommodation limited) HOBr+Br− uptake coefficientin concentrated (> 1 μmol mol−1 SO2) plume environmentssupports potential for rapid BrO formation in plumesthroughout the troposphere. However, reduced HOBr reactiveuptake may reduce the rate of BrO cycling in diluteplumes in the lower troposphere.In summary, our re-evaluation of HOBr kinetics providesa new framework for the interpretation of experimental dataand suggests that the reactive uptake of HOBr on H2SO4-acidified particles is substantially overestimated in currentnumerical models of BrO chemistry in the troposphere.

Domains: Astrophysics [astro-ph]

Tjarda Roberts, Line Jourdain, P. T. Griffiths, Michel Pirre. Re-evaluating the reactive uptake of HOBr in the troposphere with implications for the marine boundary layer and volcanic plumes. Atmospheric Chemistry and Physics, 2014, 14, pp.11185-11199. ⟨insu-01095135⟩