Re-evaluating the reactive uptake of HOBr in the troposphere with implications for the marine boundary layer and volcanic plumes
Authors: Roberts T. J. , Jourdain L. , Griffiths P. T. , Pirre M. 
Published: January 2014

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 re-examines the reaction kinetics of HOBr across a range of aerosol acidity conditions, focusing on chemistry within the marine boundary layer and volcanic plumes. We highlight that the termolecular approach to HOBr reaction kinetics, used in numerical model studies to date, is strictly only valid over a specific pH range. Here we re-evaluate the reaction kinetics of HOBr according to the general acid assisted mechanism. The rate of reaction of HOBr with halide ions becomes independent of pH at high acidity yielding an acid-independent second-order rate constant, kII. The limit of acid-saturation is poorly constrained by available experimental data, although a reported estimate for HOBr+ Br−(aq)+H+(aq), is kIIsat = 108–109 M−1 s−1, at pH &amp;lesssim; 1. By consideration of halide nucleophilic strength and re-evaluation of reported uptake coefficient data on H2SO4-acidified sea-salt aerosol, we suggest the reaction of HOBr(aq) + Cl−(aq)+H+(aq) may saturate to become acid-independent at pH ≤ 6, with kIIsat ~104 M−1 s−1. This rate constant is multiple orders of magnitude lower (a factor of 103 at pH = 3 and a factor of 106 at pH = 0) than that currently assumed in numerical models of tropospheric BrO chemistry, which are based on the termolecular approach. Reactive uptake coefficients, γHOBr, were calculated as a function of composition using the revised HOBr kinetics, with kI = kII · [X−(aq)], and X = Br or Cl. γHOBr initially increases with acidity but subsequently declines with increasing H2SO4-acidification of sea-salt aerosol. The HOBr+Cl− uptake coefficient declines due to acid-displacement of HCl(g), reducing [Cl−(aq)]. The HOBr+Br− uptake coefficient also declines at very high H2SO4:Na ratios due to dilution of [Br−(aq)]. The greatest reductions in HOBr uptake coefficients occur for small particle sizes, across which the probability of diffusion of HOBr(aq) without reaction is highest. Our new uptake calculations are consistent with all reported experimental data thus resolve previously reported discrepancies within a unified uptake coefficient framework. The following implications for BrO chemistry in the marine boundary layer are highlighted: we confirm HOBr reactive uptake is rapid on moderately acidified supramicron aerosol, but predict very low HOBr reactive uptake coefficients on the highly-acidified submicron marine aerosol fraction. This re-evaluation is in contrast to the high HOBr reactive uptake previously assumed to occur on all acidified sea-salt aerosol. Instead, our uptake evaluation indicates that particle bromide in the submicron aerosol fraction is not easily depleted by HOBr uptake, and furthermore can be augmented by deposition of gas-phase bromine released from the supramicron particles. We present this mechanism as a first explanation for the observed (but previously unexplained) Br-enhancement (relative to Na) in submicron particles in the marine environment. Further, we find HOBr reactive uptake on acidified sea-salt aerosol is driven by reaction of HOBr+Br− rather than HOBr+Cl− (γHOBr + Br− > γHOBr−+Cl−) once HCl-displacement has occurred. Thus, the reduction in γHOBr + Br− as BrO chemistry progresses (noting γHOBr + Br− is a function of aerosol Br−(aq) concentration which declines as aerosol bromide is converted into gaseous-phase reactive bromine) will have greater importance in slowing overall HOBr reactive uptake as BrO chemistry evolves than has been assumed previously. We suggest both the above factors may explain the reported overprediction of BrO cycling in the marine environment by numerical models to date. First predictions of HOBr reactive uptake on sulphate particles in tropospheric volcanic plumes are presented. High (accommodation limited) HOBr+Br− uptake coefficient in concentrated (>1 ppmv SO2) plume environments supports rapid BrO formation under all conditions. However, the HOBr + Cl− uptake coefficient exhibits an inverse temperature trend which becomes more pronounced as the plume disperses. The HOBr+Br− coefficient also declines with temperature in dilute (~ppbv SO2) plumes. We infer that BrO chemistry can readily be sustained in downwind plumes entering the mid- to-upper troposphere, e.g. either from continuous degassing from elevated volcano summits (e.g. Etna, 3.3 km a.s.l.) or episodic eruptions (e.g. Eyjafjallajökull, Iceland). However, low HOBr reactive uptake coefficients may limit sustained BrO cycling in dilute plumes in the lower troposphere. In summary, our revised HOBr kinetics that includes acid-saturation indicates that current numerical models of BrO chemistry in the troposphere substantially overestimate the rate of HOBr reactive uptake on acidic halogen rich-particles, with implications for BrO chemistry in both the marine environment and volcanic plumes, as well as the wider troposphere.