Bromine atom production and chain propagation during springtime Arctic ozone depletion events in Barrow, Alaska

摘要

Ozone depletion events (ODEs) in the Arctic are primarily controlled by abromine radical-catalyzed destruction mechanism that depends on the efficientproduction and recycling of Br atoms. Numerous laboratory and modelingstudies have suggested the importance of heterogeneous recycling of Brthrough HOBr reaction with bromide on saline surfaces. On the other hand, thegas-phase regeneration of bromine atoms through BrO–BrO radical reactions hasbeen assumed to be an efficient, if not dominant, pathway for Br reformationand thus ozone destruction. Indeed, it has been estimated that the rate ofozone depletion is approximately equal to twice the rate of the BrOself-reaction. Here, we use a zero-dimensional, photochemical model, largelyconstrained to observations of stable atmospheric species from the 2009 Ocean–Atmosphere–Sea Ice–Snowpack (OASIS)campaign in Barrow, Alaska, to investigate gas-phase bromine radicalpropagation and recycling mechanisms of bromine atoms for a 7-day periodduring late March. This work is a continuation of that presented in Thompsonet al. (2015) and utilizes the same model construct. Here, we use thegas-phase radical chain length as a metric for objectively quantifying theefficiency of gas-phase recycling of bromine atoms. The gas-phase brominechain length is determined to be quite small, at    1.5, and highlydependent on ambient O concentrations. Furthermore, we find that Bratom production from photolysis of Br and BrCl, which is predominatelyemitted from snow and/or aerosol surfaces, can account for between30 and 90 % of total Br atom production. This analysis suggests thatcondensed-phase production of bromine is at least as important as, and attimes greater than, gas-phase recycling for the occurrence of Arctic ODEs.Therefore, the rate of the BrO self-reaction is not a sufficient estimate forthe rate of O depletion.