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

Abstract

Abstract. Ozone depletion events (ODEs) in the Arctic are primarily controlled by a bromine radical-catalyzed destruction mechanism that depends on the efficient production and recycling of Br atoms. Numerous laboratory and modeling studies have suggested the importance of heterogeneous recycling of Br through HOBr reaction with bromide on saline surfaces. On the other hand, the gas-phase regeneration of bromine atoms through BrO-BrO radical reactions has been assumed to be an efficient, if not dominant, pathway for Br reformation and thus ozone destruction. Indeed, it has been estimated that the rate of ozone depletion is approximately equal to twice the rate of the BrO self-reaction. Here, we use a zero-dimensional, photochemical model, largely constrained to observations of stable atmospheric species from the 2009 Ocean-Atmosphere-Sea Ice-Snowpack (OA-SIS) campaign in Barrow, Alaska, to investigate gas-phase bromine radical propagation and recycling mechanisms of bromine atoms for a 7-day period during late March. This work is a continuation of that presented in and utilizes the same model construct. Here, we use the gas-phase radical chain length as a metric for objectively quantifying the efficiency of gas-phase recycling of bromine atoms. The gas-phase bromine chain length is determined to be quite small, at < 1.5, and highly dependent on ambient O 3 concentrations. Furthermore, we find that Br atom production from photolysis of Br 2 and BrCl, which is predominately emitted from snow and/or aerosol surfaces, can account for between 30 and 90 % of total Br atom production. This analysis suggests that condensed-phase production of bromine is at least as important as, and at times greater than, gas-phase recycling for the occurrence of Arctic ODEs. Therefore, the rate of the BrO self-reaction is not a sufficient estimate for the rate of O 3 depletion.

Springtime Bromine Activation over Coastal and Inland Arctic Snowpacks

“…It is also likely that the BrO mixing ratios decreased as a result of dilution during vertical mixing, causing the rate of reaction 7 to decrease as [BrO] 2 . This decreased reaction rate decreases the chain length and slows the propagation of the bromine chemistry . As reaction 7 slows, a greater fraction of BrO photolyzes, followed by Br loss via reactions such as reaction . While these areas of open water refreeze and form new sea ice, freshly frozen sea ice is slightly alkaline and buffered against pH changes, such that it has not been observed to produce reactive bromine .…”

Reactive bromine in the low troposphere of Antarctica. Estimations at two research sites

“…13). These ranges are in line with previous model studies for Antarctic latitudes (e.g., von Glasow et al, 2004;Saiz-Lopez et al, 2008) and in the lower limit of Artic model studies (e.g., Thompson et al, 2017).…”

“…Former works have estimated that the bromine driven ozone loss in the polar atmosphere represents 44% of the total O3 chemical loss (e.g., Liao et al, 2012;Thompson et al, 2017). Therefore, in the sites referred to in this work the shortest (i.e., 20 at highest BrOx and low wind speed) ozone chemical lifetime τO3 expected is 2.6 days at Belgrano and 0.7 days at Marambio.…”

Direct detection of atmospheric atomic bromine leading to mercury and ozone depletion

“…Alternatively, Br can be inferred from BrO, commonly measured from satellite, airborne, and ground-based instruments (19). However, Br regeneration from BrO likely plays a minor role when O 3 is depleted (22), suggesting that Br estimates from BrO observations alone may be biased.…”