Air-snowpack exchange of bromine, ozone and mercury in the springtime Arctic simulated by the 1-D model PHANTAS – Part 1: In-snow bromine activation and its impact on ozone

Abstract

Abstract. To provide a theoretical framework towards better understanding of ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs) in the polar boundary layer, we have developed a one-dimensional model that simulates multiphase chemistry and transport of trace constituents from porous snowpack and through the atmospheric boundary layer (ABL) as a unified system. In this paper, we describe a general configuration of the model and the results of simulations related to reactive bromine release from the snowpack and ODEs during the Arctic spring. The model employs a chemical mechanism adapted from the one previously used for the simulation of multiphase halogen chemistry involving deliquesced sea-salt aerosols in the marine boundary layer. A common set of aqueous-phase reactions describe chemistry both in the liquid-like (or brine) layer on the grain surface of the snowpack and in "haze" aerosols mainly composed of sulfate in the atmosphere. The process of highly soluble/reactive trace gases, whether entering the snowpack from the atmosphere or formed via gas-phase chemistry in the snowpack interstitial air (SIA), is simulated by the uptake on brine-covered snow grains and subsequent reactions in the aqueous phase while being traveled vertically within the SIA. A "bromine explosion", by which, in a conventional definition, HOBr formed in the ambient air is deposited and then converted heterogeneously to Br2, is a dominant process of reactive bromine formation in the top 1 mm (or less) layer of the snowpack. Deeper in the snowpack, HOBr formed within the SIA leads to an in-snow bromine explosion, but a significant fraction of Br2 is also produced via aqueous radical chemistry in the brine on the surface of the snow grains. These top- and deeper-layer productions of Br2 both contribute to the Br2 release into the atmosphere, but the deeper-layer production is found to be more important for the net outflux of reactive bromine. Although ozone is removed via bromine chemistry, it is also among the key species that control both the conventional and in-snow bromine explosions. On the other hand, aqueous-phase radical chemistry initiated by photolytic OH formation in the liquid-like layer is also a significant contributor to the in-snow source of Br2 and can operate without ozone, whereas the delivery of Br2 to the atmosphere becomes much smaller after ozone is depleted. Catalytic ozone loss via bromine radicals occurs more rapidly in the SIA than in the ambient air, giving rise to apparent dry deposition velocities for ozone from the air to the snow on the order of 10−3 cm s-1 under sunlight. Overall, however, the depletion of ozone in the system is caused predominantly by ozone loss in the ambient air. Increasing depth of the turbulent ABL under windy conditions will delay the build-up of reactive bromine and the resultant loss of ozone, while leading to the higher column amount of BrO in the atmosphere. If moderately saline and acidic snowpack is as prevalent as assumed in our model runs on sea ice during the spring, the shallow, stable ABL under calm weather conditions may undergo persistent ODEs without substantial contributions from blowing/drifting snow and wind-pumping mechanisms, whereas the column densities of BrO in the ABL will likely remain too low during the course of such events to be detected unambiguously by satellite nadir measurements.