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

摘要

To provide a theoretical framework towards a better understanding of ozonedepletion events (ODEs) and atmospheric mercury depletion events (AMDEs) inthe polar boundary layer, we have developed a one-dimensional model thatsimulates multiphase chemistry and transport of trace constituents fromporous snowpack and through the atmospheric boundary layer (ABL) as a unifiedsystem. This paper constitutes Part 1 of the study, describing a generalconfiguration of the model and the results of simulations related to reactivebromine release from the snowpack and ODEs during the Arctic spring. A commonset of aqueous-phase reactions describes chemistry both within theliquid-like layer (LLL) on the grain surface of the snowpack and withindeliquesced "haze" aerosols mainly composed of sulfate in the atmosphere.Gas-phase reactions are also represented by the same mechanism in theatmosphere and in the snowpack interstitial air (SIA). Consequently, themodel attains the capacity of simulating interactions between chemistry andmass transfer that become particularly intricate near the interface betweenthe atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coatedsnow grains and vertical mass transfer act simultaneously on gaseous HOBr, afraction of which enters from the atmosphere while another fraction is formedvia gas-phase chemistry in the SIA itself. A "bromine explosion", by whichHOBr formed in the ambient air is deposited and then convertedheterogeneously to Br₂, is found to be a dominant process of reactivebromine formation in the top 1 mm layer of the snowpack. Deeper in thesnowpack, HOBr formed within the SIA leads to an in-snow bromine explosion,but a significant fraction of Br₂ is also produced via aqueousradical chemistry in the LLL on the surface of the snow grains. These top-and deeper-layer productions of Br₂ both contribute to the release ofBr₂ to the atmosphere, but the deeper-layer production is found to bemore important for the net outflux of reactive bromine. Although ozone isremoved via bromine chemistry, it is also among the key species that controlboth the conventional and in-snow bromine explosions. On the other hand,aqueous-phase radical chemistry initiated by photolytic OH formation in theLLL is also a significant contributor to the in-snow source of Br₂and can operate without ozone, whereas the delivery of Br₂ to theatmosphere becomes much smaller after ozone is depleted. Catalytic ozone lossvia bromine radical chemistry occurs more rapidly in the SIA than in theambient air, giving rise to apparent dry deposition velocities for ozone fromthe air to the snow on the order of 10?3 cm s−1 duringdaytime. Overall, however, the depletion of ozone in the system is causedpredominantly by ozone loss in the ambient air. Increasing depth of theturbulent ABL under windy conditions will delay the buildup of reactivebromine and the resultant loss of ozone, while leading to the higher columnamount of BrO in the atmosphere. During the Arctic spring, if moderatelysaline and acidic snowpack is as prevalent as assumed in our model runs onsea ice, the shallow, stable ABL under calm weather conditions may undergopersistent ODEs without substantial contributions from blowing/drifting snowand wind-pumping mechanisms, whereas the column densities of BrO in the ABLwill likely remain too low in the course of such events to be detectedunambiguously by satellite nadir measurements.