Abstract
Abstract. To provide a theoretical framework towards a 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. This paper constitutes Part 1 of the study, describing 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. A common set of aqueous-phase reactions describes chemistry both within the liquid-like layer (LLL) on the grain surface of the snowpack and within deliquesced "haze" aerosols mainly composed of sulfate in the atmosphere. Gas-phase reactions are also represented by the same mechanism in the atmosphere and in the snowpack interstitial air (SIA). Consequently, the model attains the capacity of simulating interactions between chemistry and mass transfer that become particularly intricate near the interface between the atmosphere and the snowpack. In the SIA, reactive uptake on LLL-coated snow grains and vertical mass transfer act simultaneously on gaseous HOBr, a fraction of which enters from the atmosphere while another fraction is formed via gas-phase chemistry in the SIA itself. A "bromine explosion", by which HOBr formed in the ambient air is deposited and then converted heterogeneously to Br2, is found to be a dominant process of reactive bromine formation in the top 1 mm 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 LLL on the surface of the snow grains. These top- and deeper-layer productions of Br2 both contribute to the release of Br2 to 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 LLL 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 radical chemistry 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 during daytime. 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 buildup of reactive bromine and the resultant loss of ozone, while leading to the higher column amount of BrO in the atmosphere. During the Arctic spring, if moderately saline and acidic snowpack is as prevalent as assumed in our model runs on sea ice, 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 in the course of such events to be detected unambiguously by satellite nadir measurements.
Highlights
Whether or not they lead to prominent ozone depletion events (ODEs) and atmospheric mercury depletion events (AMDEs), such “BrO clouds” would indicate bromine activation in the deeper atmospheric boundary layer (ABL) associated with strong surface winds and/or in air masses elevated by low-pressure systems (Jones et al, 2010; Choi et al, 2012), if not concealed by phenomena above the ABL such as increased BrO columns near the tropopause driven by atmospheric dynamics in the upper troposphere and lower stratosphere (UTLS) (Salawitch et al, 2010)
For a better understanding of what drives the release of reactive bromine from the snowpack on sea ice, we have developed a 1-D model that describes multiphase chemistry and transport of trace constituents from the porous snowpack through the ABL as a unified system
A common set of multiphase, viz., gaseous and aqueous, chemical mechanisms is employed in the atmosphere containing “haze” aerosols and in the snowpack hosting snowpack interstitial air (SIA) between snow gains coated by the liquid-like layer (LLL)
Summary
Discoveries of rapid depletion events for ozone and mercury from the atmospheric boundary layer (ABL) in springtime polar regions of both hemispheres (e.g., Oltmans, 1981; Bottenheim et al, 1986; Schroeder et al, 1998; Wessel et al, 1998; Ebinghaus et al, 2002) have motivated many experimental, observational, and modeling studies to decipher key mechanisms behind this phenomenon (Simpson et al, 2007; Steffen et al, 2008; Abbatt et al, 2012, and references therein). On the other hand, Lehrer et al (2004) assumed that Reaction (R1) could be represented essentially by the dry deposition of HOBr from the atmosphere onto the top of the snowpack; in their one-dimensional (1-D) model of chemistry and diffusion in the ABL, it took more than 20 days from the initial buildup of reactive bromine to the near-complete depletion of ozone This timescale is much longer than 2–5 days as simulated by aforementioned box models in which bulk snowpack was assumed to interact with ventilated air (Tang and McConnell, 1996; Michalowski et al, 2000). This issue had not been addressed explicitly in most of the past modeling studies
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