Abstract
Abstract. We use the GEOS-Chem chemical transport model to examine the influence of bromine release from blowing-snow sea salt aerosol (SSA) on springtime bromine activation and O3 depletion events (ODEs) in the Arctic lower troposphere. We evaluate our simulation against observations of tropospheric BrO vertical column densities (VCDtropo) from the GOME-2 (second Global Ozone Monitoring Experiment) and Ozone Monitoring Instrument (OMI) spaceborne instruments for 3 years (2007–2009), as well as against surface observations of O3. We conduct a simulation with blowing-snow SSA emissions from first-year sea ice (FYI; with a surface snow salinity of 0.1 psu) and multi-year sea ice (MYI; with a surface snow salinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surface snow relative to seawater. This simulation captures the magnitude of observed March–April GOME-2 and OMI VCDtropo to within 17 %, as well as their spatiotemporal variability (r=0.76–0.85). Many of the large-scale bromine explosions are successfully reproduced, with the exception of events in May, which are absent or systematically underpredicted in the model. If we assume a lower salinity on MYI (0.01 psu), some of the bromine explosions events observed over MYI are not captured, suggesting that blowing snow over MYI is an important source of bromine activation. We find that the modeled atmospheric deposition onto snow-covered sea ice becomes highly enriched in bromide, increasing from enrichment factors of ∼5 in September–February to 10–60 in May, consistent with composition observations of freshly fallen snow. We propose that this progressive enrichment in deposition could enable blowing-snow-induced halogen activation to propagate into May and might explain our late-spring underestimate in VCDtropo. We estimate that the atmospheric deposition of SSA could increase snow salinity by up to 0.04 psu between February and April, which could be an important source of salinity for surface snow on MYI as well as FYI covered by deep snowpack. Inclusion of halogen release from blowing-snow SSA in our simulations decreases monthly mean Arctic surface O3 by 4–8 ppbv (15 %–30 %) in March and 8–14 ppbv (30 %–40 %) in April. We reproduce a transport event of depleted O3 Arctic air down to 40∘ N observed at many sub-Arctic surface sites in early April 2007. While our simulation captures 25 %–40 % of the ODEs observed at coastal Arctic surface sites, it underestimates the magnitude of many of these events and entirely misses 60 %–75 % of ODEs. This difficulty in reproducing observed surface ODEs could be related to the coarse horizontal resolution of the model, the known biases in simulating Arctic boundary layer exchange processes, the lack of detailed chlorine chemistry, and/or the fact that we did not include direct halogen activation by snowpack chemistry.
Highlights
Ozone depletion events (ODEs) are often observed in the springtime Arctic boundary layer (Barrie et al, 1988; Bottenheim et al, 2009, 1986; Bottenheim and Chan, 2006; Halfacre et al, 2014; Koo et al, 2012; Oltmans et al, 2012; Oltmans and Komhyr, 1986)
Four classes of substrates specific to polar regions have been proposed as a source of Br− in Reaction (R1): salty snowpack on sea ice and coastal regions (McConnell et al, 1992; Simpson et al, 2005; Toyota et al, 2011), first-year sea ice (Frieß et al, 2004; Nghiem et al, 2012; Wagner et al, 2007), sea salt aerosol (SSA) produced from frost flowers (Kaleschke et al, 2004; Rankin et al, 2002), and SSA produced from blowing salty snow (Jones et al, 2009; Yang et al, 2008)
Applying this definition to the first-year sea ice (FYI)+multi-year sea ice (MYI) Snow simulation, we find that ODEs occur up to 1 %–5 % of the time in March, increasing to up to 15 %–25 % of the time in April as sunlight extends to higher latitudes (Fig. 9c)
Summary
Ozone depletion events (ODEs) are often observed in the springtime Arctic boundary layer (Barrie et al, 1988; Bottenheim et al, 2009, 1986; Bottenheim and Chan, 2006; Halfacre et al, 2014; Koo et al, 2012; Oltmans et al, 2012; Oltmans and Komhyr, 1986). Four classes of substrates specific to polar regions have been proposed as a source of Br− in Reaction (R1): salty snowpack on sea ice and coastal regions (McConnell et al, 1992; Simpson et al, 2005; Toyota et al, 2011), first-year sea ice (Frieß et al, 2004; Nghiem et al, 2012; Wagner et al, 2007), SSA produced from frost flowers (Kaleschke et al, 2004; Rankin et al, 2002), and SSA produced from blowing salty snow (Jones et al, 2009; Yang et al, 2008) Both laboratory and outdoor chamber experiments have detected Br2 production when acidified surface saline snow was exposed to sunlight (Pratt et al, 2013; Wren et al, 2013).
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