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  1. Abstract. We use the GEOS-Chem chemical transport model to examine theinfluence of bromine release from blowing-snow sea salt aerosol (SSA) onspringtime bromine activation and O3 depletion events (ODEs) in theArctic lower troposphere. We evaluate our simulation against observations oftropospheric 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 asagainst surface observations of O3. We conduct a simulation withblowing-snow SSA emissions from first-year sea ice (FYI; with a surface snowsalinity of 0.1 psu) and multi-year sea ice (MYI; with a surface snowsalinity of 0.05 psu), assuming a factor of 5 bromide enrichment of surfacesnow relative to seawater. This simulation captures the magnitude ofobserved March–April GOME-2 and OMI VCDtropo to within 17 %, as wellas their spatiotemporal variability (r=0.76–0.85). Many of the large-scalebromine explosions are successfully reproduced, with the exception of eventsin May, which are absent or systematically underpredicted in the model. Ifwe assume a lower salinity on MYI (0.01 psu), some of the bromine explosionsevents observed over MYI are not captured, suggesting that blowing snow overMYI is an important source of bromine activation. We find that the modeledatmospheric deposition onto snow-covered sea ice becomes highly enriched inbromide, increasing from enrichment factors of ∼5 inSeptember–February to 10–60 in May, consistent with composition observations of freshly fallen snow. We propose that this progressive enrichment indeposition could enable blowing-snow-induced halogen activation to propagateinto May and might explain our late-spring underestimate in VCDtropo.We estimate that the atmospheric deposition of SSA could increase snow salinityby up to 0.04 psu between February and April, which could be an importantsource of salinity for surface snow on MYI as well as FYI covered by deepsnowpack. Inclusion of halogen release from blowing-snow SSA in oursimulations decreases monthly mean Arctic surface O3 by 4–8 ppbv(15 %–30 %) in March and 8–14 ppbv (30 %–40 %) in April. We reproduce atransport event of depleted O3 Arctic air down to 40∘ Nobserved at many sub-Arctic surface sites in early April 2007. While oursimulation captures 25 %–40 % of the ODEs observed at coastal Arctic surfacesites, it underestimates the magnitude of many of these events and entirelymisses 60 %–75 % of ODEs. This difficulty in reproducing observed surfaceODEs could be related to the coarse horizontal resolution of the model, theknown biases in simulating Arctic boundary layer exchange processes, thelack of detailed chlorine chemistry, and/or the fact that we did not includedirect halogen activation by snowpack chemistry. 
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