Spring and summertime aerosol optical depth variability over Arctic cryosphere from space-borne observations and model simulation
Abstract. The Arctic is a unique part of the Earth system that is currently undergoing a warming phase called the Arctic Amplification (AA). Changes in aerosol abundance and composition are under the influence of the AA and may be, in turn, important drivers to the AA. However, their ground and space observations are particularly difficult and spatio-temporally sparse in this region, limiting the knowledge and ability to model their variability. In this study, we have used the total aerosol optical depth (AOD) determined by the AEROSNOW algorithm using data from the AATSR satellite instrument over snow- and ice-covered regions of the Arctic. This data is then used to evaluate the global GEOS-Chem 3D chemical transport model for the period 2003–2011. Thus, the main drivers of monthly and seasonal variations in spaceborne AOD were determined by using the GEOS-Chem model-simulated aerosol components. By comparing these two AOD datasets, we examined the spring and summer AOD over Arctic snow and ice for the period of space-borne observations. The space-borne and modelled AOD show consistent spatio-temporal distributions in both seasons, with a pronounced chemical speciation in GEOS-Chem. This behaviour is attributed to the different seasonal sources of AOD. In spring, Arctic aerosols originate from long-range pollution transport from low and mid-latitudes as well as from local sources, whereas in summer natural local sources within the Arctic Circle (here defined as > 60° N) dominate. Arctic AOD is generally highest in spring and lowest in summer due to wet scavenging. In addition, carbonaceous aerosols (black carbon, BC, and organic carbon, OC) are an increasingly important contributor to total AOD over Arctic sea ice in summer due to the expected increase in boreal forest fires. The relative contribution of sulfate to total AOD over Arctic sea ice decreases while that of carbonaceous aerosols increases during the spring-summer transition. This suggests that boreal wildfires are penetrating more deeply into Arctic sea ice at higher latitudes during this study period. GEOS-Chem showed a systematically smaller AOD value compared to AEROSNOW over the Arctic sea ice region in summer. The promising results of AEROSNOW could also serve as the baseline for the evaluation and improvement of aerosol forecasts for various chemical transport models, especially over Arctic sea ice.
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