Pollution transport efficiency toward the Arctic: Sensitivity to aerosol scavenging and source regions

The processes driving current changes in Arctic atmospheric composition and climate are still uncertain. In particular the relative contributions of major source regions from the midlatitudes remain a matter of debate in the literature. The objectives of this study are to better quantify the relative contributions of different processes governing the transport of pollution from the midlatitudes to the Arctic and the relative contributions of different geopolitical source regions. We use a suite of observational data sets (including the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaigns and the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite instrument) to constrain a global aerosol simulation from the ECHAM5-HAMMOZ model. Preliminary comparison of model results with vertical profiles of sulfate and black carbon (BC) collected during the ARCTAS campaigns and with aerosol extinction time series retrieved from CALIOP indicates that the model underestimates export of aerosols from the planetary boundary layer to the free troposphere in the midlatitudes and long-range transport of aerosols from the midlatitudes toward the Arctic. In contrast, observed CO profiles are relatively well simulated, which points to a possible problem with wet scavenging. Decreasing the prescribed aerosol scavenging coefficients within the range of experimental data available in the literature significantly improves the agreement with observations. Sulfate and BC burdens in the Arctic increase by a factor 5-6. Annual global lifetimes of sulfate and BC increase from 3.1 to 4.6 days and from 4.4 to 5.9 days, respectively. Using the improved simulation, we find that 59% of sulfate in the Arctic troposphere comes from the oxidation of SO2 emitted in Siberia (19%), Europe (18%), Asia (13%), and North America (9%). Anthropogenic and biomass burning BC emitted in Siberia, Asia, Europe, and North America contributes 29, 27, 25, and 17%, respectively, to the Arctic BC burden. Emissions of aerosols (or precursors) from Siberia and Europe are more susceptible to be transported and subsequently deposited on the Arctic (about 7-12%) compared to those from North America and Asia (1-4%). Overall, the study shows that aerosol scavenging removal processes play a crucial role on the aerosol transport from the midlatitudes toward the Arctic and should be carefully characterized in aerosol models.