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Source contributions of sulfur and nitrogen deposition 1 – an HTAP II multi2 model study on hemispheric transport

With rising emissions by human activities, enhanced concentrations of air pollutants have been detected in hemispheric air flows in recent years, aggravating the regional air pollution and deposition burden. However, contributions of hemispheric air pollution to deposition at global scale have been given little attention in the literature. In this light, we assess the impact of hemispheric transport on sulfur (S) and nitrogen (N) deposition for 6 world regions: North America, Europe, South Asia, East Asia, Middle East and Russia in 2010, by using the multi-model ensemble results from the 2nd phase of Task Force Hemispheric Transport of Air Pollution (HTAP II) with and without 20% emission perturbation experiments. About 27- 58%, 26-46% and 12-23% of local S, NOx and NH3 emissions are transported and removed by deposition outside of the source regions annually, with 5% higher fraction of export in winter and 5% lower in summer. For receptor regions, 20% emission reduction in source regions affects the deposition in receptor regions by 1-10% for continental non-coastal regions and 1-15% for coastal regions and open oceans. Significant influences are found from North America to the North Atlantic Ocean (5-15%), from South Asia to western East Asia (2-10%) and from East Asia to the North Pacific Ocean (5-10%) and western North America (5-8%). The impact on deposition caused by transport between neighbouring regions (i.e. Europe and Russia) occurs throughout the whole year (slightly stronger in winter), while that by transport over long distances (i.e. from East Asia to North America) mainly takes place in spring and fall, which is consistent with the seasonality found for hemispheric transport of air pollutants. Deposition in emission intense regions such as East Asia is dominated (~80%) by own region emission, while deposition in low emission regions such as Russia is almost equally affected by own region emission (~40%) and foreign impact (~23-45%). We also find that deposition on the coastal regions or near coastal open ocean is twice more sensitive to hemispheric transport than non- coastal continental regions, especially for regions (i.e. west coast of North America) in the downwind location of major emission source regions. This study highlights the significant impact of hemispheric transport on deposition in coastal regions, open ocean and low emission regions. Further research is proposed for improving ecosystem and human health in these regions, with regards to the enhanced hemispheric transport.
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