Source contributions to sulfur and nitrogen deposition – an HTAP II multi-model study on hemispheric transport

With the rising anthropogenic emissions from human activities, elevated concentrations of air pollutants have been detected in the hemispheric air flows in recent years, aggravating the regional air pollution and deposition issues. However, the regional contributions of hemispheric air flows to deposition 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 six world regions: North America (NA), Europe (EU), South Asia (SA), East Asia (EA), Middle East (ME) and Russia (RU) in 2010, by using the multi-model ensemble results from the 2nd phase of the Task Force Hemispheric Transport of Air Pollution (HTAP II) with 20% emission perturbation experiments. About 27%–58%, 26%–46% and 12%–23% of local S, NOx and NH3 emissions and oxidation products are transported and removed by deposition outside of the source regions annually, with seasonal variation of 5% more in winter and 5% less in summer. The 20% emission reduction in the source regions could affect 1%–10% of deposition in foreign continental regions and 1%–14% in foreign coastal regions and the open ocean. Significant influences are found from NA to the North Atlantic Ocean (2%–14%), and from EA to the North Pacific Ocean (4%–10%) and to western NA (4%–6%) (20% emission reduction). The impact on deposition caused by short-distance transport between neighboring regions (i.e., from EU to RU) occurs throughout the whole year (slightly stronger in winter), while the long-range transport (i.e., from EA to NA) mainly takes place in spring and fall, which is consistent with the seasonality found for hemispheric transport of air pollutants. Deposition in the emission-intensive regions such as US, SA and EA is dominated ( ∼ 80%) by own-region emissions, while deposition in the low-emission-intensity regions such as RU is almost equally affected by foreign exported emissions (40%–60%) and own-region emissions. We also find that deposition of the coastal regions or the near-coastal open ocean is twice more sensitive to hemispheric transport than the non-coastal continental regions, especially for regions in the downwind direction of emission sources (i.e., west coast of NA). This study highlights the significant impacts of hemispheric transport of air pollution on the deposition in coastal regions, the open ocean and low-emission-intensity regions. Further research is proposed to improve the ecosystem and human health, with regards to the enhanced hemispheric air flows.