Multi-model study of HTAP II on sulfur and nitrogen deposition

This study uses multi-model ensemble results of 11 models from the second phase of Task Force Hemispheric Transport of Air Pollution (HTAP II) to calculate the global sulfur (S) and nitrogen (N) deposition in 2010. Modeled wet deposition is evaluated with observation networks in North America, Europe and East Asia. The modeled results agree well with observations, with 76-83% of stations being predicted within ±50% of observations. The models underestimate SO42-, NO₃ ^- and NH₄ ⁺ wet depositions in some European and East Asian stations but overestimate NO₃ ^- wet deposition in the eastern United States. Intercomparison with previous projects (PhotoComp, ACCMIP and HTAP I) shows that HTPA II has considerably improved the estimation of deposition at European and East Asian stations. Modeled dry deposition is generally higher than the q inferential/q data calculated by observed concentration and modeled velocity in North America, but the inferential data have high uncertainty, too. The global S deposition is 84Tg(S) in 2010, with 49% in continental regions and 51% in the ocean (19% of which coastal). The global N deposition consists of 59Tg(N) oxidized nitrogen (NO_y) deposition and 64Tg(N) reduced nitrogen (NH_x) deposition in 2010. About 65% of N is deposited in continental regions, and 35% in the ocean (15% of which coastal). The estimated outflow of pollution from land to ocean is about 4Tg(S) for S deposition and 18Tg(N) for N deposition. Comparing our results to the results in 2001 from HTAP I, we find that the global distributions of S and N deposition have changed considerably during the last 10 years. The global S deposition decreases 2Tg(S) (3%) from 2001 to 2010, with significant decreases in Europe (5Tg(S) and 55%), North America (3Tg(S) and 29%) and Russia (2Tg(S) and 26%), and increases in South Asia (2Tg(S) and 42%) and the Middle East (1Tg(S) and 44%). The global N deposition increases by 7Tg(N) (6%), mainly contributed by South Asia (5Tg(N) and 39%), East Asia (4Tg(N) and 21%) and Southeast Asia (2Tg(N) and 21%). The NH_x deposition increases with no control policy on NH₃ emission in North America. On the other hand, NO_y deposition has started to dominate in East Asia (especially China) due to boosted NO_x emission.