Seasonal and annual variations and regional characteristics of wet and dry deposition amounts in East Asian region

Emission of sulfur and nitrogen compounds in Asian region has been remarkably increased with recent rapid economical growth (Ohara et al., 2007). To appropriately assess the influence of air pollutants on the ecosystem, it is important to quantitatively determine the atmospheric deposition of air pollutants. Here, Seasonal and annual variations and regional characteristics of estimated wet and dry deposition amounts at 27 monitoring sites of Acid Deposition Monitoring Network in East Asia (EANET) from 2003 to 2009 are discussed. Wet deposition sample was collected every 24 hours or 1 week by a wet only sampler. Wet deposition amounts were calculated by the product of the volume-weighted concentrations of ionic species (SO₄^2-, NO₃^-, and NH₄⁺) in the precipitation and precipitation amount measured by a standard rain gauge at each site. Dry deposition amount was estimated by the inferential method which was originated the model developed by Wesely and Hicks (1977) and modified by Matsuda (2008). The components examined for dry deposition were sulfur compounds (gaseous SO₂ and particulate SO₄^2-) and nitrogen compounds (gaseous HNO₃ and NH₃, particulate NO₃^- and NH₄⁺). Dry deposition was calculated by the product of the deposition velocity estimated by the inferential method for forest and grass surfaces and the monitored air concentration of each compound. The mean annual dry deposition amounts for sulfur and nitrogen compounds in Japanese sites were in the range of 5-37 and 7-50 mmol m^-2 year^-1, respectively. The regional characteristics of dry deposition amounts in Japan were similar between sulfur and nitrogen compounds, which showed higher deposition in the Sea of Japan side and the western Japan. The mean annual total (wet + dry) deposition amounts for sulfur and nitrogen compounds in Japanese sites were in the range of 28-77 and 22-130 mmol m^-2 year^-1, respectively. The contributions of dry deposition to the total deposition amounts were 10-55% and 13-56% for sulfur and nitrogen compounds, respectively. The regional characteristic of total deposition in Japan was different between sulfur and nitrogen compounds, which showed higher total deposition amounts for sulfur compounds at remote sites caused by long-range transboundary air pollution. When it is focused on the annual trend, the total deposition amounts of sulfur and nitrogen compounds increased remarkably at some sites in Japan, especially in the coast of Sea of Japan. Average dry and wet deposition amounts of sulfur or nitrogen compounds among Japanese EANET sites, 78 CASTNET sites in USA and 2447 EMEP domain grids in Europe were compared. Averages of total S (54 mmol m^-2 year^-1) and N (77 mmol m^-2 year^-1) deposition amounts in Japanese sites were larger than those in CASTNET and EMEP because of remarkably high wet deposition amounts. Especially, the higher deposition amounts of sulfur compounds in Japan were possibly caused by the long-range transboundary air pollution and natural emission sources, such as volcanic eruptions in the Japanese Archipelago. The recently increasing SO₂ and NOx emissions in East Asian region would cause higher atmospheric depositions in Japan than those in other networks.