Emission and distribution characteristics of ammonia from nitrogen application in farmland of Tianjin

Nitrogen is not only a necessary nutrient for crop growth, but also an import source of ammonia volatilization. The volatilization of ammonia occurs in the process of nitrogen fertilizer application. As an important precursor of fine particulates and a main component of atmospheric acid deposition, ammonia threatens environment health due to its volatilization caused by nitrogen fertilization. To analyze ammonia emission and distribution characteristics from nitrogen fertilizer application in 2014 of Tianjin City, we estimated ammonia emission amounts from different nitrogen fertilizer, different crops and in different areas, as well as emission factors by using emission factor method. In the ammonia emission calculation process, nitrogen fertilizer application levels of different nitrogen fertilizers used in different crops came from Statistic Yearbook, while the emission factors were derived from guidelines published by the Ministry of Environmental Protection of China, and the temperature was the annual average temperature of Tianjin City. Results showed that 17 999.91 tons of ammonia was emitted form nitrogen application in Tianjin City with the emission intensity of 3.27 t.km^-2. Among five types of nitrogen fertilizer (urea, ammonium bicarbonate, ammonium nitrate, thiamine, and others), urea was the largest resource of ammonia emission, with 83.13% contribution rate, followed by ammonium bicarbonate with contribution rate of 13.83%, and the other nitrogen fertilizers was the least resource with only 3.04%. For crop types, vegetables was the largest resource of ammonia emission with 38.91% contribution rate, followed by maize with contribution rate of 29.43% and wheat with contribution rate of 19.66%, and the others occupied 12.00%. Because ammonia emissions were positively correlated with temperature, in this study, ammonia emission showed strong time-characteristics with maximum emissions in midday and August and the minimum at night and in January. In all counties/districts, Wuqing District emitted the most ammonia with contribution rate of 27.06%, while Jinnan District emitted the least with contribution rate of 1.14%. And Baodi District and Jixian District had greater contribution rate with 20.71% and 17.86%, respectively. Ammonia emission had strong spatial characteristics, and the large ammonia emission occurred mainly in farmland. To control ammonia emission from farmland in Tianjin City, the scientific application of nitrogenous fertilizer in Wuqing District, Baodi District and Jixian District in the process of vegetable cultivation in June to August should be strengthened. The results could provide a scientific basis for haze treatment in Tianjin City.