Ammonia Deposition Around Paddy Fields In Subtropical Hilly Area

Ammonia（NH₃）is an important alkaline gas in the atmosphere and an important precursor of secondary aerosols.Ammonia（NH₃）also has a large dry deposition rate and is an important component of atmospheric nitrogen deposition.According to research,there is a large range of subsidence around the ammonia emission source,and studying its near-source subsidence characteristics is of great significance for clarifying its whereabouts after emission.In this study,a typical double-season rice area with an area of about 0.6 ha in the subtropical hilly region（Jinjing Town,Changsha City,Hunan Province）of China was used to monitor the downwind area of its ammonia emission source in different periods.The rice area was developed in the early and late rice seasons.The study of NH₃ concentration and its deposition amount within 100 m of downwind direction is to quantify the ammonia deposition amount around typical agricultural emission sources in the subtropical zone,and to clarify the main influencing factors of near-source ammonia deposition to help clarify the characteristics of ammonia deposition and analyze its Spatiotemporal variation and impact mechanisms.The main contents and results of this study are as follows:（1）In the subtropical hilly region of China,the monitoring results of ammonia gas deposition in the range of 50 m upwind and 100m downwind around a rice field（about0.6 ha）in the late rice season show that the monthly average of ammonia in the range of50 m upwind The concentration was 3.8-12.1μg N m^-3,and the average ammonia concentration within the range of 100 m in the downwind direction was 9.1-21.9μg N m^-3,and all of them were the highest in July when the paddy fertilizer was applied.During the rice season,the north,south,east,west,north upwind,and south upwind direction settlements were 5.7,3.5,4.9,0.4,and 1.1 kg,respectively,and the area averages were 2.5,1.5,2.5,0.6,and 1.7 kg N ha^-1,Which indicates that there is a high ammonia gas deposition around the rice area.The ambient ammonia deposition intensity is mainly affected by the ammonia concentration at the emission source,and the ammonia concentration at the emission source is positively related to the wind speed.Studies have shown that near-source deposition of ammonia is an important trend for ammonia emissions from rice fields.（2）The ammonia concentration in the downwind direction of the rice season in the morning and evening in this study,the average ammonia concentration during its growth period has a certain temporal and spatial dynamic change.Taking the range of100m downwind in the early and late rice seasons as an example,the average ammonia concentration showed 1 to 2 peaks,and the ammonia concentration in other non-fertilization periods was stable and low.Among them,the peak ammonia concentration in the early rice season appeared in late April after basal fertilizer application,and the ammonia concentration was relatively low after topdressing and other periods.The peak ammonia concentration in the late rice season appeared in late July and topdressing fertilizer application.In early August,other periods were lower.After the application of the base fertilizer,the ammonia concentration in the rice field in the morning and evening rice seasons increased significantly.Within one week after the base fertilizer application,the ammonia concentration in the downwind direction remained at a high level,and the ammonia concentration at different distances first increased.And then began to decline.From the second week after the base fertilizer application to the 10 d before the top dressing,the ammonia concentration at different distances in the downwind direction began to decrease,basically below 20μg N m^-3,and the changes at 0 m and 20 m were the most obvious.Within one week after the topdressing,the ammonia concentration at different distances increased to varying degrees,and then fell to a stable level.And the ammonia concentration from 0m to 20 m,50 m,and 100 m decreased gradually as the distance from the rice field emission source increased,and the ammonia concentration at upwind 50 m remained at a low level for a long time.（3）In the subtropical hilly region of China,the monitoring results of NH₃deposition in a range of rice fields（approximately 0.6 ha）around the morning and evening rice seasons within a range of 50 m upwind and 100 m downwind indicate that the downwind direction of the paddy field is within 15 d of applying nitrogen fertilizer.Produces higher atmospheric NH₃ concentrations.The concentrations at 0 m,20 m,50m,and 100 m can reach 5.0-27 times and 1.3-9.7 times the background point after applying base fertilizer and topdressing,respectively,and increase with distance from the paddy field.The concentration index decreased.Excluding the background NH₃sedimentation,the total amount of ammonia sedimentation in the range of 100m downwind from the early and late rice seasons was 2.7 kg and 3.5 kg,and the near-source sedimentation accounted for 79%and 81%of the ammonia volatilization.The results show that the near-source deposition of NH₃ from paddy fields in the subtropical hilly region is an important way to remove NH₃.