Trends and formation mechanisms of summertime particulatnitrate in the North China Plain

【摘要】：Nitrate composes a significant fraction of fine particles and plays a key role in regional air quality and climate.To obtain the spatial and temporal distribution and the formation mechanisms of fine nitrate over the North China Plain(NCP) – the most industrialized and polluted region in northern China, intensive field observations in urban, rural and mountain areas had been conducted during summertime of 2014–2015.Elevated fine nitrate concentrations were observed at all three areas despite the distinct temporal and spatial variations.The ratios of nitrate/PM_(2.5) and nitrate/sulfate have significantly increased in Ji'nan during 2005–2015 and at Mt.Tai from 2007 to 2014, indicating the worsening situation of regional nitrate pollution.A multi-phase chemical box model(RACM/CAPRAM) was deployed and constrained by in-situ observations to elucidate the nitrate formation mechanisms.The principal formation route was the partitioning of HNO_3 to aerosol phase at daytime, whilst the nocturnal nitrate formation was dominated by the heterogeneous hydrolysis of N_2O_5.The daytime nitrate production was mainly sensitive to the availability of NO_2 and to a lesser extent O_3 and NH_3, and the nighttime formation was limited by both NO_2 and O_3.NH_3 promotes significantly the nitrate formation at daytime but plays a slightly negative role in the nighttime.Overall, the present study provides some useful implications for the future pollution control in the North China Plain.