Summertime fine particulate nitrate pollution in the North China Plain: increasing trends, formation mechanisms and implications for control policy

Nitrate aerosol makes up a significant fraction of fine particles and plays a key role in regional air quality and climate. The North China Plain (NCP) is one of the most industrialized and polluted regions in China. To obtain a holistic understanding of the nitrate pollution and its formation mechanisms over the NCP region, intensive field observations were conducted at three sites during summertime in 2014-2015. The measurement sites include an urban site in downtown Jinan - the capital city of Shandong Province -, a rural site downwind of Jinan city, and a remote mountain site at Mt. Tai (1534 m a.s.l.). Elevated nitrate concentrations were observed at all three sites despite distinct temporal and spatial variations. Using historical observations, the nitrate / PM_2.5 and nitrate / sulfate ratios have statistically significantly increased in Jinan (2005-2015) and at Mt. Tai (from 2007 to 2014), indicating the worsening situation of regional nitrate pollution. A multiphase chemical box model (RACM-CAPRAM) was deployed and constrained by observations to elucidate the nitrate formation mechanisms. The principal formation route is the partitioning of gaseous HNO₃ to the aerosol phase during the day, whilst the nocturnal nitrate formation is dominated by the heterogeneous hydrolysis of N₂O₅. The daytime nitrate production in the NCP region is mainly limited by the availability of NO₂ and to a lesser extent by O₃ and NH₃. In comparison, the nighttime formation is controlled by both NO₂ and O₃. The presence of NH₃ contributes to the formation of nitrate aerosol during the day, while there is slightly decreasing nitrate formation at night. Our analyses suggest that controlling NO_x and O₃ is an efficient way, at the moment, to mitigate nitrate pollution in the NCP region, where NH₃ is usually in excess in summer. This study provides observational evidence of a rising trend of nitrate aerosol as well as scientific support for formulating effective control strategies for regional haze in China.