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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
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Research article 02 Jul 2018

Research article | 02 Jul 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Two years online measurement of fine particulate nitrate in western Yangtze River Delta: Influences of thermodynamics and N2O5 hydrolysis

Peng Sun1, Wei Nie1,2, Xuguang Chi1,2, Yuning Xie1, Xin Huang1,2, Zheng Xu1,2, Ximeng Qi1,2, Zhengning Xu1, Lei Wang1, Tianyi Wang1, Qi Zhang3, and Aijun Ding1,2 Peng Sun et al.
  • 1Joint International Research Laboratory of Atmospheric and Earth System Sciences, and School of Atmospheric Sciences, Nanjing University, Nanjing, 210023, China
  • 2Jiangsu Provincial Collaborative Innovation Center of Climate Change, Nanjing, 210023, China
  • 3Department of Environmental Toxicology, University of California, Davis, CA 95616, USA

Abstract. Particulate nitrate contributes a large fraction of secondary aerosols. Despite understanding of its important role in regional air quality and global climate, long-term continuous measurements are rather limited in China. In this study, we conducted online measurement of PM2.5 nitrate for two years from March 2014 to February 2016 using the Monitor for Aerosols and Gases in ambient Air (MARGA) in the western Yangtze River Delta (YRD), eastern China, and investigate the main factors that influenced its temporal variations and formation pathways. Compared to other sites in China, an overall high concentration of particulate nitrate was observed with a mean value of 15.8µgm−3 (0.5 to 92.6µgm−3). Nitrate on average accounted for 32% of the total mass of water-soluble ions and the proportion increased with PM loading, indicating that nitrate is a major driver of haze pollution episodes in this region. Sufficient ammonia drove most nitrate into the particle phase in the form of ammonium nitrate. A typical seasonal cycle of nitrate was observed with the concentrations in winter on average two times higher than those in summer mainly due to different meteorological conditions. In summer, the diurnal variation of particulate nitrate was determined by the thermodynamic equilibrium, resulting in a much lower concentration during daytime despite of a considerable photochemical production. Air masses from polluted YRD and biomass burning region contributed to the high nitrate concentration during summer. In winter, particulate nitrate didn’t reveal an evident diurnal variation. Regional transport from northern China played an important role in enhancing nitrate concentration. Eighteen nitrate episodes were selected to understand the processes that drive the formation of high concentration of nitrate. Rapid nitrate formation was observed during the pre-episode (the day before nitrate episode day) nights, and dominated the increase of total water-soluble ions. Calculated nitrate from N2O5 hydrolysis was highly correlated to and accounted for 80 percent of the observed nitrate, suggesting that N2O5 hydrolysis was a major contributor to the nitrate episodes. Our results suggested that rapid formation of nitrate could be a main cause for extreme aerosol pollution events in YRD during winter, and illustrated the urgent needs to control the NOx emission.

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Short summary
Two-year online measurement of particulate nitrate was conducted at the SORPES station in western Yangtze River delta, east China. Nitrate was found to be the major driver of haze pollution, and behaved differently in different season. In summer, thermodynamic equilibrium and photochemical processes controlled the nitrate formation. In winter, N2O5 hydrolysis was demonstrated to be a major contributor to the nitrate episodes.
Two-year online measurement of particulate nitrate was conducted at the SORPES station in...