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. Particulate nitrate contributes a large fraction of secondary aerosols.Despite understanding of its important role in regional air quality andglobal climate, long-term continuous measurements are rather limited inChina. In this study, we conducted online measurement of PM 2.5 (particulate matter with diameters less than 2.5  µm ) nitratefor 2 years from March 2014 to February 2016 using the Monitor for AeRosolsand Gases in ambient Air (MARGA) in the western Yangtze River Delta (YRD),eastern China, and investigate the main factors that influenced its temporalvariations and formation pathways. Compared to other sites in China, anoverall high concentration of particulate nitrate was observed, with a meanvalue of 15.8  µg m−3 (0.5 to 92.6  µg m−3 ). Nitrateon average accounted for 32 % of the total mass of water-soluble ions andthe proportion increased with PM loading, indicating that nitrate is a majordriver of haze pollution episodes in this region. Sufficient ammonia drovemost nitrate into the particle phase in the form of ammonium nitrate. Atypical seasonal cycle of nitrate was observed, with the concentrations inwinter on average 2 times higher than those in summer mainly due todifferent meteorological conditions. In summer, the diurnal variation ofparticulate nitrate was determined by thermodynamic equilibrium,resulting in a much lower concentration during daytime despite aconsiderable photochemical production. Air masses from the polluted YRD andbiomass burning region contributed to the high nitrate concentration duringsummer. In winter, particulate nitrate did not reveal an evident diurnalvariation. Regional transport from northern China played an important role inenhancing nitrate concentration. A total of 18 nitrate episodes were selected tounderstand the processes that drive the formation of high concentration ofnitrate. Rapid nitrate formation was observed during the pre-episode (the daybefore nitrate episode day) nights, and dominated the increase of totalwater-soluble ions. Calculated nitrate from N2O5 hydrolysis washighly correlated to and accounted for 80 % of the observed nitrate,suggesting that N2O5 hydrolysis was a major contributor to thenitrate episodes. Our results suggested that rapid formation of nitrate couldbe a main cause for extreme aerosol pollution events in the YRD during winter,and illustrated the urgent need to control NOx emission.

atmospheric chemistry and physics

Two years of online measurement of fine particulate nitrate in the western Yangtze River Delta: influences of thermodynamics and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;5&amp;lt;/sub&amp;gt; hydrolysis