Heterogeneous formation of particulate nitrate under ammonium-rich regimes during the high-PM2.5 events in Nanjing, China

Abstract

Particulate nitrate ( NO 3 - ) not only influences regional climates but also contributes to the acidification of terrestrial and aquatic ecosystems. In 2016 and 2017, four intensive online measurements of water-soluble ions in PM 2.5 were conducted in Nanjing City in order to investigate the potential formation mechanisms of particulate nitrate. During the sampling periods, NO 3 - was the predominant species, accounting approximately for 35 % of the total water-soluble inorganic ions, followed by SO 4 2 - (33 %) and NH 4 + (24 %). Significant enhancements of nitrate aerosols in terms of both absolute concentrations and relative abundances suggested that NO 3 - was a major contributing species to high-PM 2.5 events (hourly PM 2.5≥150 μ g m -3). High NO 3 - concentrations mainly occurred under NH 4 + -rich conditions, implying that the formation of nitrate aerosols in Nanjing involved NH3. During the high-PM 2.5 events, the nitrogen conversion ratios ( F n) were positively correlated with the aerosol liquid water content (ALWC; R 0.72 and p <0.05). Meanwhile, increasing NO 3 - concentrations regularly coincided with increasing ALWC and decreasing O x ( O x Combining double low line O 3 + NO 2 ). These results suggested that the heterogeneous reaction was probably a major mechanism of nitrate formation during the high-PM 2.5 events. Moreover, the average production rate of NO 3 - by heterogeneous processes was estimated to be 12.6 % h -1 (4.1 μ g m -3 h -1), which was much higher than that (2.5 % h -1; 0.8 μ g m -3 h -1) of gas-phase reactions. This can also explain the abrupt increases in nitrate concentrations during the high-PM 2.5 events. Utilizing the ISORROPIA II model, we found that nitrate aerosol formation in Nanjing during the high-PM 2.5 events was HNO3 limited. This indicated that control of NO x emissions will be able to efficiently reduce airborne particulate nitrate concentrations and improve the air quality in this industrial city..