Seasonal modeling analysis of nitrate formation pathways in Yangtze River Delta region, China

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Nitrate (NO3-) has been the dominant and the least reduced chemical component of fine particulate matter (PM2.5) since the stringent emission control implemented in China in 2013. The formation pathways of NO3- vary seasonally and differ substantially in daytime vs. nighttime. They are affected by precursor emissions, atmospheric oxidation capacity, and meteorological conditions. Understanding NO3- formation pathways provides insights for the design of effective emission control strategies to mitigate NO3- pollution. In this study, the Community Multiscale Air Quality (CMAQ) model was applied to investigate the impact of regional transport, predominant physical processes, and different formation pathways to NO3- and total nitrate (TNO3, i.e., HNO3+NO3-) production in the Yangtze River Delta (YRD) region during the four seasons of 2017. NO3-/PM2.5 and NO3-/TNO3 are the highest in the winter, reaching 21 % and 94 %, respectively. Adjusted gas ratio (adjGR = ([NH3] + [NO3-])/([HNO3] + [NO3-])) in YRD is generally greater than two in different seasons across most areas in YRD, indicating that YRD is mostly in the NH3-rich regime and NO3- is limited by HNO3 formation. Local emissions and regional transportation contribute to YRD NO3- concentrations by 50–62 % and 38–50 %, respectively. Majority of the regional transport of NO3- concentrations is contributed by indirect transport (i.e., NO3- formed by transported precursors reacting with local precursors). Aerosol (AERO, including condensation, coagulation, new particle formation and aerosol growth) processes are the dominant source of NO3- formation. In summer, NO3- formation is dominated by AERO and total transport (TRAN, sum of horizontal and vertical transport) processes. The OH+NO2 pathway contributes to 60–83 % of the TNO3 production, and the N2O5 heterogeneous (HET N2O5) pathway contributes to 10–36 % in YRD. HET N2O5 contribution becomes more important in cold seasons than warm seasons. Within the planetary boundary layer in Shanghai, the TNO3 production is dominated by the OH+NO2 pathway during the day (98 %) in the summer and spring, and by the HET N2O5 pathway during the night (61 %) in the winter. Local contribution dominates the OH+NO2 pathway for TNO3 production during the day, while indirect transport dominates the HET N2O5 pathway at night.