Heterogeneous Nitrate Production Mechanisms in Intense Haze Events in the North China Plain

Yuk-Chun Chan, Mathew J. Evans, Pengzhen He, Christopher D. Holmes, Lyatt Jaeglé, Prasad Kasibhatla, Xue-Yan Liu, Tomás Sherwen, Joel A. Thornton, Xuan Wang, Zhouqing Xie, Shuting Zhai, Becky Alexander

Research output: Contribution to journalArticlepeer-review

Abstract

Abstract Studies of wintertime air quality in the North China Plain (NCP) show that particulate-nitrate pollution persists despite rapid reduction in NOx emissions. This intriguing NOx-nitrate relationship may originate from non-linear nitrate-formation chemistry, but it is unclear which feedback mechanisms dominate in NCP. In this study, we re-interpret the wintertime observations of 17O excess of nitrate (∆17O(NO3−)) in Beijing using the GEOS-Chem (GC) chemical transport model to estimate the importance of various nitrate-production pathways and how their contributions change with the intensity of haze events. We also analyze the relationships between other metrics of NOy chemistry and [PM2.5] in observations and model simulations. We find that the model on average has a negative bias of −0.9‰ and −3617O(NO3−) and [Ox,major] (≡ [O3] + [NO2] + [p-NO3−]), respectively, while overestimating the nitrogen oxidation ratio ([NO3−]/([NO3−] + [NO2])) by +0.12 in intense haze. The discrepancies become larger in more intense haze. We attribute the model biases to an overestimate of NO2-uptake on aerosols and an underestimate in wintertime O3 concentrations. Our findings highlight a need to address uncertainties related to heterogeneous chemistry of NO2 in air-quality models. The combined assessment of observations and model results suggest that N2O5 uptake in aerosols and clouds is the dominant nitrate-production pathway in wintertime Beijing, but its rate is limited by ozone under high-NOx-high-PM2.5 conditions. Nitrate production rates may continue to increase as long as [O3] increases despite reduction in [NOx], creating a negative feedback that reduces the effectiveness of air pollution mitigation.
Original languageEnglish
Article numbere2021JD034688
JournalJournal of Geophysical Research: Atmospheres
Volume126
Issue number9
DOIs
Publication statusPublished - 5 May 2021

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Keywords

  • Δ17O(NO3−)
  • GEOS-Chem
  • heterogeneous chemistry
  • nitrate
  • NO2 uptake
  • winter air quality

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