Joint Effects of N2O5 Heterogeneous Chemistry and Chloride Sources on the Nocturnal Nitrogen Cycle

The nocturnal nitrogen cycle strongly influences wintertime air quality by governing the formation of particulate nitrate and nitryl chloride (ClNO₂), but it remains difficult to simulate accurately in chemical transport models because of uncertainties in heterogeneous N₂O₅ chemistry and chloride availability. This study investigates how these two factors jointly affect model representation of nighttime nitrogen chemistry and whether their combined revision can improve simulations of particulate nitrate and ClNO₂. Using field observations and CMAQ simulations, we revised two key components of the nocturnal nitrogen cycle. First, observation-constrained parameterizations were applied to update the N₂O₅ uptake coefficient and ClNO₂ yield. Second, chloride source representation was improved by incorporating additional anthropogenic and natural chlorine emissions. The revised framework was then used to examine the individual and combined effects of chemical and emission updates on the nighttime conversion of NO_x to particulate nitrate and ClNO₂. The results show that revising either heterogeneous chemistry or chloride sources alone provided only partial improvement, whereas the most substantial improvement was obtained when both were updated together. In particular, correcting the N₂O₅ uptake coefficient and ClNO₂ yield reduced biases in nocturnal heterogeneous processing, while improved chloride emissions provided a more realistic reservoir for ClNO₂ formation. Together, these changes led to a more consistent representation of the nocturnal nitrogen cycle and improved simulations of both particulate nitrate and ClNO₂. These findings highlight that accurate prediction of wintertime particulate nitrate requires the coupled treatment of heterogeneous N₂O₅ chemistry and chloride source availability.

 

Acknowledgment: This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MSIT) (No. RS-2025-16070879).