Nocturnal N2O5 and NO3 chemistry and its impact on wintertime HNO3 in Seoul during ASIA-AQ campaign

초록

Continuous measurements of dinitrogen pentoxide (N2O5) and nitrate radical (NO3) were conducted at an urban site in Seoul during the ASIA-AQ campaign (February-March 2024). Observed concentrations of N2O5 and NO3 ranged from the detection limit to 1047 and 9.7 pptv, with mean values of 58.7 and 0.4 pptv, respectively. To investigate nocturnal nitrate chemistry, a one-dimensional (1-D) model was applied using in-situ constraints. The model yielded a representative N2O5 uptake coefficient (gamma N2O5) of 0.021 (95 % confidence interval: 0.015-0.029), indicating significant heterogeneous loss. While the model successfully reproduced the general diurnal variations of key species, it tended to underestimate concentrations of N2O5 and NO3 during pre-dawn hours (01:00 to 04:00 Local Time) under the influence of mountain breezes. Simulations revealed the accumulation of high concentrations of NO3 and N2O5 in the upper residual layer, with heterogeneous N2O5 hydrolysis contributing 10.2-19.5 % of the total column nitric acid (HNO3) production but only 2.5-5.5 % near the surface. Although surface contributions were limited, vertical profiles indicated that this pathway becomes important when NO3 and N2O5 in the residual layer are transported downward. Such downward transport, driven by mountain breezes, enhanced early morning NO3 and N2O5 concentrations by a factor of two to three. These findings highlight the important role of nocturnal vertical transport in nitrate radicals budgets in complex urban areas, including Seoul and other megacities.

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