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

Continuous measurements of dinitrogen pentoxide (N 2 O 5 ) and nitrate radical (NO 3 ) were conducted at an urban site in Seoul during the ASIA-AQ campaign (February–March 2024). Observed concentrations of N 2 O 5 and NO 3 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 N 2 O 5 uptake coefficient (γN 2 O 5 ) 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 N 2 O 5 and NO 3 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 NO 3 and N 2 O 5 in the upper residual layer, with heterogeneous N 2 O 5 hydrolysis contributing 10.2-19.5% of the total column nitric acid (HNO 3 ) production but only 2.5-5.5% near the surface. Although surface contributions were limited, vertical profiles indicated that this pathway becomes important when NO 3 and N 2 O 5 in the residual layer are transported downward. Such downward transport, driven by mountain breezes, enhanced early morning NO 3 and N 2 O 5 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. • N 2 O 5 (58.7 pptv) and NO 3 (0.4 pptv) observed near the surface. • Aerosol uptake coefficient of N 2 O 5 estimated at 0.021. • Heterogeneous hydrolysis contributed 2.5-5.5% to HNO 3 at 28m. • Pre-dawn NO 3 and N 2 O 5 enhanced by a factor of 2-3. • Nocturnal vertical transport influenced the nitrate radical budgets in Seoul.