A case study of heavy PM_2.5 secondary formation by N₂O₅ nocturnal chemistry in Seoul, Korea in January 2018: Model performance and error analysis

Heterogeneous hydrolysis of dinitrogen pentoxide (N₂O₅) plays an important role in nighttime nitrate (NO₃⁻) formation in urban areas, and sometimes influences the occurrence of heavy PM_2.5 pollution the next day in the Seoul Metropolitan Area (SMA), Korea. Here, we discuss the heavy PM_2.5 wintertime episode of January 13–15, 2018, which was mainly induced by nighttime N₂O₅ heterogeneous reaction in the SMA. In our case, we confirmed that nighttime N₂O₅ hydrolysis is the most critical factor in the rapid formation of aerosol nitrate at high levels during the night, which prevailed in the morning of the next day. Our Integrated Process Rate (IPR) analysis showed that nighttime nitrate production in the episode was almost solely attributable to N₂O₅ chemistry, with hourly mean production rates of 0.8 ± 0.4 μg/m³ per hour in SMA, which is comparable to the daytime nitrate photochemical production rates of 0.9 ± 0.5 μg/m³ per hour. We also carried out a series of assessment of N₂O₅–driven nitrate formation sensitivity, and relevant errors were quantified by applying different N₂O₅ uptake coefficients in the WRF-CMAQ model. The potential errors of nighttime-average nitrate concentrations induced by N₂O₅ uptake process were assessed from a linear perspective for the planetary boundary layer (PBL) variances caused by four different PBL parameterization schemes: YSU, ACM2, MYJ, and QNSE. The potential error ranges by N₂O₅ uptake process were analyzed to be 2.3 to 3.5 μg/m³ (~10% relative to the nighttime-average), while biases of PBL simulations from 4 parameterization schemes were 2.3 ± 1.0 μg/m³, showing similar ranges in our episode. Although N₂O₅–driven heavy PM_2.5 episodes do not occur often in SMA, our findings suggest the importance of N₂O₅ chemistry in vigorous wintertime nitrate formation and operational prediction errors of such PM_2.5 episodes, under the premise of enhanced PBL simulation capabilities.