Influence of the nocturnal residual layer on nitrate formation in the Seoul Metropolitan Area using the Korea Air Quality Observation-Based Box Model (KAB)

Fine particulate matter has declined across South Korea. Yet haze episodes remain frequent in the Seoul Metropolitan Area. Understanding these events requires a mechanistic analysis of nitrate formation, a major component of fine particulate matter.

Previous box model studies have not fully represented boundary layer evolution. They also have not captured the nighttime influence of residual layer entrainment and dilution. Even multi-layer box model frameworks often assume a fixed boundary layer height. Most related studies have focused on China. As a result, quantitative evidence for the role of boundary layer mixing in the Seoul Metropolitan Area is still limited.

Here we quantified how the residual layer affects nitrate production and loss over the Seoul Metropolitan Area using KAB (Korea Air Quality Observation-Based Box Model). KAB is an emissions- and observation-constrained box model derived from the 3D chemical transport model CMAQ. We extended the conventional single-layer configuration to a two-layer structure. We also diagnosed boundary layer and residual layer heights from ERA5 reanalysis to capture day–night differences. During daytime, we assumed a well-mixed layer. During nighttime, we prescribed distinct concentrations in the two layers to represent vertical gradients and multi-layer effects.

Our results show that residual layer development and boundary layer mixing exert substantial control on nitrate variability in the Seoul Metropolitan Area. Residual layer entrainment increases surface nitrate by transporting aerosol aloft down to the ground. It also enhances aerosol formation. This occurs when undiluted precursors are converted to particulate nitrate during subsequent mixing. These findings indicate that residual layer mixing is a key driver of high haze events in this region.

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