When and why PM_2.5 is high in Seoul, South Korea: Interpreting long-term (2015-2021) ground observations using...

Seoul has high PM_2.5 concentrations and has not attained the national annual average standard so far. To understand the reasons, we analyzed long-term (2015-2021) hourly observations of aerosols (PM_2.5, NO₃^-, NH₄⁺, SO₄^2-, OC, and EC) and gases (CO, NO₂, and SO₂) from Seoul and Baekryeong Island, a background site in the upwind region of Seoul. We applied the weather normalization method for meteorological conditions and a 3-dimensional chemical transport model, GEOS-Chem, to identify the effect of policy implementation and aerosol formation mechanisms. The monthly mean PM_2.5 ranges between about 20 μg m^-3 (warm season) and about 40 μg m^-3 (cold season) at both sites, but the annual decreasing rates were larger at Seoul than at Baengnyeong (-0.7 μg m^-3 a^-1 vs. -1.8 μg m^-3 a^-1) demonstrating the effectiveness of the local air quality policies including the Special Act on Air Quality in the Seoul Metropolitan Area (SAAQ-SMA) and the seasonal control measures. The weather-normalized monthly mean data shows the highest PM_2.5 concentration in March and the lowest concentration in August throughout the 7 years with NO₃^- accounting for about 40 % of the difference between the two months at both sites. Taking together with the GEOS-Chem model results, which reproduced the elevated NO₃^- in March, we concluded the elevated atmospheric oxidant level increases in HNO₃ (which is not available from the observation) and the still low temperatures in March promote rapid production of NO₃^-. We used O_x (≡ O₃ + NO₂) from the observation and OH from the GEOS-Chem as a proxy for the atmospheric oxidant level which can be a source of uncertainty. Thus, direct observations of OH and HNO₃ are needed to provide convincing evidence. This study shows that reducing HNO₃ levels through atmospheric oxidant level control in the cold season can be effective in PM_2.5 mitigation in Seoul.