Why is ozone in South Korea and the Seoul Metropolitan Area so high and increasing?

Abstract

Abstract. Surface ozone pollution in South Korea has increased over the past two decades, despite efforts to decrease emissions, and is pervasively in exceedance of the maximum daily 8-hr average (MDA8) standard of 60 ppb. Here, we investigate the 2015–2019 trends in surface ozone and NO2 concentrations over South Korea and the Seoul Metropolitan Area (SMA), focusing on the 90th percentile MDA8 ozone as an air quality metric. We use a random forest algorithm to remove the effect of meteorological variability on the 2015–2019 trends and find an emission-driven ozone increase of up to 1.5 ppb a-1 in April–May while NO2 decreases by 22 %. GEOS-Chem model simulations including recent chemical updates can successfully simulate surface ozone over South Korea and China as well as the very high free tropospheric ozone observed above 2 km altitude (mean 75 ppb in May–June), and can reproduce the observed 2015–2019 emission-driven ozone trend over the SMA including its seasonality. Further investigation of the model trend for May, when meteorology-corrected ozone and its increase are the highest, reveals that a decrease in South Korea NOx emissions is the main driver for the SMA ozone increase. Although this result implies that decreasing volatile organic compound (VOC) emissions is necessary to decrease ozone, we find that SMA ozone would still remain above 80 ppb even if all anthropogenic emissions in South Korea were shut off. China contributes only 8 ppb to this elevated South Korea background and ship emissions contribute only a few ppb. Zeroing out all anthropogenic emissions in East Asia in the model indicates a remarkably high external background of 56 ppb, consistent with the high concentrations observed in the free troposphere, implying that the air quality standard in South Korea is not practically achievable unless this background external to East Asia can be decreased.

Particulate Nitrate Photolysis as a Possible Driver of Rising Tropospheric Ozone

“…This bias is remedied by including pNO 3 − photolysis in the model, as also found by Colombi et al. (2023) and Yang et al. (2023) in comparison with ozonesonde and aircraft observations over South Korea in May–June 2016.…”

“…Ozone increases in the free troposphere have a larger effect on climate than increases near the surface (Lacis et al., 1990). Free tropospheric ozone also contributes to ozone pollution at the surface (Colombi et al., 2023; Lin et al., 2017). However, global atmospheric chemistry models cannot account for the observed ozone increase (Christiansen et al., 2022; Gaudel et al., 2020; Skeie et al., 2020; H. Wang et al., 2022), implying that key processes are missing.…”

Coarse particulate matter air quality in East Asia: implications for fine particulate nitrate

“…The MEIC also reports a 15% decrease of NH3 emissions over China from 2015 to 2019 (19% for the NCP), while the PKU-NH3 emission inventory reports a 6% decrease over China from 2015 to 2018 (Liao et al, 2022). Observations of surface NO2 and SO2 over the SMA imply a 22% decrease of NOx emissions and a 40% decrease of SO2 emissions during 2015-2019 (Bae et al, 2021;Colombi et al, 2022). Coarse PM decreased by 33% over the NCP and by 31% over SMA averaged for winter and summer.…”

Airborne observation with a low-cost hyperspectral instrument: Retrieval of NO2 VCD and the satellite sub-grid variability over industrial point sources

“…To enhance our understanding and obtain explicit depictions of atmospheric pollutants, improving the feasibility of airborne observations is important. The value of airborne NO2 VCD observations can be further highlighted in East Asia, where the NOx emissions are still substantial, and tropospheric O3 levels are increasing (Colombi et al, 2023;Li et al, 2020).…”