Changes in surface ozone in South Korea on diurnal to decadal time scale for the period of 2001–2021

Abstract. Increasing trends of tropospheric ozone in South Korea in the last decades have reported in several studies, based on various metrics. In this study, we derived the trends of surface ozone in South Korea utilizing the daily maximum 8-hours average ozone concentrations (MDA8) measured at the surface from 2001 to 2021 and analyzed diurnal, seasonal, multi-decadal variations of this parameter at city, province, and background sites. The 4^th highest MDA8 values have positive trends at 7 cities and 8 provinces throughout 2001–2021 with approximately 1–2 ppb yr^-1 and were greater than 70 ppb after early 2010 for all sites, despite decreases of its precursor NO₂ and CO. The Seoul Metropolitan Area (SMA) and the background sites have different diurnal and seasonal characteristics of MDA8 exceedances defined in this study (percentage of the data points with MDA8 > 70 ppb among all data points). SMA have much higher exceedances during summer than spring, while the background sites have much higher exceedances during spring than summer highlighting efficient local production of ozone in SMA during summer and strong influence of long-range transport during spring. The exceedances during spring and summer are similar for the rest of sites. The peaks of exceedances occur at 4–5 PM in SMA and most of locations, while exceedances mainly occur at 7–8 PM through night at the background sites. During spring of the COVID-19 pandemic (2020–2021), the MDA8 ozone exceedances decreased for most of locations with large NO_x reductions in South Korea and China compared to 2010–2019. The large decreases of the MDA8 ozone exceedances occur in particular at the background sites during spring. In Gosung, Gangwondo (~600 m above sea level), the exceedances drop to ~5 % from 30 % in springtime during the COVID-19 pandemic. The concept of decreases of ozone in the boundary layer in Seoul and Gangwon-do to reductions in the emissions was confirmed by regional model simulations. The reductions of ozone exceedances did not occur at the major cities and provinces during summer of the COVID-19 pandemic with much smaller decreases of NO_x in South Korea and China compared to spring. This study demonstrates distinctions between spring and summer in the formation and transport of surface ozone in South Korea and the need of monitoring and modeling with focus on different processes in each season or a finer time scale.