A comprehensive multi-tiered approach to assessing weather penalties on O3 levels and exceedance days in Seoul using generalized additive models (2001–2019).

Over the past two decades, ambient O 3 air pollution in Seoul, the capital city of South Korea, has increased. As a secondary air pollutant, O 3 is affected not only by precursor gas emissions but also by meteorological conditions. This study examined the influence of weather changes in Seoul for 2001–2019 on the long-term daily maximum 8-h O 3 concentration (MDA8 O 3) trends measured at 25 monitoring sites. As the relationship between O 3 and meteorological variables may not be linear, we used a generalized additive model framework to estimate O 3 trends, incorporating quantile and logistic regressions for continuous O 3 data and O 3 exceedance days (>60 ppb), respectively. Over the 19-year period, the O 3 concentrations in Seoul increased by 14.7 (SD = 3.2) and 13.1 (SD = 3.1) ppb before and after adjusting for local meteorology, respectively, resulting in an average weather penalty of 1.55 ppb. Seasonal variations in the penalty were substantial, showing a greater penalty (4.5 ppb) during the warm season (May–October) than during the cold season (November–April). Furthermore, the increase in O 3 concentration was more pronounced on days with comparatively high O 3 levels. During the warm season, the weather penalties on O 3 trends reached 3.7–4.0 ppb on high-O 3 days (75th, 95th, and 99th percentiles). From 2001 to 2019, the O 3 exceedance days started 2.66 days earlier and ended 2.30 days later per year, extending the peak-O 3 season by approximately 89 days in total. Weather changes accelerated the upward trend in the odds ratio of O 3 exceedance days by 3.8 years. Our findings indicate that O 3 exceedance days can occur in nearly all months owing to weather changes. The significant weather penalties on O 3 concentrations and exceedance days emphasize the need for a comprehensive O 3 air pollution mitigation strategy, considering non-emission factors that are increasingly being recognized in the context of climate change. [Display omitted] • Ambient ozone air pollution in Seoul has increased over the past two decades. • Influence of weather changes on long-term O 3 pollution trends was explored. • Before and after adjusting for meteorology, O 3 increased by 14.7 and 13.1 ppb. • Peak-O 3 season has increased by 89 days driven by weather changes. [ABSTRACT FROM AUTHOR]