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 O₃ air pollution in Seoul, the capital city of South Korea, has increased. As a secondary air pollutant, O₃ 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 O₃ concentration (MDA8 O₃) trends measured at 25 monitoring sites. As the relationship between O₃ and meteorological variables may not be linear, we used a generalized additive model framework to estimate O₃ trends, incorporating quantile and logistic regressions for continuous O₃ data and O₃ exceedance days (>60 ppb), respectively. Over the 19-year period, the O₃ 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 O₃ concentration was more pronounced on days with comparatively high O₃ levels. During the warm season, the weather penalties on O₃ trends reached 3.7-4.0 ppb on high-O₃ days (75^th, 95^th, and 99^th percentiles). From 2001 to 2019, the O₃ exceedance days started 2.66 days earlier and ended 2.30 days later per year, extending the peak-O₃ season by approximately 89 days in total. Weather changes accelerated the upward trend in the odds ratio of O₃ exceedance days by 3.8 years. Our findings indicate that O₃ exceedance days can occur in nearly all months owing to weather changes. The significant weather penalties on O₃ concentrations and exceedance days emphasize the need for a comprehensive O₃ air pollution mitigation strategy, considering non-emission factors that are increasingly being recognized in the context of climate change.