Long-Term Trends and Regional Differences in Background Ozone Concentrations in Korea: Insights from EEMD Analysis

Background ozone is highlighted as a factor that reflects the impact of intercontinental transport and international emissions of precursors as well as regional photochemical pollution. In this study, we used daily mean, MDA8 (max. daily 8-hour average), and MinDA8 (min. daily 8-hour average) ozone concentrations obtained from three national background air pollution monitoring stations—Jeju, Ulleung, and Baengnyeong—to investigate the long-term trends of Korea's background ozone concentrations. Ensemble Empirical Mode Decomposition (EEMD) was applied to decompose ozone time series of these concentrations into long-term, medium-termeasonal, and short-term variability for 2001 – 2022. Here, we define the background ozone variations as the long-term component extracted from the daily mean time series to compare them with those estimated in the west coast of the United States and Europe in the previous study.

The results showed that Jeju's background ozone concentration has steadily increased at the rate of 0.26 ppb per year since 2001 similar to those observed in urban monitoring stations, whereas both Ulleung and Baengnyeong stations initially showed increasing trends but shifted to declining trends after 2015, with annual rates of -0.37 and -0.25 ppb per year, respectively. To explore these differences, we defined daily ozone production (DOP) as the difference between MDA8 and MinDA8. The long-term components of DOP in urban stations, extracted using EEMD, was approximately 10 ppb higher than at background stations due to precursor emissions from anthropogenic sources, with little variation over time. In contrast, background DOP has steadily decreased since 2015, with decreasing rates of 0.26 and 0.25 ppb per year in Jeju and Ulleung, respectively.

In California, the contribution of background ozone to the ozone design value (ODV) has steadily increased, reaching approximately 70% in 2022 due to exponential decrease in anthropogenic ozone production. By comparison, the contribution of background ozone in background regions began to increase, starting in 2016, and reached about 50% in 2022. These findings underscore the importance of addressing background ozone in national air quality management strategies and offer a scientific basis for establishing effective mitigation policies.

Acknowledgments

This work was funded by the Korea Meteorological Administration Research and Development Program under Grant RS-2024-00404042.