Recent ozone trends and changepoints in the Greater Region: The role of meteorology and NOₓ emission reductions

Abstract. We analysed long-term records of NO₂ and O₃ in Luxembourg and the Greater Region with the Bayesian Estimator of Abrupt change, Seasonality, and Trend (BEAST) to identify probabilistic changepoints in time series. Seasonal components of NO₂ and long-term components of O₃ revealed a significant changepoint in 2017/2018 that is more prominent than the effect of the short-term emission reductions in 2020/2021 and the subsequent post-Covid rebound in urban areas. NO₂ concentrations decrease by at least 50 % for all station categories from 2009 to 2024. Linear trends for O₃ during this period based on quantile regression and Theil-Sen trend estimator were positive, with highest values at urban stations (~0.75 µg m^-3 yr^-1), while increases at peri-urban, rural, and forest sites were smaller (~0.4–0.6 µg m^-3 yr^-1). Spring and autumn are the dominant seasons contributing to the O₃ increase. The decline of the positive weekend effect, together with the reduced MDA8 O₃ sensitivity to maximal air temperatures above ~20 °C after 2018, indicates an ongoing transition from predominantly VOC-limited towards NOₓ-limited chemistry, particularly at peri-urban, rural, and forest stations during summer. However, our results suggest that the impact of reduced O₃ titration by NO, particularly at low to moderate temperatures has sustained increasing O₃ in the Greater Region. Simultaneously, the reduction in peak O₃ due the influence of NO_x-limited chemistry was partly offset by the increase in daytime maximum boundary layer height observed after the 2017/2018 changepoint.