Explaining trends and changing seasonal cycles of surface ozone in North America and Europe over the 2000–2018 period: a global modelling study with NO _<i>x</i> and VOC tagging

Abstract. Surface ozone, with its long enough lifetime, can travel far from its precursor emissions, affecting human health, vegetation, and ecosystems on an intercontinental scale. Recent decades have seen significant shifts in ozone precursor emissions: reductions in North America and Europe, increases in Asia, and a steady global rise in methane. Observations from North America and Europe show declining ozone trends, a flattened seasonal cycle, a shift in peak ozone from summer to spring, and increasing wintertime levels. To explain these changes, we use TOAST 1.0, a novel ozone tagging technique implemented in the global atmospheric model CAM4-Chem which attributes ozone to its precursor emissions fully by NOx or VOC+CO+CH4 sources and perform multi-decadal model simulations for 2000–2018. Model-simulated maximum daily 8 h ozone (MDA8 O3) agrees well with rural observations from the TOAR-II database. Our analysis reveals that declining local NOx contributions to peak-season ozone (PSO) in North America and Europe are offset by rising contributions from natural NOx (due to increased O3 production), and foreign anthropogenic- and international shipping NOx due to increased emissions. Transported ozone dominates during spring. Methane is the largest VOC contributor to PSO, while natural NMVOCs become more important in summer. Contributions from anthropogenic NMVOCs remain smaller than those from anthropogenic NOx. Despite rising global methane levels, its contribution to PSO in North America and Europe has declined due to reductions in local NOx emissions. Our results highlight the evolving drivers of surface ozone and emphasize the need for coordinated global strategies that consider both regional emission trends and long-range pollutant transport.