Abstract. Analyses have been undertaken of the spatial and temporal trends and drivers of the distributions of ground-level O₃ concentrations associated with potential impacts on human health and vegetation using measurements at the two UK European Monitoring and Evaluation Program (EMEP) supersites of Harwell and Auchencorth. These two sites provide representation of rural O₃ over the wider geographic areas of south-east England and northern UK respectively. The O₃ exposures associated with health and vegetation impacts were quantified respectively by the SOMO10 and SOMO35 metrics and by the flux-based POD_Y metrics for wheat, potato, beech and Scots pine. Statistical analyses of measured O₃ and NO_x concentrations were supplemented by analyses of meteorological data and NO_x emissions along air-mass back trajectories.

The findings highlight the differing responses of impact metrics to the decreasing contribution of regional O₃ episodes in determining O₃ concentrations at Harwell between 1990 and 2013, associated with European NO_x emission reductions. An improvement in human health-relevant O₃ exposure observed when calculated by SOMO35, which decreased significantly, was not observed when quantified by SOMO10. The decrease in SOMO35 is driven by decreases in regionally produced O₃ which makes a larger contribution to SOMO35 than to SOMO10. For the O₃ vegetation impacts at Harwell, no significant trend was observed for the POD_Y metrics of the four species, in contrast to the decreasing trend in vegetation-relevant O₃ exposure perceived when calculated using the crop AOT40 metric. The decreases in regional O₃ production have not decreased POD_Y as climatic and plant conditions reduced stomatal conductance and uptake of O₃ during regional O₃ production.

Ozone concentrations at Auchencorth (2007–2013) were more influenced by hemispheric background concentrations than at Harwell. For health-related O₃ exposures this resulted in lower SOMO35 but similar SOMO10 compared with Harwell; for vegetation POD_Y values, this resulted in greater impacts at Auchencorth for vegetation types with lower exceedance ("Y") thresholds and longer growing seasons (i.e. beech and Scots pine). Additionally, during periods influenced by regional O₃ production, a greater prevalence of plant conditions which enhance O₃ uptake (such as higher soil water potential) at Auchencorth compared to Harwell resulted in exacerbation of vegetation impacts at Auchencorth, despite being further from O₃ precursor emission sources.

These analyses indicate that quantifications of future improvement in health-relevant O₃ exposure achievable from pan-European O₃ mitigation strategies are highly dependent on the choice of O₃ concentration cut-off threshold, and reduction in potential health impact associated with more modest O₃ concentrations requires reductions in O₃ precursors on a larger (hemispheric) spatial scale. Additionally, while further reduction in regional O₃ is more likely to decrease O₃ vegetation impacts within the spatial domain of Auchencorth compared to Harwell, larger reductions in vegetation impact could be achieved across the UK from reduction of hemispheric background O₃ concentrations.