Trends and drivers of ozone human health and vegetation impact metrics from UK EMEP supersite measurements (1990-2013)

Abstract

Analyses have been undertaken of the spatial and temporal trends and drivers of the distributions of ground-level O 3 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 O 3 over the wider geographic areas of south-east England and northern UK respectively. The O 3 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 O 3 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 O 3 episodes in determining O 3 concentrations at Harwell between 1990 and 2013, associated with European NO x emission reductions. An improvement in human health-relevant O 3 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 O 3 which makes a larger contribution to SOMO35 than to SOMO10. For the O 3 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 O 3 exposure perceived when calculated using the crop AOT40 metric. The decreases in regional O 3 production have not decreased POD Y as climatic and plant conditions reduced stomatal conductance and uptake of O 3 during regional O 3 production. Ozone concentrations at Auchencorth (2007-2013) were more influenced by hemispheric background concentrations than at Harwell. For health-related O 3 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 O 3 production, a greater prevalence of plant conditions which enhance O 3 uptake (such as higher soil water potential) at Auchencorth compared to Harwell resulted in exacerbation of vegetation impacts at Auchencorth, despite being further from O 3 precursor emission sources. These analyses indicate that quantifications of future improvement in health-relevant O 3 exposure achievable from pan-European O 3 mitigation strategies are highly dependent on the choice of O 3 concentration cut-off threshold, and reduction in potential health impact associated with more modest O 3 concentrations requires reductions in O 3 precursors on a larger (hemispheric) spatial scale. Additionally, while further reduction in regional O 3 is more likely to decrease O 3 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 O 3 concentrations.