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

摘要

Analyses have been undertaken of the spatial and temporal trends and driversof the distributions of ground-level O₃ concentrations associated withpotential impacts on human health and vegetation using measurements at thetwo UK European Monitoring and Evaluation Program (EMEP) supersites of Harwell and Auchencorth. These two sites providerepresentation of rural O₃ over the wider geographic areas ofsouth-east England and northern UK respectively. The O₃ exposuresassociated with health and vegetation impacts were quantified respectivelyby the SOMO10 and SOMO35 metrics and by the flux-based POD_Y metricsfor wheat, potato, beech and Scots pine. Statistical analyses of measuredO₃ and NO_x concentrations were supplemented by analyses ofmeteorological data and NO_x emissions along air-mass back trajectories.The findings highlight the differing responses of impact metrics to thedecreasing contribution of regional O₃ episodes in determining O₃concentrations at Harwell between 1990 and 2013, associated with EuropeanNO_x emission reductions. An improvement in human health-relevantO₃ exposure observed when calculated by SOMO35, which decreasedsignificantly, was not observed when quantified by SOMO10. The decrease inSOMO35 is driven by decreases in regionally produced O₃ which makes alarger contribution to SOMO35 than to SOMO10. For the O₃ vegetationimpacts at Harwell, no significant trend was observed for the POD_Ymetrics of the four species, in contrast to the decreasing trend invegetation-relevant O₃ exposure perceived when calculated using thecrop AOT40 metric. The decreases in regional O₃ production have notdecreased POD_Y as climatic and plant conditions reduced stomatalconductance and uptake of O₃ during regional O₃ production.Ozone concentrations at Auchencorth (2007–2013) were more influenced byhemispheric background concentrations than at Harwell. For health-relatedO₃ exposures this resulted in lower SOMO35 but similar SOMO10 comparedwith Harwell; for vegetation POD_Y values, this resulted in greaterimpacts 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, agreater prevalence of plant conditions which enhance O₃ uptake (such ashigher soil water potential) at Auchencorth compared to Harwell resulted inexacerbation of vegetation impacts at Auchencorth, despite being furtherfrom O₃ precursor emission sources.These analyses indicate that quantifications of future improvement inhealth-relevant O₃ exposure achievable from pan-European O₃mitigation strategies are highly dependent on the choice of O₃concentration cut-off threshold, and reduction in potential health impactassociated with more modest O₃ concentrations requires reductions inO₃ precursors on a larger (hemispheric) spatial scale. Additionally,while further reduction in regional O₃ is more likely to decreaseO₃ vegetation impacts within the spatial domain of Auchencorth comparedto Harwell, larger reductions in vegetation impact could beachieved across the UK from reduction of hemispheric background O₃concentrations.