Abstract
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 PODY metrics for wheat, potato, beech and Scots pine. Statistical analyses of measured O3 and NOx concentrations were supplemented by analyses of meteorological data and NOx 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 NOx 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 PODY 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 PODY 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 PODY 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.
Highlights
As part of the European Monitoring and Evaluation Program (EMEP) monitoring network, the UK operates two level II “supersites” at Harwell (80 km west of London, Fig. 1) and Auchencorth (17 km south of Edinburgh, Fig. 1) (Tørseth et al, 2012)
During periods influenced by regional O3 production, a greater prevalence of plant conditions which enhance O3 uptake 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 spatial scale
The chemical climate statistics derived for the O3 human health and vegetation impacts at Harwell and Auchencorth are presented as data sheets in Tables S1–S12 in the Supplement
Summary
As part of the European Monitoring and Evaluation Program (EMEP) monitoring network, the UK operates two level II “supersites” at Harwell (80 km west of London, Fig. 1) and Auchencorth (17 km south of Edinburgh, Fig. 1) (Tørseth et al, 2012). The distinct impacts of one of the constituents measured at Harwell and Auchencorth, ground-level ozone (O3), on human health and vegetation have been widely studied (REVIHAAP, 2013; RoTAP, 2012), but changes in the recommended metrics by which O3 exposure relevant to these impacts is quantified (see below) necessitates new analyses of supersite measurement data. The analyses in this study are based on the chemical climatology concept introduced by chemist Robert Angus Smith in “Air and rain: The beginnings of a chemical climatology” (Angus Smith, 1872). (i) an “impact” of the atmospheric composition, often characterised through a metric; (ii) the “state” of relevant atmospheric composition variation (temporal, spatial and covariance) producing instances of the impact; and (iii) the “drivers” of this state, which could include meteorology, source proximity and emission profiles. A chemical climate has temporal boundaries (time period) and spatial boundaries (geographical extent); where there is identification of a significant change in the impact resulting from significant change to the drivers and state, these may be classified as different phases of the chemical climate
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