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Scorched earth: How will changes in the strength of the vegetation sink to ozone deposition affect human health and ecosystems?

by L. D. Emberson, N. Kitwiroon, S. Beevers, P. Büker, S. Cinderby
Atmospheric Chemistry and Physics ()
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This study investigates the effect of ozone (O-3) deposition on ground\nlevel O-3 concentrations and subsequent human health and ecosystem risk\nunder hot summer ``heat wave{''} type meteorological events. Under such\nconditions, extended drought can effectively ``turn off{''} the O-3\nvegetation sink leading to a substantial increase in ground level O-3\nconcentrations. Two models that have been used for human health (the\nCMAQ chemical transport model) and ecosystem (the DO3SE O-3 deposition\nmodel) risk assessment are combined to provide a powerful policy tool\ncapable of novel integrated assessments of O-3 risk using methods\nendorsed by the UNECE Convention on Long-Range Transboundary Air\nPollution. This study investigates 2006, a particularly hot and dry year\nduring which a heat wave occurred over the summer across much of the UK\nand Europe. To understand the influence of variable O-3 dry deposition\nthree different simulations were investigated during June and July: (i)\nactual conditions in 2006, (ii) conditions that assume a perfect\nvegetation sink for O-3 deposition and (iii) conditions that assume an\nextended drought period that reduces the vegetation sink to a minimum.\nThe risks of O-3 to human health, assessed by estimating the number of\ndays during which running 8 h mean O-3 concentrations exceeded 100 mu g\nm(-3), show that on average across the UK, there is a difference of 16\ndays exceedance of the threshold between the perfect sink and drought\nconditions. These average results hide local variation with exceedances\nbetween these two scenarios reaching as high as 20 days in the East\nMidlands and eastern UK. Estimates of acute exposure effects show that\nO-3 removed from the atmosphere through dry deposition during the June\nand July period would have been responsible for approximately 460\npremature deaths. Conversely, reduced O-3 dry deposition will decrease\nthe amount of O-3 taken up by vegetation and, according to flux-based\nassessments of vegetation damage, will lead to a reduction in the impact\nof O-3 on vegetation across the UK. The new CMAQ-DO3SE model was\nevaluated by comparing observation vs. modelled estimates of various\nhealth related metrics with data from both urban and rural sites across\nthe UK; although these comparisons showed reasonable agreement there\nwere some biases in the model predictions with attributable deaths at\nurban sites being over predicted by a small margin, the converse was\ntrue for rural sites. The study emphasises the importance of accurate\nestimates of O-3 deposition both for human health and ecosystem risk\nassessments. Extended periods of drought and heat wave type conditions\nare likely to occur with more frequency in coming decades, therefore\nunderstanding the importance of these effects will be crucial to inform\nthe development of appropriate national and international policy to\nmitigate against the worst consequences of this air pollutant.

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