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

Abstract. This study investigates the effect of ozone (O₃) deposition on ground level O₃ concentrations and subsequent human health and ecosystem risk under hot summer "heat wave" type meteorological events. Under such conditions, extended drought can effectively "turn off" the O₃ vegetation sink leading to a substantial increase in ground level O₃ concentrations. Two models that have been used for human health (the CMAQ chemical transport model) and ecosystem (the DO₃SE O₃ deposition model) risk assessment are combined to provide a powerful policy tool capable of novel integrated assessments of O₃ risk using methods endorsed by the UNECE Convention on Long-Range Transboundary Air Pollution. This study investigates 2006, a particularly hot and dry year during which a heat wave occurred over the summer across much of the UK and Europe. To understand the influence of variable O₃ dry deposition three different simulations were investigated during June and July: (i) actual conditions in 2006, (ii) conditions that assume a perfect vegetation sink for O₃ deposition and (iii) conditions that assume an extended drought period that reduces the vegetation sink to a minimum. The risks of O₃ to human health, assessed by estimating the number of days during which running 8 h mean O₃ concentrations exceeded 100 μg m⁻³, show that on average across the UK, there is a difference of 16 days exceedance of the threshold between the perfect sink and drought conditions. These average results hide local variation with exceedances between these two scenarios reaching as high as 20 days in the East Midlands and eastern UK. Estimates of acute exposure effects show that O₃ removed from the atmosphere through dry deposition during the June and July period would have been responsible for approximately 460 premature deaths. Conversely, reduced O₃ dry deposition will decrease the amount of O₃ taken up by vegetation and, according to flux-based assessments of vegetation damage, will lead to a reduction in the impact of O₃ on vegetation across the UK. The new CMAQ-DO₃SE model was evaluated by comparing observation vs. modelled estimates of various health related metrics with data from both urban and rural sites across the UK; although these comparisons showed reasonable agreement there were some biases in the model predictions with attributable deaths at urban sites being over predicted by a small margin, the converse was true for rural sites. The study emphasises the importance of accurate estimates of O₃ deposition both for human health and ecosystem risk assessments. Extended periods of drought and heat wave type conditions are likely to occur with more frequency in coming decades, therefore understanding the importance of these effects will be crucial to inform the development of appropriate national and international policy to mitigate against the worst consequences of this air pollutant.