Under such conditions, extended drought can effectively “turn off” the ozone vegetation sink, leading to a substantial increase in ground-level ozone 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 ozone 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. Three different simulations were investigated during June and July: (i) actual conditions in 2006, (ii) conditions that assume a perfect vegetation sink for ozone deposition, and (iii) conditions that assume an extended drought period that reduces the vegetation sink to a minimum.

The models show that on average across the UK, ozone levels exceeded the threshold for human safety for 16 days, and as many as 20 days in the East Midlands and eastern UK. Estimates of acute exposure effects show that ozone removed from the atmosphere through dry deposition during the June and July period would have been responsible for approximately 460 premature deaths.

Conversely, reduced ozone dry deposition will decrease the amount of ozone taken up by vegetation and, according to flux-based assessments of vegetation damage, will lead to a reduction in the impact of ozone 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 emphasizes the importance of accurate estimates of ozone 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.

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