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

  • Emberson L
  • Kitwiroon N
  • Beevers S
 et al. 
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This study investigates the effect of ozone (O-3) deposition on ground
level O-3 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 O-3
vegetation sink leading to a substantial increase in ground level O-3
concentrations. Two models that have been used for human health (the
CMAQ chemical transport model) and ecosystem (the DO3SE O-3 deposition
model) risk assessment are combined to provide a powerful policy tool
capable of novel integrated assessments of O-3 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 O-3 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 O-3 deposition and (iii) conditions that assume an
extended drought period that reduces the vegetation sink to a minimum.
The risks of O-3 to human health, assessed by estimating the number of
days during which running 8 h mean O-3 concentrations exceeded 100 mu g
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
O-3 removed from the atmosphere through dry deposition during the June
and July period would have been responsible for approximately 460
premature deaths. Conversely, reduced O-3 dry deposition will decrease
the amount of O-3 taken up by vegetation and, according to flux-based
assessments of vegetation damage, will lead to a reduction in the impact
of O-3 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 O-3 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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