An aerodynamic correction for the European ozone risk assessment methodology

In Europe the risk of ozone damage to vegetation is assessed using two different metrics, the concentration-based AOT40 index and the flux-based AFstY index. An important part of the definition of both these indices is that ozone concentrations must be known at the top of the vegetation canopy. An estimate of canopy-top concentrations entails an estimate of the above-canopy concentration gradient, which is affected by the roughness sublayer (RSL) present above an aerodynamically rough surface. A calculation method is derived to correct the aerodynamic resistance for the effect of RSL. This correction results in systematically higher canopy-top concentrations as compared to standard Monin–Obukhov similarity theory. The effect of the modified concentration profile is quantified based on data from the chemical transport model of the European Monitoring and Evaluation Programme, with calculations for crops and forests at six different locations in Europe. Although the average change in ozone concentrations is not very large, the resulting changes in AOT40 and AFstY metrics can be significant. The RSL leads to increases in AOT40 of 6–13% for forests across the example sites, and 11–24% for crops. AFst1.6 only increases by 3–4% for forests, but for crops AFst6 increases by 16–54%.