Non-Stomatal Ozone Deposition to Vegetation: New Insights and Models

Tropospheric ozone is a natural constituent of the troposphere but man-made emissions of NOx and VOCs have led to an increase in concentrations globally. Ozone plays an important role in the Earth System, e.g. damaging vegetation, modifying the oxidation potential of the atmosphere and by acting as a greenhouse gas. Dry deposition of ozone to terrestrial surfaces governs its potential to cause damage and provides an important atmospheric sink. Ozone is very reactive so readily deposits on most surfaces. It is also taken in through stomata as the plant respires and this stomatal uptake can cause damage to the plant. Stomatal uptake is estimated using existing stomatal conductance models that have been well characterized and evaluated. However non-stomatal deposition has not been well described in land-atmosphere exchange models to date. Several sets of ozone flux measurements have been collated to examine the non-stomatal deposition process. The data showed ozone deposition increased to dry canopies as temperature or solar radiation increased, consistent with the thermal decomposition of ozone on leaf cuticles or reactions with emitted biogenic VOCs. It was also found that the presence of surface water increased ozone deposition, probably due to aqueous chemical reactions with compounds in the water; therefore this may depend on the composition of cuticular waxes or the presence of other deposited compounds. A new parameterisation for non-stomatal ozone deposition to grassland and coniferous forests is derived and incorporated into a standard resistance type deposition model. This new model is evaluated using data reserved from the parameterization process and compared to other existing models.