Reactive uptake of ozone at simulated leaf surfaces: implications for ‘non-stomatal’ ozone deposition.

The reaction of ozone (O3) with α-pinene has been studied as a function of temperature and relative humidity and in the presence of wax surfaces that simulate a leaf surface. The objective was to determine whether the presence of a wax surface, in which α-pinene could dissolve and form a high surface concentration, would lead to enhanced reaction with O3. The reaction of O3 itself with the empty stainless steel reactor and with aluminium and wax surfaces demonstrated an apparent activation energy of around 30 kJ mol-1 for all the surfaces, similar to that observed in long-term field measurements of O3 deposition to vegetation. However the absolute reaction rate was 14 times greater for aluminium foil and saturated hydrocarbon wax surfaces than for stainless steel, and a further 5 times greater for beeswax than hydrocarbon wax. There was no systematic dependence on either relative or absolute humidity for these surface reactions. Reaction of O3 with α-pinene occurred at rates close to those predicted for the homogeneous gas-phase reaction, and was similar for both the empty reactor and in the presence of wax surfaces. The hypothesis of enhanced reaction at leaf surfaces caused by enhanced surface concentrations of α-pinene was therefore rejected. Comparison of surface decomposition reactions on different surfaces as reported in the literature with the results obtained here demonstrate that the loss of ozone at the earth’s surface by decomposition to molecular oxygen (i.e. without oxidative reaction with a substrate) can account for measured ‘non-stomatal’ deposition velocities of a few mm s-1. In order to quantify such removal, the effective molecular surface area of the vegetation/soil canopy must be known. Such knowledge, combined with the observed temperature dependence, provides necessary input to global-scale models of O3 removal from the troposphere at the earth’s surface.