Measurements of ozone removal by Scots pine shoots: calibration of a stomatal uptake model including the non-stomatal component

Abstract

The calculation of ozone uptake to vegetation requires the determination of both the stomatal uptake and the non-stomatal sinks. This paper analyses the relative contribution of stomatal and non-stomatal sinks at the shoot level based on direct measurements of ozone flux that included both components. Ozone deposition was estimated for Scots pine shoots enclosed in chambers, connected to a gas-exchange system, at the SMEAR II field station, in southern Finland. For the analysis of stomatal conductance, we utilised the formulation within the dry deposition module of the photochemical EMEP model, for which we revised the parameterisation for Scots pine. In addition, we introduced modifications in the treatment of spring phenology and parameter estimation. Provided these modifications, the parameter values estimated from our data fell close to the default values, although presenting a less sensitive VPD response during the summer. A major proportion of the measured removal was due to the non-stomatal component. We introduced a non-stomatal term dependent on the ambient relative humidity, which significantly improved the agreement between the calculated and measured values of total shoot conductance. The non-stomatal contribution to total conductance was in the order of 50%.