Suitability of a combined stomatal conductance and photosynthesis model for calculation of leaf-level ozone fluxes

Currently, the most important source of uncertainty in stomatal ozone flux (FO3) modelling is the stomatal conductance (g(st)) factor. Hence FO3 model accuracy will strongly depend on the g,t model being implemented. In this study the recently developed semi-empirical g(st) model of Dewar was coupled to the widely known biochemical photosynthesis (A(n)) model of Farquhar. The g(st) performance of this model combination was evaluated with a 4-month time series of beech (Fagus sylvatica L.) measurements. The g(st) model was hereto optimized in two steps to a 4-day and a 8-day period. A comparison between the modelled and measured g(st) to O-3 (g(stO3)) revealed a rather good overall performance (R-2 = 0.77). Errors between the model combination and the measurements are thought to be largely caused by a moderate performance of the A(n) model, due to poor parameterization. Two 2-day periods with distinctly differing soil and meteorological conditions were chosen to give a picture of the daily g, performance. Although instant relative differences between modelled and measured g(stO3) are sometimes high, the model combination is able to simulate the rough daily courses of g(stO3) and hence FO3 reasonably well. Further improvement on full parameterization of the g, model and a well-parameterized A, model to be linked to are needed to draw founded conclusions about its performance. Future efforts hereto are certainly justified since the model's mechanistic nature makes it a tool able to model g(st) variation in space and time, O-3 effects on g(st), and effective FO3.