Modelling stomatal ozone flux across Europe

A model has been developed to estimate stomatal ozone flux across Europe Fo r a number of important species. An initial application of this model is il lustrated for two species, wheat and beech, The model calculates ozone flux using European Monitoring and Evaluation Programme (EMEP) model ozone conc entrations in combination with estimates of the atmospheric, boundary layer and stomatal resistances to ozone transfer. The model simulates the effect of phenology, irradiance, temperature, vapour pressure deficit and soil mo isture deficit on stomatal conductance. These species-specific microclimati c parameters are derived from meteorological data provided by the Norwegian Meteorological Institute (DNMI), together with detailed land-use and soil type maps assembled at the Stockholm Environment Institute (SEI). Modelled fluxes are presented as mean monthly flux maps and compared with maps descr ibing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb or nl l(-1)), highlighting the spatial differences between these two i ndices. In many cases high ozone fluxes were modelled in association with o nly moderate AOT40 values. The factors most important in limiting ozone upt ake under the model assumptions were vapour pressure deficit (VPD), soil mo isture deficit (for Mediterranean regions in particular) and phenology. The limiting effect of VPD on ozone uptake was especially apparent, since high VPDs resulting in stomatal closure tended to co-occur with high ozone conc entrations. Although further work is needed to link the ozone uptake and de position model components, and to validate the model with field measurement s, the present results give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation. (C) 2000 E lsevier Science Ltd. All rights reserved.