Development and evaluation of an ozone deposition scheme for coupling to a terrestrial biosphere model

Ozone is a toxic air pollutant that can damage plant leaves and substantially affect the plant’s gross primary production (GPP) and health. Realistic estimates of the effects of tropospheric anthropogenic ozone on GPP are thus potentially important to assess the strength of the terrestrial biosphere as a carbon sink. To better understand the impact of ozone damage on the terrestrial carbon cycle, we developed a module to estimate ozone uptake and damage of plants for the state of the art global terrestrial biosphere model OCN. Our approach accounts for ozone damage by calculating (a) ozone transport from the free troposphere to leaf level, (b) ozone flux into the leaf, and (c) ozone damage of photosynthesis as a function of the accumulated ozone uptake over the life-time of a leaf. A comparison of modelled canopy conductance, GPP, and latent heat to FLUXNET data across European forest and grassland sites shows a general good performance of OCN. In comparison to literature values, we demonstrate that the new model version produces realistic stomatal flux ratios as well as ozone surface resistances and depo- sition velocities. A sensitivity study reveals that key metrics of the air-to-leaf ozone transport and ozone deposition, in particular the stomatal ozone update are reason- ably robust against uncertainty in the underlying parameterisation of the deposition scheme. Correctly estimating canopy conductance plays a pivotal role in the estimate of cumulative ozone uptake. When applied at the European scale, we find that the added complexity of the ozone uptake simulation substantially affects simulated ozone uptake and accumulation, be- cause aerodynamic resistance and non-stomatal ozone destruction reduce the predicted ozone concentrations outside the leaves. Ozone impacts on GPP and transpiration in a Europe-wide simulation indicate that tropospheric ozone impacts the regional carbon and water cycling less than expected from previous studies