Cumulative ozone effect on canopy stomatal resistance and the impact on boundary layer dynamics and CO2 assimilation at the diurnal scale: A case study for grassland in the Netherlands

Biological, chemical, and dynamical processes occurring at the surface strongly interact at diurnal scales. Therefore, this study examines the seasonal ozone impact on stomatal resistance, surface energy balance, boundary layer dynamics, and CO2 assimilation at this (sub)diurnal scale under changing conditions. We combine a seasonal canopy resistance module with a surface-boundary layer model that solves the diurnal evolution of dynamical and chemical variables in a well-mixed, convective boundary layer. The model is constrained with observations from Cabauw (Netherlands) for the dry year 2003, representing a well-mixed boundary layer at midlatitudes over water-stressed grassland. To quantify the ozone impact, the Cumulative Uptake of Ozone is calculated over a growing season, which gives an estimate of the reduction in stomatal aperture and photosynthesis. From a sensitivity analysis with mixed-layer temperature and soil moisture content we conclude that drought is the dominant factor that determines the surface energy partitioning and limits CO2 assimilation. Although drought causes stomatal closure, the results indicate that ozone damage, nevertheless, occurs. A second sensitivity analysis with CO2 and ozone shows that ozone damage causes an increase in stomatal resistance of up to 40% under high ozone levels and that CO2-induced stomatal closure limits ozone damage. The impact on boundary layer development through the effect of CO2 and ozone on the stomatal resistance is much smaller. At the diurnal scale soil moisture influences the surface energy partitioning, which affects the entrainment of ozone-rich air. Due to ozone damage, the CO2 assimilation flux is reduced by about 15%.