Ozone dry deposition through plant stomata: Multi-model comparison with flux observations and the role of water stress as part of AQMEII4 Activity 2.

A substantial portion of tropospheric O 3 dry deposition occurs after diffusion of O 3 through plant stomata. Simulating stomatal uptake of O 3 in 3D atmospheric chemistry models is important in the face of increasing drought induced declines in stomatal conductance and enhanced ambient O 3 . Here, we present a comparison of the stomatal component of O 3 dry deposition eg s from chemical transport models and estimates of eg s from observed CO 2 , latent heat, and O 3 flux. The dry deposition schemes were configured as single-point models forced with data collected at flux towers. We conducted sensitivity analyses to study the impact of model parameters that control stomatal moisture stress on modeled eg s . Examining six sites around the northern hemisphere, we find that the seasonality of observed flux-based eg s agrees with the seasonality of simulated eg s at times during the growing season with disagreements occurring during the later part of the growing season at some sites. We find that modeled water stress effects are too strong in a temperate-boreal transition forest. Some single-point models overestimate summertime eg s in a seasonally water-limited Mediterranean shrubland. At all sites examined, modeled eg s was sensitive to parameters that control the vapor pressure deficit stress. At specific sites that experienced substantial declines in soil moisture, the simulation of eg s was highly sensitive to parameters that control the soil moisture stress. The findings demonstrate the challenges in accurately representing the effects of moisture stress on the stomatal sink of O 3 during observed increases in dryness due to ecosystem specific plant-resource interactions.