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

Abstract. A substantial portion of tropospheric O₃ dry deposition occurs after diffusion of O₃ through plant stomata. Simulating stomatal uptake of O₃ in 3D atmospheric chemistry models is important in the face of increasing drought induced declines in stomatal conductance and enhanced ambient O₃. Here, we present a comparison of the stomatal component of O₃ dry deposition (eg_s) from chemical transport models and estimates of eg_s from observed CO₂, latent heat, and O₃ 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 O₃ during observed increases in dryness due to ecosystem specific plant-resource interactions.