We investigate here inter-annual variability in seasonal and diurnal cycles of ozone dry deposition velocity using nine years of hourly ozone eddy covariance measurements at Harvard Forest, a broadleaf deciduous forest in western Massachusetts, USA. We use coincident water vapor eddy covariance and other micrometeorological measurements to infer stomatal conductance in order to separate variability in stomatal versus non-stomatal ozone deposition.
For all months of the year, monthly daytime mean ozone dry deposition velocity at Harvard Forest during the lowest vs. highest year differs approximately by a factor of two. These observed year-to-year differences are not apparent in nine years of simulated ozone dry deposition velocity for the model grid box containing Harvard Forest in a state-of-the-art chemistry-transport model using a modified Wesely scheme and driven by observed meteorology. We find that inter-annual variability in ozone dry deposition velocity during the growing season at Harvard Forest is driven by non-stomatal uptake. This suggests that the traditional Wesely scheme could be improved by incorporating time-varying non-stomatal conductance.