Nonstomatal uptake controls inter-annual variability in ozone dry deposition at a northern mid-latitude broadleaf deciduous forest

Our understanding of ozone removal by northern mid-latitude broadleaf deciduous forests is largely based on short-term observational studies. Thus, year-to-year variations of this sink have received little attention. The specific pathways for ozone dry deposition include stomatal uptake and other non-stomatal processes that are poorly understood. Given the importance of ozone dry deposition to model the tropospheric ozone budget and regional air quality accurately, an improved mechanistic understanding of this ozone sink is needed.

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.