Evaluation of a flux-gradient method for determining O3 deposition velocities over a mixed hardwood forest

As anthropogenic tropospheric ozone (O₃) production has been reduced in recent years, natural O₃ sources and sinks have become more important to the O₃ cycle. A significant pathway of O₃ removal in the boundary layer is dry deposition in forests, a process in which molecules adhere to vegetation and soils. Despite the importance of dry deposition to the O₃ budget, global chemistry model estimates of deposition can disagree by up to a factor of two. These large differences are driven by uncertainties in the response of deposition to environmental variables controlling seasonal and interannual trends. Multi-decadal O₃ fluxes (FO₃) in forests would provide us with the additional constraints needed to inform model parameters and understand how O₃ dry deposition may change in the future. We determined deposition from FO₃ calculated using the aerodynamic flux-gradient method (AGM) for the available record of O₃ concentrations above the canopy, removing times when failures in the inlet pressure control hardware caused uncorrectable biases in O₃ profile concentrations. We evaluated the method against FO₃ measured at the Harvard Forest Long-term Ecological Research Site using the eddy covariance (EC) method from June 1992-December 2001. We also calculated FCO₂^AGM, comparing it against FCO₂^EC to develop a correction factor to apply to FO₃^AGM for years beyond the EC record. Using this new 30-year FO₃ record, we evaluated trends in and the environmental drivers of O₃ deposition above the mixed hardwood forest.