Ozone deposition and stomatal uptake at flux tower sites

We develop and evaluate a method for estimating O3 deposition and stomatal O3 uptake at FLUXNET tower sites. The method combines standard micrometeorological measurements that are made with the eddy covariance method with a gridded dataset of surface O3 concentration. We evaluate the method at 3 sites where O3 fluxes were measured. This comparison shows that our method reproduces 69-92% of the variability with modest bias. The remaining unexplained variance is due to stomatal conductance filtering methods, non-stomatal parameterizations, representation error in the gridded O3 data and measurement error in . Using our O3 deposition method, we calculate the stomatal O3 uptake at 41 FLUXNET sites in the US and 55 FLUXNET sites in Europe. The spatial patterns in stomatal O3 uptake are driven by the gradients in humidity, which promote stomatal conductance rather than spatial patterns in O3 concentrations alone. Factors such as land cover, land management, and plant functional types are also important underlying factors in the spatial variability in the stomatal O3 uptake throughout the FLUXNET network. With the synthetic O3 flux data we examine the relationship with common concentration-based metrics for plant injuries due to O3, such as mean concentration, AOT40 and W126. We show that concentration-based metrics are poor predictors of O3 flux (R2 ≤ 0.11), when applied across different biomes, but if they must be used, they should be limited to cases where VPD < 1.5 kPa (R2 ≤ 0.28). The synthetic O3 flux data enable evaluation of atmospheric chemistry models at many additional sites.