11 September 2018

Synthetic Ozone Deposition and Stomatal Uptake at Flux Tower Sites

Ozone harms plants and reduces global photosynthesis, but little is known about Ozone deposition rates, hampering model development.

Science

We developed an Ozone (O3) deposition product, SynFlux, for 43 AmeriFlux eddy-covariance sites across the United States, and 60 sites in Europe, with a total of 926 site years of data. The method combines standard micrometeorological flux measurements, which constrain O3 deposition velocity and stomatal conductance, with a gridded dataset of observed surface O3 concentrations.    

Impact

This dataset, which is now public, dramatically expands the number and types of sites where O3 fluxes can be used for ecosystem impact studies and evaluation of air quality and climate models. The methods proposed will be further developed to expand the number of sites involved, and ultimately to provide global, data-informed, estimates of stomatal deposition of O3.

Summary

We develop and evaluate a method to estimate O3 deposition and stomatal O3 uptake across networks of eddy covariance flux tower sites where O3 concentrations and O3 fluxes have not been measured. Measurement errors are propagated through all calculations to quantify O3 flux uncertainties. We evaluate the method at three sites with O3 flux measurements: Harvard Forest, Blodgett Forest, and Hyytiälä Forest. The method reproduces 83% or more of the variability in daily stomatal uptake at these sites with modest mean bias (21% or less). At least 95% of daily average values agree with measurements within a factor of 2 and, according to the error analysis, the residual differences from measured O3 fluxes are consistent with the uncertainty in the underlying measurements.

Across these sites, the mean stomatal conductance and O3 deposition velocity is 0.03–1.0 cm s−1. The stomatal O3 flux during the growing season (typically April–September) is 0.5–11.0 nmol O3 m−2 s−1 with a mean of 4.5 nmol O3 m−2 s−1 and the largest fluxes generally occur where stomatal conductance is high, rather than where O3 concentrations are high. The conductance differences across sites can be explained by atmospheric humidity, soil moisture, vegetation type, irrigation, and land management. These stomatal fluxes suggest that ambient O3 degrades biomass production and CO2 sequestration by 20% – 24% at crop sites, 6% – 29% at deciduous broadleaf forests, and 4% – 20% at needleleaf forests in the United States and Europe.

Contact
William J. Riley
Lawrence Berkeley National Laboratory (LBNL)
Publications
Ducker, J, C Holmes, T Keenan, S Fares, A Goldstein, I Mammarella, J Munger, and J Schnell.  2018.  "Synthetic Ozone Deposition and Stomatal Uptake at Flux Tower Sites."  Biogeosciences 15(17): 5395-5413.  https://doi.org/10.5194/bg-15-5395-2018.