ESTIMATING CANOPY CONDUCTANCE TO OZONE UPTAKE FROM OBSERVATIONS OF EVAPOTRANSPIRATION AT THE CANOPY SCALE AND AT THE LEAF SCALE

Stomatal uptake by vegetation is often the major sink for the destruct ion of tropospheric ozone. Using data obtained during the summer of 19 91 at a grape vineyard and a cotton field in the San Joaquin Valley of California, we compare canopy (stomatal) conductances to ozone estima ted (1) from eddy covariance ozone nux data (2) from eddy covariance e vapotranspiration data and (3) by scaling leaf transpirational conduct ance to the canopy level using a canopy radiative transfer model. Thes e simultaneous data, obtained at two levels of biological organization and for two trace gases, allow us to contrast the pathways for canopy -atmosphere exchange of water vapour and ozone, to evaluate limitation s to scaling from leaf to canopy, and to predict ozone uptake paramete rs from those governing transpiration. At the vineyard site the eddy c ovariance ozone results underestimate the ET-based (eddy covariance an d leaf scaling) approaches between 25% and 36%. At the cotton site the ozone-based results overestimate the ET-based approaches between 9% a nd 62%. A number of modelling and measurement uncertainties are of app ropriate magnitude to reconcile these estimates. Some of the possible causes for these discrepancies that are discussed include NO effects, mesophyll resistances to ozone uptake and flaws in the K-theory (first -order closure) approach on which the canopy-scale analysis is based. Nevertheless, both canopy and single leaf measurements of conductance for water vapour provide acceptable estimates of conductance for ozone , but further experiments in which all are measured simultaneously are suggested.