SURFACE CONDUCTANCES FOR OZONE UPTAKE DERIVED FROM AIRCRAFT EDDY-CORRELATION DATA

Plants and soils act as major sinks for tropospheric ozone, especially during daylight hours when plant stomata are thought to provide the d ominant pathway for ozone uptake. The present study, as part of the la rger California Ozone Deposition Experiment, uses aircraft eddy covari ance measurements taken during the summer of 1991 in the San Joaquin V alley of California to estimate the surface conductance for ozone upta ke. To explore for possible sources of discrepancies between the aircr aft-derived and tower-based surface conductances a comparison is first made between tower-based fluxes and aircraft fluxes at three tower-ba sed sites. On the average the momentum and surface energy fluxes (sens ible and latent heat) observed between 30 and 33 m altitude with an ai rcraft agreed to within +/- 10% with simultaneously measured tower-bas ed fluxes (observed between 4 and 10 m at a vineyard, a cotton and a g rassland site). However, comparisons of the aircraft- and tower-based ozone fluxes indicate that between about 4 and 33 m there is an averag e loss of ozone flux with height of about 18%. It is suggested that ei ther (or both) soil NO emissions or entrainment of ozone free air at t he top of the mixed layer may be responsible for this relatively large r discrepancy in the ozone fluxes. Nevertheless, in spite of any relat ively larger uncertainties associated with the ozone flux, the tower-b ased and aircraft-based conductances are in good agreement. The aircra ft-derived conductances display a similar magnitude and range of varia tion as the tower-based conductances and the regression coefficient be tween the two sets of conductances is 0.9 +/- 0.08. Therefore, results from this study suggest that the aircraft can be used to estimate sur face conductances of ozone deposition; however, these conductances are subject to large uncertainties.