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 dominant pathway for ozone uptake. The present study, as part of the larger California Ozone Deposition Experiment, uses aircraft eddy covariance measurements taken during the summer of 1991 in the San Joaquin Valley of California to estimate the surface conductance for ozone uptake. To explore for possible sources of discrepancies between the aircraft-derived and tower-based surface conductances a comparison is first made between tower-based fluxes and aircraft fluxes at three tower-based sites. On the average the momentum and surface energy fluxes (sensible and latent heat) observed between 30 and 33 m altitude with an aircraft agreed to within ± 10% with simultaneously measured tower-based fluxes (observed between 4 and 10 m at a vineyard, a cotton and a grassland site). However, comparisons of the aircraft- and tower-based ozone fluxes indicate that between about 4 and 33 m there is an average loss of ozone flux with height of about 18%. It is suggested that either (or both) soil NO emissions or entrainment of ozone free air at the top of the mixed layer may be responsible for this relatively larger discrepancy in the ozone fluxes. Nevertheless, in spite of any relatively larger uncertainties associated with the ozone flux, the tower-based and aircraft-based conductances are in good agreement. The aircraft-derived conductances display a similar magnitude and range of variation as the tower-based conductances and the regression coefficient between the two sets of conductances is 0.9 ± 0.08. Therefore, results from this study suggest that the aircraft can be used to estimate surface conductances of ozone deposition; however, these conductances are subject to large uncertainties.