Areal Measurements of Ozone,Water, and Heat Fluxes Over Land With Different Surface Complexity,Using Aircraft

Contemporary models addressing issues of air quality and/oratmospheric deposition continue to exploit air-surface exchangeformulations originating from single-tower studies. In reality,these expressions describe situations that are rare in the realworld – nearly flat and spatially homogeneous. There have beenseveral theoretical suggestions about how to extend from single-point understanding to areal descriptions, but so far thecapability to address the problem experimentally has beenlimited. In recent years, however, developments in sensingtechnology have permitted adaptation of eddy-correlation methodsto low-flying aircraft in a far more cost-effective manner thanpreviously. A series of field experiments has been conducted,ranging from flat farmland to rolling countryside, employing arecently modified research aircraft operated by the US NationalOceanic and Atmospheric Administration (NOAA). The resultsdemonstrate the complexity of the spatial heterogeneity question,especially for pollutants (ozone in particular). In general, theuncertainty associated with the adoption of any single-pointformulation when describing areal averages is likely to be in therange 10% to 40%. In the case of sensible and latent heatfluxes, the overall behavior is controlled by the amount ofenergy available. For pollutant deposition, there is noconstraint equivalent to the net radiation limitation onconvective heat exchange. Consequently, dry deposition rates andair-surface exchange of trace gases in general are especiallyvulnerable to errors in spatial extrapolation. The resultsindicate that the susceptibility of dry deposition formulationsto terrain complexity depends on the deposition velocity itself. For readily transferred pollutants (such as HNO₃), a factorof two error could be involved.