Ozone deposition

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Removal of tropospheric ozone from polluted airbasins by deposition to vegetation may be an important determinant of regional air quality. The physiological and physical processes that determine the magnitude of deposition, and the relative contributions of uptake by photosynthetic tissues versus ozone destruction on plant and soil surfaces, are not well understood. The California Ozone Deposition Experiment (CODE) demonstrated substantial deposition of ozone to several vegetated surfaces in the San Joaquin Valley. Using data collected during CODE, we examine the roles of stomatal conductance and leaf wetness from dew in mediating ozone deposition to an extensive field of irrigated cotton. Stomatal conductance, photon flux density, leaf area index (L), leaf wetness, ozone deposition, and canopy photon extinction coefficient (K) were measured. Single leaf measurements were scaled to canopy values of stomatal conductance to ozone (gc). Deposition velocity (Vd) and surface conductance (gsurf) were strongly positively correlated with gc. Under dry canopy conditions gc < < gsurf indicating a significant residual conductance of a non-stomataL pathway for ozone deposition (gr), possibly reflecting reaction of O3 with nitric oxide emitted from fertilized soil. Dewfall reduced ozone deposition and eliminated gr. However, gr may actually have been increased by dew in amphistomatous cotton, as it was in hypostomatous grape during CODE. If so, canopy wetness reduced the stomatal pathway (gc) by occlusion of adaxial pores sufficiently to offset both the nitric oxide titration and the true wetness enhancement of gr. We conclude that ozone deposition to cotton is largely controlled by stomatal responses. Stomatal responses may readily be modelled, potentially providing sufficient information to infer ozone deposition. In contrast to the case for hypostomatous grape, ozone uptake (particularly by the stomatal pathway) is reduced by leaf surface wetness in amphistomatous cotton. Alternative models of single leaf stomatal conductance and expected errors of ± 20% in model parameters did not affect these conclusions.