Partitioning ozone fluxes to sparse grass and soil and the inferred resistances to dry deposition

A two-source (Penman-Monteith type) model, used in a preceding companion study as a diagnostic tool to partition objectively half-hourly measurements of evapotranspiration into bare soil and plant components and to derive in situ estimates of the bulk plant and soil resistances to evaporation, is extended to include ozone deposition. At the time this study was performed, the total leaf area index (LAI) of the site varied between 0.5 and 0.8. Live plant material accounted for 60–75% of the LAI while the remaining LAI was dead plant material. For present purposes this two-source model augments measurements of the major components of the surface energy balance and other micrometeorological measurements with measurements of the ambient ozone concentration and eddy correlation measurements of the total dry depositional flux of ozone. This study employs the bulk canopy resistances estimated previously with this model along with additional ozone measurements: (1) to partition the ozone dry deposition flux into bare soil and plant components in a region of partial canopy cover and (2) to estimate the intrinsic soil resistance to ozone destruction. The results of this study suggest: (a) that the plant component probably receives no more than 25% of the total ozone depositional flux and this percentage decreases as the soil water available to the plants decreases and (b) that the soil resistance to ozone destruction has a near-surface boundary layer component of about 0.7 s cm⁻¹ and an intrinsic component of about 1.0 s cm⁻¹.