Atmospheric Environment Vol, 27A, No. 2, pp. 167-174, 1993. 0004-6981/93 $6.00+0.00 Printed in Great Britain. Pergamon Press Ltd PARTITIONING OZONE FLUXES TO SPARSE GRASS AND SOIL AND THE INFERRED RESISTANCES TO DRY DEPOSITION W. J. MASSMAN USDA/Forest Service, Rocky Mountain Forest and Range Experiment Station, 240 West Prospect Street, Fort Collins, CO 80526, U.S.A. (First received 23 March 1992 and in final form 4 September 1992) Abstraet--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 leafarea 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 measure- ments 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- t and an intrinsic component of about 1.0 s em- t. Key word index: Ozone dry deposition, short-grasssteppe, eddy covariance fluxes, inferred bulk resistances. I. INTRODUCTION The dry deposition of ozone and other gaseous pollu- tants to plant-covered surfaces can have deleterious effects upon the physiological functioning of plant species (Runeckles and Chevone, 1992; Chappelka and Chevone, 1992). Because of the changing nature of the present chemical environment to which many plants are exposed, dry deposition has become an increasingly important scientific concern (e.g. Businger, 1986; Galbally et al., 1986; Chang et al., 1987; Wesely, 1989; Hicks et al., 1989). Central to many of these studies are the transfer resistances that describe the interaction between the surface and the atmosphere. Recently a method was proposed to infer the bulk soil and plant resistances to water vapor transfer using measurements of the major components of the surface energy balance and measurements of ambient atmospheric meteorological variables (Mass- man, 1992a). The approach, which was applied to a region of partial canopy cover, used a two-source (plant and bare soil) model of evapotranspiration (Shuttleworth and Wallace, 1985) augmented with an empirically derived relationship between the soil (eva- porative) resistance and the soil Bowen ratio. By assuming that these two soil parameters do not vary much during any given day, Massman (1992a) estim- ated their daily values by a nonlinear optimization procedure which exploited the diurnal changes in the micrometeorological data. Once the soil (evaporation) resistance and soil Bowen ratio were determined, Massman then partitioned the available energy and evapotranspiration data into soil and plant compon- ents from which the in situ soil and plant resistances could be deduced. The intent of the present study is to extend this work to include the transfer of ozone. Specifically, this study employs the previously deter- mined bulk plant resistances (Massmaa, 1992a) to deduce the subcaaopy and soil resistances associated with the dry deposition of ozone and to quantify the partitioning of ozone flux between the bare soil and plant canopy in a region of partial plant cover. However, it is important to comment here that the present approach (the two-source Penman-Monteith model) is based on K-theory or the gradient-diffusion model. For application to forests, the gradient- diffusion model is likely to be inappropriate (Den- mead and Bradley, 1985). But for sites dominated by short sparse grasses (such as discussed in this study), the gradient-diffusion model is a reasonable descrip- tion of the turbulent exchange processes (Dolman and Wallace, 1991). AE(A) 27:2-0 167