~rnospkric l?~~~iromunr Vol. 21. No. I. pp. 91-101. 1987 Printed in Gnxr B&in. ooo4-69al/s?s1.00+0.00 Pergmon Jourmlr Ltd. A CANOPY STOMATAL RESISTANCE MODEL FOR GASEOUS DEPOSITION TO VEGETATED SURFACES DENNIS D. BALDOCCHI, BRUCE B. HICKS and PAMELA CAMARA* Atmospheric Turbulence and Diffusion Division, Air Resources Laboratory/NOAA, P.O. Box 2456. Oak Ridge, TN 37831. U.S.A. (First received 24 December 1985 and in_fi~ljon 27 June 1986) Abstract-A gaseous deposition model, based on a realistic canopy stomata1 resistance submodel, is described, anal@ and tested. This model is designed as one of a hierarchy of simulations, leading up to a “big-leaf” model of the processes contributing to the exchange of trace gases between the atmosphere and vegetated surfaas. Computations show that differences in plant species and environmental and physiological conditions can affect the canopy stomatal resistance by a factor of four. Canopy stomata1 r&stances to water vapor transfer computed with the present model arc compared against values measured with a poromcter and computed with the Penman-Montcith equation. Computed stomata1 resistances from a soybean canopy in both well-watered and water-stressed conditions yield good agreement with test data. The stomata1 resistance submodcl responds well to changing environmental and physiological conditions. Model predictions of deposition velociticsareevaluated for the case of ozone, transferred to maize. Calculated deposition velocities of 0, overestimate measured values on the average by about 30%. probably largely as a consequence of uncertainties in leaf area index, soil and cuticle resistances, and other modeling parameters, but also partially due to imperfect measurement of O3 deposition velocities. Key word index: Dry deposition, stomata1 conductance, environmental physiology, micrometeorology. I. INTRODUCTION The consequence of deposition of gaseous pollutants to vegetated surfaces is perceived by many to be one of the major environmental problems of our time. Gaseous deposition is popularly identified as a major factor damaging forests in eastern North America and Europe, and as a possible contributor to the elimi- nation of aquatic life in some Adirondack and Scandanavian Lakes. Assessments of area-wide de- position budgets are presently hindered by the in- ability to compute (or model) trace gas exchange with specific kinds of vegetation; the ability to decide on the comparative adequacy of potential emission control strategies is correspondingly limited. An important task confronting the atmosphere-surface exchange community is therefore to develop better methods to quantify flux densities of gaseous pollutants to land- scapes, both for inclusion in larger scale numerical models and for use in interpreting air chemistry observations made in areas of special interest. The flux density of a gaseous pollutant that is known to be depositing at the surface can be expressed as the product of the mean concentration of the pollutant (C) and an appropriate deposition velocity (IQ). For many chemical species of interest, mean concentrations atn he measured using available technology. The de- position velocity, on the other hand, is difficult to l Formerly Lowell University, presently State University of New York, Albany. determine, since it depends on the chemical species and is a function on many meteorological, biological and surface variables (see Sehmel, 1980; Hosker and Lindberg, 1982; Hosker, 1986). Most efforts to model and parameterize Up employ a “big-leaf “, multiple-resistance analog model (Wesely and Hicks, 1977; O’Dell et al, 1977; Unsworth, 1980; Sehmel, 1980; Hosker and Lindberg, 1982; Hicks, 1984). Such big-leaf models are one-dimensional and are most applicable over relatively tlat, horizontally homogeneous terrain. The most important individual resistances to pollutant transfer are usually identified as an aerodynamic resistance (R,,) associated with atmospheric turbulence, a quasi-laminar boundary layer resistance (RJ which is influenced by the diffu- sivity of the material being transferred, and a net canopy resistance (&) which is dominated by surface factors (mainly biological, see Fig l).t To a large extent, it is the uncertainty surrounding specification of the canopy resistance which has limited abilities to infer dry deposition rates from air concentration data. Although this uncertainty is often large, big-leaf models are being applied to infer rates of pollutant uptake in a trial dry deposition monitoring network presently operational in North America (Hicks er al.. 1985). The inclusion of simple ecophysiological con- t For clarity, upper case symbols will be used to signify rcaistanas asaociatcd with the whok anopy, as in a big-kaf model. Lower aae symbols are used for the corresponding quantities expressed as an individual-kaf basis. 911 c STONE B OTHER -- MATEd?IALS PLANT TISSUE WATER SURFACES Fig. 1. Pathway of resistances to the deposition of gaseous pollutants. The dotted line illustrates a probable route for SO2 transfer. cepts into such models is needed to make them adaptable to different vegetated surfaces and regions. The canopy resistance is a function of environ- meatai and physiological conditions, surface wetness and chemistry, leaf area index, and diffusivity of the pollutant (Jarvis, 1971, I976; Turner et al., 1973; Sehmel, 1980; Unsworth, 1980; Hosker and Linderg, 1982). In comparison, the aerodynamic and quasi- laminar components are relatively simple products of factors not strongly influenced by the physiology of the surface. For a gas such as SO*, R, is known to be influenced by stomatal (R,) and (somewhat less cer- tainly) mesophyll (R_) resistances, which are in series with each other, and are in paraIM with resistances exerted by the leaf cuticle fk), the soil CR,&, surface wetness (R,,& and any other surface material CR,& (Unsworth, 1986 Hosker and Lindberg, 1982; Hicks, 1984; Hosker, 1986). High ambient pollutant concentrations can ah0 influence the canopy resistance to pollutant uptake by increasing or decreasing