MODELED AND OBSERVED DRY DEPOSITION VELOCITY OF O-3 ABOVE A DECIDUOUS FOREST IN THE WINTER

Meteorological and air quality data were collected within and above a leafless deciduous forest during March and April of 1990. Included were measurements of O3 fluxes, using the eddy correlation technique, to obtain dry deposition velocities. These observations were used to investigate the capability of the dry deposition module that forms part of an existing Eulerian air quality model, known as ADOM (Acid Deposition and Oxidant Model). The observed dry deposition velocities of O3 (V(d)) had an average daytime value of 0.3 cms-1, compared to the summer value of 1.0 cm s-1, reported by Padro et al. (1991, Atmospheric Environment 25A, 1689-1704). For the diurnal cycle, the model overestimated V(d) by about 215% in the nighttime and 75% in the daytime. After making adjustments to canopy parameters and formulating the canopy resistance in terms of surface air temperature the errors reduced to 115% in the nighttime and 34% in the daytime. A further modification to ADOM, incorporating Wesely's (1989, Atmospheric Environment 23, 1293-1304) canopy resistance model, yielded V(d) values for the diurnal cycle that were smaller than the observations by about 24% in the nighttime and 25% in the daytime. The possible influences of the solar radiation, the surface air temperature, and the canopy wetness on V(d) are discussed by analysing their time-average diurnal cycles and their temporal variations for some days that are classified by sky and ground conditions.