DRY DEPOSITION VELOCITY ESTIMATES OF SO2 FROM MODELS AND MEASUREMENTSOVER A DECIDUOUS FOREST IN WINTER

Measurements were made of concentrations and vertical flux densities ( using the eddy correlation technique) for SO2 over a deciduous forest during March and April 1990. These were compared with estimates of dry deposition velocities, obtained from resistance analogue models that employ measured meteorological data as input. The models include: (1) the dry deposition module that forms part of a larger Eulerian air qua lity model, known as ADOM (Acid Deposition and Oxidant Model), (2) a m odified version of ADOM and (3) a new parameterization for R(c). The o bservations yielded V(d) values of about 0.5 cm s-1 for the daytime an d 0.1 cm s-1 at night. ADOM's estimates were much larger, averaging ab out 3.0 cm s-1 during the daytime and about 2.0 cm s-1 at night. When the ADOM canopy resistance was increased by modifying its formulation, the modified dry deposition estimates were slightly larger during the day than at night, averaging about 0.5 cm s-1. The new parameterizati on for R(c) yielded smaller V(d) estimates with no diurnal variation. An attempt is made to relate the observed diurnal cycle of the dry dep osition velocity to meteorological parameters.