Description and evaluation of a model of deposition velocities for routineestimates of dry deposition over North America. Part II: review of past measurements and model results

Numerical sensitivity tests and four months of complete model runs have bee n conducted for the Routine Deposition Model (RDM). The influence of indivi dual model inputs on dry deposition velocity as a function of land-use cate gory (LUC) and pollutant (SO2, O-3, SO42- and HNO3) were examined over a re alistic range of values for solar radiation, stability and wind speed. Spat ial and temporal variations in RDM deposition velocity (V-d) during June - September 1996 time period generated using meteorological input from a meso scale model run at 35 km resolution over north-eastern North America were a lso examined. Comparison of RDM Vd values to a variety of measurements of d ry deposition velocities of SO2, O-3, SO42- and NHO3 that have been reporte d in the literature demonstrated that RDM produces realistic results. Over northeastern NA RDM monthly averaged dry deposition velocities for SO2 vary from 0.2 to 3.0 cm s(-1) with the highest deposition velocities over water surfaces, For O-3 the monthly averaged dry deposition velocities are from 0.05 to 1.0 cm s(-1) with the lowest values over water surfaces and the hig hest over forested areas. For HNO3 the monthly averaged dry deposition velo cities have the range of 0.5 to 6 cm s(-1), with the highest values for for ested areas. For SO42-, they range from 0.05-1.5 cm s(-1), with the lowest values over water and the highest over forest, The monthly averaged dry dep osition velocities for SO2 and O-3 are higher in the growing season compare d to the fall, but this behaviour is not apparent for HNO3 and sulphate. In the daytime, the hourly averaged dry deposition velocities for SO2, O-3, S O42- and HNO3 are higher than that in the nighttime over most of the vegeta ted area. The diurnal variation is most evident for surfaces with large val ues for leaf area index (LAI), such as forests. Based on the results presen ted in this paper, it is concluded that RDM V-d values can be combined with measured air concentrations over hourly, daily or weekly periods to determ ine dry deposition amounts and with wet deposition measurements to provide seasonal estimates of total deposition and estimates of the relative import ance of dry deposition. (C) 1999 Elsevier Science Ltd, All rights reserved.