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

Abstract

Numerical sensitivity tests and four months of complete model runs have been conducted for the Routine Deposition Model (RDM). The influence of individual model inputs on dry deposition velocity as a function of land-use category (LUC) and pollutant (SO 2, O 3, SO 2− 4 and HNO 3) were examined over a realistic range of values for solar radiation, stability and wind speed. Spatial and temporal variations in RDM deposition velocity ( V d) during June – September 1996 time period generated using meteorological input from a mesoscale model run at 35 km resolution over north-eastern North America were also examined. Comparison of RDM V d values to a variety of measurements of dry deposition velocities of SO 2, O 3, SO 2− 4 and NHO 3 that have been reported in the literature demonstrated that RDM produces realistic results. Over northeastern NA RDM monthly averaged dry deposition velocities for SO 2 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 highest over forested areas. For HNO 3, the monthly averaged dry deposition velocities have the range of 0.5 to 6 cm s −1, with the highest values for forested areas. For SO 2− 4, 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 deposition velocities for SO 2 and O 3 are higher in the growing season compared to the fall, but this behaviour is not apparent for HNO 3 and sulphate. In the daytime, the hourly averaged dry deposition velocities for SO 2, O 3, SO 2− 4 and HNO 3 are higher than that in the nighttime over most of the vegetated area. The diurnal variation is most evident for surfaces with large values for leaf area index (LAI), such as forests. Based on the results presented 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 determine dry deposition amounts and with wet deposition measurements to provide seasonal estimates of total deposition and estimates of the relative importance of dry deposition.