Measuring and Modelling Dry Deposition in Mountainous Areas

Abstract

A trial programme has been initiated to test methods for estimating dry deposition rates from measurements of air concentration, using selected surface and atmospheric data to specify the appropriate deposition velocities. Results obtained in the testing programme are used here to compare a site on the slopes of Whiteface Mountain, New York, and a similarly forested site distant from mountains. Data are presented on atmospheric resistances to turbulent exchange, and on the surface resistance associated with the dry deposition of sulphur dioxide. All considerations involving the concept of a deposition velocity combining surface and vertical-diffusion components are limited in generality, because of the over-riding assumption that the aerodynamic transfer is dominated by vertical diffusion, therefore neglecting advective effects such as blowthrough. For the case of sulphur dioxide, the increase in computed dry deposition resulting when such advective effects are taken into account is found to be small. This result from the fact that SO² exchange is largely controlled by surface rather than atmospheric resistance; hence a similar result is expected for other trace gases most strongly influenced by surface resistance, such as ozone, and could be expected to extend to particle deposition in some size ranges. However, for nitric acid vapour (and presumably for all other trace gases having surface resistance small in comparison to atmospheric resistance) the consequences of surface heterogeneity and topographic complexity on deposition velocities could be very large.