Development of an atmospheric particle dry deposition model

A model to predict the atmospheric dry deposition velocities of particles h as been developed that is similar to a model developed for the prediction o f particle deposition velocities in vertical pipes. The model correlates th e particle deposition velocity (V-d) with Stokes settling velocity (V-st), friction velocity (V*), dimensionless inertial deposition velocity (V-di(+) ), and dimensionless Brownian diffusion deposition velocity (V-dd(+)). V-di (+) is a function of flow Reynolds number (Re) and dimensionless relaxation time (tau (+)), while V-dd(+) is a function of Schmidt number (Sc). The at mospheric particle mass size distribution and dry deposition flux measured simultaneously with a wide range aerosol classifier (WRAC) and a smooth gre ased surface were used to calculate V-d and V-di(+). The relationship betwe en V-di(+), Re, and tau (+) for particles between 1 and 100 mum in diameter was determined using the least square method to obtain coefficients for a sigmoid curve. A sensitivity analysis of the model revealed three distinct particle size ranges: for tau (+) > 0.2 and aerodynamic diameter (d(pa)) > 8 mum, the controlling parameter is tau (+); for 0.005 < <tau>(+) < 0.2 (1 < d(pa) < 8 <mu>m), the controlling parameters are tau (+) and Re; and for tau (+) < 0.005 (d(pa) < 1 mum), the controlling parameters are Sc and Re. The model was evaluated by determining the ratio between the average calcul ated flux and the flux measured with a smooth surrogate surface. The new mo del was found to better fit the experimental data (ratio 1.05 +/- 0.45) tha n the Sehmel-Hodgson model (0.34 +/- 0.18), which is often used to predict particle deposition velocities. A sensitivity analysis for the ambient model revealed three size ranges of model application based on particle diameter. Of the three physical paramet ers (tau (+), Re, Sc) used in the model, not more than two parameters contr ol the deposition in any one of these size ranges. Based on this analysis, a reduced deposition model for d(pa) > 8 mum, which is a function of V-st, V*, tau (+), has been developed.