Pergamon

J. Aerosol Sci. Vol. 29, Suppl. 1, S122941230, 1998 pp. 0 1998 Published by Elsevier Science Ltd. All tights reserved

Printed in Great Britain 0021-8502/98 $19.00+ 0.00

DRY DEPOSITION MODEL FOR ATMOSPHERIC PARTICLES

MSAFIRI M. JACKSON, KENNETH E. NOLL, and THOMAS M. HOLSEN

Department of Chemical and Environmental Engineering, Illinois Institute of Technology, 1OW. 33’d Street, Chicago, IL 60616

A model to predict the atmospheric dry deposition of particles has been developed that is similar to a model developed for particle deposition in vertical pipes (Muyshondt et al 1996). The model correlates the particle deposition velocity (Vd) with Stokes settling velocity (V,,), friction velocity (V*), dimensionless inertial deposition velocity (Vd:) (which is a function of flow Reynolds number, Re, and dimensionless relaxation time, r+), and dimensionless Brownian diffusion deposition velocity (V&j+) (which is a function of Schmidt number, SC). V&+ (obtained from data collected in the atmosphere with a particle size classifier system and a smooth greased plate), Re, and zc for particles between 1 and 100 urn diameter (Figure 1) were fit with a sigmoid curve using the least square procedure to obtain coefficients for the sigmoid curve.

Figurel. Dimensionless Deposition Velocity (Vdi+) for Deposition Data Collected in Chicago (1992).

The correlation coefficient, R2, between the sigmoid curve and experimental data is 0.83. A statistical analysis (paired t-test) of the data showed that, there was no significant difference between dimensionless deposition velocity predicted by the sigmoid curve and atmospheric data at 95 percent confidence interval.

Brownian diffusion deposition velocity is incorporated by using a theoretical model (Claver and Yates, 1975) which describe Vdd+ as a function of SC. The overall deposition model is:

S1229

S1230 Abstracts of the 5th International Aerosol Conference 1998

Re-a2 2 In,+ - lna5

-0.5(-p I2 vd=

C,(Pp-PfkdZp +

1% V*{ale

-0.5”-a} +a4e a6 +

o.084sc-” 667 } Where p,, and pf are particle and fluid densities, g is the acceleration due to gravity, n is the absolute viscosity of fluid, d, is the particle diameter and C, is the Cunningham slip correction factor. The coeffkients are: al = 0.024175, a2 = 40300, as = 3833.25, a= 1.4911534, as = 18, a6 = 1.7. A sensitivity analysis for the model has revealed three distinct particle size ranges of model application: For r* > 0.2 (d,, > 8 pm) controlling parameter is r+; for 0.005 < r+ < 0.2 (1 < d,, < 8 pm) controlling parameters are 7’ and Re; for ri < 0.005 (d,, < 1 pm) controlling parameters are SC and Re.

The model generated flux predictions that agreed well with atmospheric measurements. It was evaluated by determining the ratio (average calculated flux/measured flux) for atmospheric deposition data (Figure 2). Sehmel-Hogson wind tunnel model (Sehmel and Hodgson, 1978) was used for comparison purposes. The ratios were as follows: Ambient model (new), 1.05f0.45; Sehmel-Hodgson model, 0.34&O. 18.

2.7 10

Wind Velocity (U), m/s

4.6 6.4 8.2

0.1 9000 12000150001800021000240002700030000

Reynold Number (Re)

Figure 2. Comparison of Measured and Modeled Flux Using Sehmel-Hodgson Model and Ambient Model. A linear Regression Line and 99% Confedence Intervals are Shown.

REFERENCES

Cleaver, J. W. and Yates, B. (1975) A Sub layer Model for the Deposition of Particles from a Turbulent Flow. Chem. Eng. Sci. 30,983 - 992.

Muyshondt, A., Arnand, N. K., and McFarland, R. M. (1996). Turbulent Deposition of Aerosols Particles in Large Transport Tubes. J. Aerosol Sci., 24: 107- 116.

Sehmel, G. A. and Hodgson, W. H. (1978) A model for Predicting Dry Deposition of Particles and Gases to Environmental Surfaces. DOE report PNL - SA - 6721, Pacific Northwest Laboratory, Richland, WA.