Raindrop-Aerosol Collision Efficiency

The collision efficiency E(Dp,dp) is by definition equal to the ratio of the total number of collisions occurring between droplets and particles to the total number of particles in an area equal to the droplet s effective cross-sectional area. A value of E = 1 implies that all particles in the geometric volume swept out by a falling drop will be collected. Usually 1, although E can exceed unity under certain conditions (charged particles). Experimental data suggest that all particles that hit a hydrometeor stick, and therefore, a sticking efficiency of unity is assumed. 

Theoretical solution of the Navier-Stokes equation for prediction of the collision efficiency, E(Dp,dp), for the general raindrop-aerosol interaction case is a difficult undertaking. Complications arise because the aerosol size varies over orders of magnitude, and also because the large raindrop size results in complicated flow patterns (drop oscillations, wake creation, eddy shedding, etc.) Pruppacher and Klett (1997) present a critical overview of the theoretical attempts for the solution of the problem. A detailed discussion of these efforts is outside our scope. However, it is important to understand at least qualitatively the various processes involved. 

Interception and inertial impaction are closely related, but interception occurs as a result of particle size neglecting its mass, while inertial impaction is a result of its mass neglecting its size. 

On the basis of these equations, Slinn (1983) proposed the following correlation for E that fits experimental data  

For particles of density different from 1 gem the last term in (20.53) should be scaled by (p /p ,)l /2. The first term in (20.53) is the contribution from Brownian diffusion, the second is due to interception, and the third represents impaction. 

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Using expressions obtained for the collision efficiency for E(Dp, dp) in(20.53)and (20.54), one can estimate the scavenging coefficient, and the scavenging rate for a rain event. The calculation requires knowledge of the size distributions of the raindrops and the below-cloud aerosols. 

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