Interstitial Aerosol Scavenging by Cloud Droplets

Interstitial aerosol particles collide with cloud droplets and are removed from cloud interstitial air. The coagulation theory of Chapter 12 can be used to quantify the rate and effects of such removal. If n(Dp,t) is the aerosol number distribution and nj(Dp,t) the droplet 

FIGURE 15.24 Predicted aerosol size-composition distributions before and after a fog episode (Pandis et al., 1990b). 

If the scavenging coefficient did not vary with time and were equal to A Dp), then the evolution of the number distribution would be given by 

Assuming for the time being that cloud droplets are stationary, then particles are captured by Brownian diffusion. The collection of particles by a falling drop will be discussed when we consider wet deposition in Chapter 20. The collection coefficient K Dp, x) can then be estimated by (12.57). Let us estimate this collection rate assuming that the cloud has a liquid water content of 0.5 g m and that all drops have diameters of 10 /im, resulting in a number concentration of Nj = 955 cm For such a monodisperse droplet population (15.95) simplifies to 

Assuming for the time being that cloud droplets are stationary, then particles are captured by Brownian diffusion. The collection of particles by a falling drop will be 

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