Raindrop size

The dependence of washout on raindrop diameter results from the fact that as diameter decreases, the surface to volume ratio increases and surface adsorption becomes proportionately more important. Note also that a rainfall will have a distribution of raindrop sizes, therefore Eq. 13 must be integrated... 

Fig. 7-11. Effect of raindrop size and acidity on the retention of (a) plutonium-citrate and (b) plutonium-nitrate by Phaseolus vulgaris foliage (redrawn from Cataldo et at., 1981).

During rain, soluble species that exist below clouds dissolve into falling raindrops and are removed from the atmosphere. We would like to estimate the rate of removal of these species based on rain event characteristics (rain intensity, raindrop size) and species physical and chemical properties. 

Finally, the rainfall rate po (mm h 1) is related to the raindrop size distribution by... 

While routine measurements of po are available, knowledge of reliable raindrop size distributions remains a problem because of their variability from event to event as well as during the same rainstorm. 

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. 

The scavenging coefficient A(dp) given by (20.50) describes the rate of removal of particles of diameter dp by rain with a raindrop size distribution N(Dp ). If one assumes that all raindrops have the same diameter Dp, and a number concentration Afo, then (20.50) simplifies to... 

To calculate the scavenging coefficient A, we need to specify a raindrop size distribution n(Dp). A frequently used raindrop size distribution, as we saw in Chapter 15, is that of Marshall and Palmer (1948), (15.112),... 

FIGURE 20.3 Differential scavenging coefficient for the Marshall-Palmer raindrop size distribu-... 

TABLE 20.2 Scavenging Coefficients A(h for Below-Cloud Irreversible Scavenging of HNO3 Based on a Marshall-Palmer Raindrop Size Distribution... 

Marshal JS, Palmer WM (1948) The distribution of rain-drops with size. J Atmos Sci 5 165-166 Mircea M, Stefan S, Fuzzi S (2000) Precipitation scavenging coefficient influence of measured aerosol and raindrop size distributions. Atmos Environ 34 5169-5174 Monte MJS, Santos LMNBF, Fulem M, Fonseca IMS, Sousa CAD (2006) New static apparatus and vapor pressure of reference materials naphthalene, benzoic acid, benzophenone, and ferrocene. J Chem Eng Data 51 757-766... 

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