Scavenging coefficient

Numerical modellers often find it convenient to describe wet deposition by a scavenging coefficient, actually a first order rate constant for... 

Caltech unified GCM (Global) Bulk equilibrium, ZSR equation, no hysteresis None Autoconversion nucl. scavenging with prescribed scavenging coefficient for sea-salt and dust and a first-order precipitation-dependent parametrization for other aerosols precip. rate independent of aerosols First-order precipitation-dependent bulk parametrization calculated scavenging efficiency with size dependence Implicitly accounted for in a parametrization of the limiting autoconversion rate... 

GEM-AQ only has a simplified aqueous phase reaction module for oxidation of SO2 to sulphate. Thus, for the gas phase species, wet deposition processes are treated in a simphfied way. Only below-cloud scavenging of gas phase species is considered in the model. The efficiency of the rainout is assumed to be proportional to the precipitation rate and a species-specific scavenging coefficient. The coefficients apphed are the same as those used in the MATCH model (Multiscale Atmospheric Transport and Chemistry Model) used by the Swedish Meteorological and Hydrological Institute (SMHl) (Langner et al. 1998). 

In theory, the removal of fine particles by collision processes results in chemical concentrations in the air mass following an exponential decay law. In actual clouds, both collision and nucleation may contribute to particle removal, and their combined effects often are lumped into a single scavenging coefficient, A [T 3] ... 

Assume the dry deposition velocity over a New England forest for particles emanating from a midwestern U.S. power plant averages 0.5 cm/sec on a certain day. The scavenging coefficient in a rain-forming cloud over the forest is 10 3/sec. If the cloud is 500 m in vertical extent, what is the equivalent wet deposition velocity, Vw, for rainout of the particles ... 

If the scavenging coefficient does not vary with time and is equal to A(Dp), then the evolution of the number distribution is given by... 

If the atmosphere below the cloud is homogeneous, then one can also define an average scavenging coefficient Ag so that... 

Below-Cloud Scavenging Coefficient The concentration of gas A is lOpgm 3 below a raining cloud. Assuming a constant scavenging coefficient of 3.3 h "1, calculate the concentration of A in the atmosphere after 30 min of rain, and the overall wet deposition flux. Assume a cloud base at 2 km. 

Assuming that the scavenging coefficient remains constant with time, we can solve the above equation to get... 

This example illustrates that the scavenging coefficient can be a useful parameter provided that it can be estimated reliably and one is aware of its physical meaning and limitations. Use of a scavenging coefficient in (20.2) implies linear, irreversible transport of a species into droplets. 

The parameters Kc and Ut are given in Table 20.1 as a function of the droplet diameter. The scavenging coefficients in Table 20.1 have been estimated for p0 = 1 mmh-1 assuming that all the raindrops have the same size Dp. As the scavenging coefficient according to (20.25) depends linearly on po, one just needs to multiply the A values in Table 20.1 with... 

TABLE 20.1 Estimation of the Scavenging Coefficient A for Irreversible Scavenging in a Homogeneous Atmosphere (p0 = l mm b )... 

Because of the reversibility of the scavenging process, one can define a scavenging coefficient only if CgH RT This is valid when the initial raindrop species concentration is much lower than the equilibrium concentration corresponding to the below-cloud atmospheric conditions. If this is valid, then... 

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... 

The scavenging coefficient for this simplified scenario is shown in Figure 20.7 for Po = 1 mmh 1 for drops of diameters Dp = 0.2 and 2 mm. This figure indicates the sensitivity of the scavenging coefficient to sizes of both aerosols and raindrops, suggesting the need for realistic size distributions for both aerosol and drops in order to obtain useful estimates for ambient scavenging rates. 

FIGURE 20.7 Scavenging coefficient for monodisperse particles as a function of their diameter collected by monodisperse raindrops with diameters 0.2 and 2 mm assuming a rainfall intensity of 1 mmh-1. 

Our calculations can be simplified by defining a mean mass scavenging coefficient Am so that ... 

Using this, calculate the scavenging coefficient for the Marshall-Palmer distribution (17.108) (Marshall and Palmer 1948)... 

C Assuming a rainfall with intensity po = lOmmh-1 and a monodisperse raindrop distribution with Dp =4 mm, calculate the mass scavenging coefficients for the model continental, marine, and urban distribution of Chapter 8. 

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),... 

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