Precipitation scavenging of particles

FIGURE 20.5 Fall of a drop through a layer containing CO2, S02, HNO3, NH3, O3. aod H202. [SO4 ] as a function of fall distance in cases A (0 D in text. 

The rate of mass accumulation of these particles by a single falling drop can be calculated by simply replacing the number distribution n(dp) by the mass distribution nM(dp) to obtain 

The total rate of collection of mass of all particles of diameter dp is obtained by integrating (20.46) over the size distribution of collector drops 

Therefore the below-cloud scavenging (rainout) rate of aerosol particles of diameter dp can be written as 

Calculation therefore of the aerosol scavenging rate, for a given aerosol diameter dp, requires knowledge of the droplet size distribution N(Dp) and the scavenging efficiency E(DP, dp). 

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Scavenging of particles or gases may take place in clouds (rainout) by cloud droplets or below clouds(washout) by precipitation. A scavenging ratio or washout ratio W can be defined as... 

There has not been a measured decrease in rain concentrations of PAHs over the period from 1991 to 1998 [ 148], unlike the observed decreases in concentration of these same compounds in the gas phase. This is likely due to the dominance of particle scavenging on the observed precipitation concentrations. PAHs are present in both the gas and particle phases, and Cortes et al. [33] showed that only gas-phase PAH concentrations showed a decrease with time. Particle bound atmospheric PAH concentrations exhibited no such decrease, and precipitation does not as well. Simcik et al. [148] compared the profiles of individual compounds and found the distribution in precipitation closely resembles that associated with particles in the air (r2 = 0.925) but not the gas phase (r2 = 0.085). They went on to to explain that precipitation is an effective scavenger of particle phase PAHs, and so the lack of decrease in particle-bound PAHs in the air has lead to the same lack of decrease in PAH concentration in rain. Earlier measurements revealed concentrations of 109 to 459ngL-1 across all Great Lakes on the US Canada border [149], and support the conclusion that concentrations of PAHs in precipitation are not decreasing appreciably. 

Transport from the atmosphere to land and water Dry deposition of particulate and gaseous pollutants Precipitation scavenging of particulate and gaseous pollutants Adsorption of gases onto particles and subsequent diy and wet deposition Transport within the atmosphere Turbulent dispersion and convection Atmospheric transformation Diffusion to the stratosphere Photochemical degradation Oxidation by free radicals and ozone Gas-to-particle conversion... 

Offenberg and Baker (2002) explained the difficulty in the apphcation of Eq. 14 due to the complexities in attributing particles of various sizes to their original atmospheric diameter and inefficiencies in separating small particles in rainwater. As such, Eq. 16 could be simplified as Eq. 17 and apphed to assess the precipitation scavenging of SVOCs on those samples collected in Singapore. 

As discussed above, SVOCs are removed from the atmosphere and transported to the waters by precipitation scavenging of atmospheric vapors and particles, which are incorporated into the rain within or below the clouds. After SVOCs are deposited into the bulk seawater, partitioning in water column can affect the distribution of pollutants between the dissolved aqueous and the solid phases and eventually impact the fate of these compounds in oceans (Luo et al. 2004). In addition, air-sea exchange can make SVOCs diffuse across the air-sea interface however, the sea surface microlayer (SML), a unique compartment at the air-sea boundary defined operationally as the upper millimeter (1-1,000 pm)... 

Upward diffusion of water vapor through the cold temperatures of the tropopause is very inefficient in fact, the upper limit of cloud formation often occurs at the tropopause. Thus the stratosphere is so dry as to prevent rain formation, and particles and gases have very much longer residence times there than in the troposphere. Stratospheric removal requires diffusion back through the tropopause, which then may be followed by precipitation scavenging. 

Wurzler, S., A. I. Flossmann, H. R Pruppacher, and S. E. Schwartz, The Scavenging of Nitrate by Clouds and Precipitation. I. A Theoretical Study of the Uptake and Redistribution of NaNO, Particles and HNO, Gas by Growing Cloud Drops Using an Entraining Air Parcel Model, J. Atmos. Chem., 20, 259-280 (1995). 

Wet deposition may be an important source of PCBs to the Great Lakes. Unfortunately, like dry particle deposition, it is difficult to measure directly because of the low PCB concentrations, the problem of sampling artifacts, and the distribution of rain and PCB concentrations over space and time. Some trends are evident, however [ 189]. Snow has been shown to be much more effective than rain in scavenging PCBs from both vapor and particulate phases at a site in Minnesota [190,191]. This leads to a seasonal effect, in that precipitation concentrations of PCBs are highest in the winter in Chicago (Hites, personal communication), as with the concentrations of many other persistent organic pollutants [192], Possible reasons include sorption to an aqueous or... 

Poster and Baker [91] modified Eq. 8 to include the scavenging of submicron particles that are present in the filtrate of precipitation samples ... 

Poster, D.L. and J.E. Baker. 1996b. Influence of submicron particles on hydrophobic organic contaminants in precipitation. 2. Scavenging of polycyclic aromatic hydrocarbons by rain. Environ. Sci. Technol. 30 349-354. 

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