Rain washout

Another form of removal through the water-atmosphere interphase is aerosol formation in marine surface waters where salt spray tends to remove mainly NaCl, NaS04, and MgS04. These are generally known as cyclic salts due to their potential return to the sea through rain washout. 

In the atmosphere, ammonia is estimated to have a half-life of several days. The primary fate process is reaction of ammonia with acid air pollutants and removal of the resulting ammonium compounds by dry or wet deposition. Rain washout and reaction with photochemically produced hydroxyl radicals are also expected to contribute to the atmospheric fate of vapor-phase ammonia. In water and soil, ammonia will volatilize to the atmosphere and be removed by microbial processes, by adsorption to sediment and soil matrices as well as by plant uptake. 

Initial rainfall from a given event is the most acid, because of rain washout of atmospheric acid gases and aerosols. This effect produces initial rain pH s below 3 in central Pennsylvania, for example. In cities such as Los Angeles and London, when fog has been present and the air stagnant, pH values between 2 and 3 have been observed, due in part to evaporative concentration, with serious consequences for people with respiratory problems. 

The vapor-phase reaction of ammonia with photochemically produced hydroxyl radicals is known to occur. The rate constants for this reaction have been determined to be 1.6x10 cm molecule-sec, which translates to a calculated half-life of 100 days at a hydroxyl radical concentration of 5x10 molecules/cm (Graedel 1978). This process reportedly removes 10% of atmospheric ammonia (Crutzen 1983). Since ammonia is very soluble in water, rain washout is expected to be a dominant fate process. The half-life for ammonia in the atmosphere was estimated to be a few days (Brimblecombe and Dawson 1984 ... 

TREATABILITY/REMOVABILITY Process, Removable Range (%), Avg. Achievable Cone. (pg/L)) Aerated lagoons, >60, <10 Aerated lagoons (based on synthetic wastewater), 0, not available Powdered activated carbon adsorption (based on synthetic wastewater), 97,1800 rain washout may be an important fate process because of its high water mobility... 

Deposition. The products of the various chemical and physical reactions in the atmosphere are eventually returned to the earth s surface. Usually, a useful distinction is made here between wet and dry deposition. Wet deposition, ie, rainout and washout, includes the flux of all those components that are carried to the earth s surface by rain or snow, that is, those dissolved and particulate substances contained in rain or snow. Dry deposition is the flux of particles and gases, especially SO2, FINO, and NFl, to the receptor surface during the absence of rain or snow. Deposition can also occur through fog, aerosols and droplets which can be deposited on trees, plants, or the ground. With forests, approximately half of the deposition of SO(, NH+,andH+ occurs as dry deposition. 

The contribution of atmospheric dust to surface dust depends on the dust falling to the earth. This occurs either as dry dust fall or wet washout with rain, snow or hail (1-6,8-10). Dry dust fall occurs by s imentation, impaction, interception or diffusion. Sedimentation, the fall under gravity, may be estimated using Stoke s law which relates the density and diameter of particles to their falling velocity. A particle of density 1.0 g cm"3 and diameter around 0.1 pm would fall with a velocity of around 9 x 10" cm s" ... 

It is dispersed by wind and removed by gravitational settling (sedimentation), dry deposition (inertial impaction characterized by a deposition velocity), washout by rain (attachment to droplets within clouds), and rainout (scrubbing action below clouds) (Schroeder et al. 1987). The removal rate and distance traveled from the source depends on source characteristics (e.g., stack height), particle size and density, and meteorological conditions. 

The washout factor of the particulate phase (Wp) is important in rain and snow scavenging of particle-bound POP (Cair particuiate) [61] ... 

Tin may be transported in the atmosphere by the release of particulate matter derived from the combustion of fossil fuels and solid wastes. The vapor pressure of elemental tin is negligible (Cooper and Stranks 1966). Tin in aerosol samples that existed in particulate-carbon masses was removed from the atmosphere predominantly by gravitational settling (Byrd and Andreae 1986). The half- life of airborne particles is usually on the order of days, depending on the size of the particle and atmospheric conditions (Nriagu 1979). Removal by washout mechanisms (such as rain) is thought to be unimportant. 

Only limited information is available on the washout of radioiodine vapour by rain. Measurements of stable iodine in rain, compared with iodine vapour in air (Whitehead, 1984), give a value about 30 for the washout ratio W, defined as the ratio (I per kg rain)/(I per kg air). For a moderately heavy rainfall of 1 mm h-1, this would imply a velocity of deposition in rain of 7 mm s-1, which is the same order of magnitude as the velocity of dry deposition. The washout ratio of particulate fission products is typically about 500 (Table 2.12), so radioiodine deposited in... 

Washout by rain becomes fully effective only when the cloud of activity extends up to the height of the rain-forming clouds. Within less than 10 km from a near-ground source, dry deposition is usually more effective than washout. 

Chamberlain, A.C. Eggleton, A.E.J. (1964) Washout of tritiated water vapour by rain. International Journal of Air, Water Pollution, 8,135-49. 

Rain Scavenging Ratios (RS) and Percent Washout Due to Particulates (%W) for CDDs and CDFs in Ambient Air in Two Midwest Cities... 

The value of H provides the means to describe vapour phase equilibrium between water and air. This can be used to help define vapour phase washout by rain, and also the oceanic gas phase exchange between marine air masses and associated surface water. 

Washout ratios for aerosol S04= and for total NO3" (HNO3 + aerosol NC ") are presented in Table 3. These washout ratios presuppose, in accordance with Barrie s treatment (18), that SO2 does not contribute to the rain SO and that both HNO3 and aerosol NO3" contribute to the rain NO3. Accordingly, the sulfate ratios are upper limits on W and the NC " ratios reported are close to, and only slightly less than, WNQ for HNO3 alone since HNO3 is highly soluble and dominates the total NC ". Thus from Table 3,... 

Typical values of scavenging ratio lie within the range 300-2000. Scavenging ratios are rather variable, dependent upon the ehemieal nature of the trace substance (particle or gas, soluble or insoluble, etc) and the type of atmospheric precipitation. Incorporation of gases and particles into rain can occur both by in-cloud scavenging (also termed rainout) and below-cloud scavenging (termed washout). 

Rainwater is responsible for the washing and cleaning processes of the atmosphere by means of dissolution of gases and salts, and the transport of substances and particles onto the surface of the Earth. The particles of minerals and salts washed out by rain usually have diameters below 1 p.m. Other particles included in the washout are microorganisms, such as bacteria, that are suspended in air through wind erosion and carryover. 

For a consideration of wet deposition mechanism it is useful to make a distinction between processes transferring material to cloud droplets before they begin their descent as a raindrop, known collectively as rain-out and processes transferring material to falling raindrops known as washout, There are five mechanisms [8] by which particulate and gaseous compounds may be captured by cloud or rain drop diffusiophoresis, brownian diffusion, impact and interception, solution and oxidation of gaseous species (notably SO2 and NO2) and the cloud condensation nuclei (CCN) pathway. 

The washout of many pollutants by rain is rather small. Some of these pollutants are adsorbed to particles and aerosols and are returned to the surface of the earth by settling and precipitation. 

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