Deposition in rain

Pesticides can be transported away from the site of appHcation either in the atmosphere or in water. The process of volatili2ation that transfers the pesticide from the site of appHcation to the atmosphere has been discussed in detail (46). The off-site transport and deposition can be at scales ranging from local to global. Once the pesticide is in the atmosphere, it is subject to chemical and photochemical processes, wet deposition in rain or fog, and dry deposition. 

The gases radon (222Rn) and thoron (220Rn) are formed as progeny of uranium and thorium in rocks and soil. They are emitted from the ground into the atmosphere, where they decay and form daughter products, isotopes of polonium, bismuth and lead, which either remain airborne till they decay, or are deposited in rain and by diffusion to the ground. 

Table 2.15. Interception by foliage of particulate activity deposited in rain (analysis of results of Hoffman et al., 1989)...

Hoffman, F.O. et al. (1989) Pasture grass interception and retention of1311, 7Be and insoluble microspheres deposited in rain. Oak Ridge National Laboratory Report ORNL 6542, Springfield, Va. N.T.I.S. 

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... 

Harteck (1954) measured the amount of T in the lower atmosphere before H-bomb tests had added significantly to natural activity, and found 4000 and 3 TU in hydrogen and water vapour respectively. The amount of H2 in the atmosphere has increased in recent years due to industrial production (Schmidt, 1974). Circa 1950 there were about 1.5 x 1011 kg of H as H2, compared with 1.4 x 1015 kg as H20. Thus Harteck s values for natural T correspond to tropospheric inventories of 1.8 g as HT and 13 g as HTO. Because HTO is deposited in rain and by vapour transfer to the sea, its residence time in the troposphere is only about a week, similar to that of 137Cs (Fig. 2.8), and more than 90% of the atmospheric inventory is in the stratosphere. By contrast, the residence time of HT in the troposphere is several years (Mason Ostlund, 1979), and most of the atmospheric inventory of HT is in the troposphere. It is only necessary for a small fraction of naturally produced T to form HT to account for the high specific activity of hydrogen gas. 

Precipitation in the vicinity of Allegheny Mountain is about 107 cm per year (19), or close to the rate recorded in Table 2. If the ratio between deposition by rain and deposition to dew during the sampling period is also representative of the year, then it follows that the annual total acid deposited in rain is very roughly 60 times as great as that deposited to dew or in settled fogwater. 

NO2 and SO2 are ultimately deposited in rains as nitric, sulfurous and sulfuric acids Nitric and sulfuric acids are strong acids, while sulfurous acid is rather weak. 

A major sink for compounds released into the environment is the soil or sediment in aquatic systems. Some compounds (e.g., a herbicide) may be applied directly to the soil, other compounds applied to plants may reach the soil as leaves fall. Compounds in the atmosphere may be deposited in rain-out or as particulates settle. Waste material will ultimately be consigned to a landfill. Consequently, the processes by which compounds distribute into and are bound in soil or sediment will influence their environmental distribution. 

Acid rain The deposition of strongly acidic pollutants, especially sulfuric and nitric acids, from the atmosphere to the earth s surface. The term strictly refers only to wet deposition (in rain, snow, etc.) but is often used to include dry deposition processes (i.e., due to settling or turbulent transfer). 

Study, much cropland would be destroyed direcdy by fast-moving fires. Also of special concern are the heavy deposits of air pollutants from the atmosphere which would take place in the months during and following the war. If an appreciable fraction of the NOx formed in the nuclear explosions and in the resulting fires were to be deposited in rain, the rainwater would be highly acidic with an average pH of less than 4. 

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