Acid Deposition Atmospheric Processes in Eastern North America (1983) / Chapter Skim
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2. Atmospheric Processes
2. Atmospheric Processes
Pages 29-54
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From page 29... ...
VI. Wet deposition Heterogeneous chemical processes may occur between gases and liquids adsorbed on solid surfaces, although these are generally considered to be less important in the development of acid deposition than the homogeneous processes.
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From page 30... ...
Transport phenomena directly determine where the pollution goes before it is deposited and therefore affect the atmospheric residence time of pollutant materials. Dry deposition, for example, is often limited by the speed at which the atmosphere can vertically transport pollution to the proximity of the surface.
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From page 31... ...
Quite often, for example, atmospheric transport models incorporate diffusionlike terms to account for time-averaging of meandering plumes, when in fact the physical processes described have little to do with actual intermixing of materials. Similar treatments often arise in transport models using grids to approximate the desired solutions numerically.
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From page 32... ...
Early, very crude attempts to simulate long-range phenomena simply employed local wind roses and straight trajectories from the sources in question. The obvious deficiencies associated with this approach prompted further efforts to develop curved-trajectory simulations, which were driven by conceptualized, timeevolving wind fields.
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From page 33... ...
On the basis of thermodynamic arguments, it is expected that vertical motions of air parcels should adhere rather closely to constant-entropy surfaces in the atmosphere, and from this a few ~isentropic~ trajectory simulations have evolved as well. However, vertical motions caused by the heat released or absorbed during cloud formation are not taken into account by either method.
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From page 34... ...
The difficulties associated with wind shears and vertical motions could be largely overcome if the vertical motions were indeed known. Several weather prediction models produce such information, albeit prognostic in nature, rather coarse in scale, and dedicated more to upper regions of the troposphere than to the planetary boundary layer.
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From page 35... ...
The average flows across North America are shown in Figure 2.4, which illustrates that the region in which acid deposition is currently thought to be an environmental problem is also a region of intense interaction between tropical marine and arctic air masses. CHEMICAL TRANSFORMATION During transport through the atmosphere, SO2, NOX, hydrocarbons, and their oxidation products participate in complex chemical reactions that transform the primary pollutants into sulfates and nitrates.
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From page 36... ...
Figure 2.5 which SO2 and NOx are and aqueous-phase acids. Field studies indicate that the relative importance of gas- and liquid-phase reactions depends on meteorological conditions, such as the presence of clouds, relative humidity, intensity of solar radiation, and the presence and concentrations of other pollutants.
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From page 37... ...
37 o ~n I ~n ~ ._ ._ , .O o ' 3 CD r~ o I O Ce, I _ ~ ' ._ ~ ~_ ~ o o ~ ' O ·- x Q o o =5 o X ~ ~ ~ O I I O ase4d sea o z I .= .O .
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From page 38... ...
The hydroxy radical can be formed in the troposphere by a number of reactions. ~ ~~ ~ A common process begins wltn dissociation of NCk by absorption of sunlight, which forms a highly reactive oxygen atom that combines quickly with a diatomic oxygen molecule to form the triatomic oxygen molecule, ozone (O3)
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From page 39... ...
. According to current understanding, then, the rates at which sulfuric and nitric acids are formed in homogeneous gas-phase reactions depend on ambient concentrations of the hydroxy radical.
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From page 40... ...
As indicated in Appendix A, equilibrium between gas-phase SC2 and total dissolved sulfur is established quickly, so S(IV) in cloud droplets or liquid aerosol particles is a function of pH and the ambient gaseous concentration of SO2.
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From page 41... ...
Because the pH of aerosol droplets and cloud water is generally measured to be below 5, H2O2 is currently regarded as the most important oxidizing agent in the aqueous-phase chemistry of the formation of sulfuric acid.
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From page 42... ...
If homogeneous air masses containing H202, O3, and SO2 encounter aqueous aerosol droplets, cloud water, or precipitation, solution-phase oxidation of the SC2 is favored because of the high conversion rates of these reactions. However, the optimal conditions for formation of H2O2 and O3 in the troposphere differ, so the relative importance of the two aqueous-phase oxidants will depend on conditions in the gas phase that determine the relative rates of production of the two oxidants.
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From page 43... ...
The results of field and laboratory studies suggest that although rates of oxidation of SC2 in the gas phase are relatively slow, the relative importance of gas-phase and solution-phase oxidation varies, depending on a variety of meteorological conditions, such as the extent of cloud cover, relative humidity, presence and concentrations of various pollutants, intensity of solar radiation, and amount of precipitation. Although solution-phase conversion rates can be considerably higher than those in the gas phase, air masses over the eastern United States are likely to be relatively free of clouds and precipitation a large fraction of the time, so both gaseous- and aqueous-phase processes must in general be regarded as contributing to acid formation.
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From page 44... ...
and the effects of introducing a cloud with 1 g/m3 of liquid water at 1400 h. In theory the insertion of cloud water causes dramatic decreases in atmospheric concentrations of H2O2, HNO3, SO2, and SO4.
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From page 45... ...
(1982~. deposition takes place at the Earth's surface and thus is inactive in the volume of the atmosphere in which chemical transformation and processes leading to wet deposition occur.
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From page 46... ...
All of these results render the concept of a simple boundary condition approach to dry deposition somewhat questionable; the corresponding uncertainties are again large. These difficulties combine to give a number of widely varying estimates for the temporal and spatial scales of dry deposition of specific pollutants.
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From page 48... ...
Wet Deposition The term wet deposition encompasses all processes by which atmospheric pollutants are transported to the Earth's surface in one of the many forms of precipitation: rain, snow, or fog, for example. Wet deposition therefore involves attachment of pollutants to atmospheric water and includes chemical reactions in the aqueous phase as well as the precipitation process itself.
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From page 50... ...
Nucleation is a kinetic process in which water molecules condense from the vapor phase onto a suitable surface. Dust and pollutant aerosol particles provide such surfaces in the air.
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From page 51... ...
In this case, the hydrometeor falls under the influence of gravity, sweeping out a volume in space. Collisions occur between the falling hydrometeors and some aerosol particles, resulting in attachment.
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From page 52... ...
The resulting minimum in capture efficiency as a function of particle size, shown schematically in Figure 2.10, is known as the Greenfield gap. Depending on circumstances, there are several additional attachment mechanisms (including accretion via the two-stage nucleation-impaction mechanism mentioned earlier)
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From page 53... ...
1982. Trends in SO2 emissions and associated release height for Ohio River Valley Power Plants.
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From page 54... ...
Reidel Publishing Company. Sling, WeGeNe 1983e Precipitation scavenging.
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