Atmosphere-Biosphere Interactions Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion (1981) / Chapter Skim
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5 Atmospheric Transport, Transformation, and Deposition Processes
5 Atmospheric Transport, Transformation, and Deposition Processes
Pages 57-86
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From page 57... ...
The processes substances undergo in the atmosphere, their pathways into the biosphere, and their atmospheric residence times depend upon such characteristics as physical state, particle size, and chemical reactivity. Figure 5.1 gives examples of atmospheric constituents with various residence times and the corresponding distances they may typically be transported.
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From page 58... ...
I_/ FIGURE 5.1 Dispersion of pollutants introduced into the atmosphere as determined by residence time. Man-made sulfur compounds, including fine particles, are distributed on a continental scale.
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From page 59... ...
Organic pollutants such as PCBs, hexachlorobenzene, and pesticides, have been detected in the atmosphere over remote parts of the oceans (Atlas and Giam 1981~. In examining atmospheric processes, it is convenient to group pollutants in terms of their residence time or zone of influence.
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From page 60... ...
The following night, the cycle is repeated: pollutants well mixed from the previous day remain above the newly formed surface layer, and new pollutants are injected into the lower, stable layer. Although this picture is rather simplified and pertains primarily to fair-weather conditions in nonpolar, continental areas, it does indicate the complexity of the atmospheric processes controlling pollutant behavior and the difficulty in modeling these processes.
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From page 62... ...
62 For example, at chemical equilibrium, there would be less than one H2S molecule in the entire atmosphere' Measurements have shown that the true concentration is over 1033 molecules and that the oxidation of H2S to sulfate aerosols occurs within about 12 hours of its emission (Jaechke et al.
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From page 63... ...
and the reorganization, which is called the change in entropy, S The atmosphere, then, is not at equilibrium; the observed levels of atmospheric constituents are controlled by reaction rates, especially those modulated through the OH free radical.
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From page 64... ...
Atmospheric residence times for gaseous organic species may be defined as: 1 ~ = ~ ki xi where Xi are the concentrations of reactive species such as the hydroxyl radical, ozone, etc., and ki are the corresponding second-order reaction-rate constants. On the basis of known tropospheric concentrations for OH, ozone, etc., and of the known rate constants for their reactions, it appears that atmospheric lifetimes
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From page 65... ...
For typical atmospheric concentrations of about 106 OH radicals per cm3, most organic compounds, except haloalkanes, have atmospheric lifetimes of less than one month, with the most reactive compounds, including terpenes, being chemically transformed and removed in a matter of hours. Photooxidation of gaseous organics in the atmosphere, initiated by reaction with the OH radical, involves removal of a hydrogen atom (paraffins)
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From page 66... ...
These deficiencies arise, in large part, from an inability to measure accurately the mass transfers occurring by several of the deposition processes, particularly dry deposition. Deposition, or removal, processes are conveniently separated into two categories: those which involve precipitation, called wet deposition processes, and those which do not involve precipitation and may go on all the time, called dry deposition processes.
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From page 67... ...
67 ·0 ^ O ~ O v ~ ~ V ,= ~ X =_ ° ~ ~ 8 ~ vie o ~ V C-)
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From page 68... ...
68 CH3 CHID - ~ CH3 ,OH + OH ,OH it H CH3 |
From page 69... ...
69 H E ARC 11 TIC H +OH: H (C- +o2,+lO TIC—OH ~ H NO2 Ki -O I~1 o +~ H / o H fc=0 COO H H Ki-0 1 TIC—OH H H rc=o C=0 OH H ~C=0 TIC—OH 1 H FIGURE 5.3d Simplified photooxidation mechanisms for cyclic monoterpenes H 1 - HO2~( C=0 1 H
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From page 70... ...
Gases diffuse to and across the air/drop interface, followed by dissolution and possibly chemical reaction within the drop. Dry Deposition Processes Particles that are relatively large are deposited by gravitational sedimentation.
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From page 71... ...
71 loo l to o sol LLJ 0.001 Canopy: L igh t ( lu x )
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From page 72... ...
Whether a substance is present as a gas or as a solid or liquid particle will determine which removal processes are operative. Particle size.
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From page 73... ...
73 10 1 o > O 10o Lo 10-2 1 1 1 1 10-2 10-1 1 10 1o2 DIAMETER OF PARTICLE (,um) FIGURE 5.5 Velocity of deposition of particles onto short grass.
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From page 74... ...
have modeled the transfer of SO2 and other gases, the lack of diffusion coefficients and estimates of chemical reactivity for gases of anthropogenic origin makes the application of such models difficult. RESIDENCE TIMES FOR SUBSTANCES IN THE ATMOSPHE~ The mode of emission, rate of supply, transformations in the atmosphere, and factors affecting the deposition processes act together to determine the residence time of a substance in the atmosphere, which, in turn, determines how far the substance is likely to be dispersed from its source.
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From page 75... ...
component, ret, the plant cuticular component, rC2, and the soil component, rc3 W refers to the situation with pure water on foliage when normal paths of uptake are short-circuited. The boundary layer resistance, rb, is in series with the aerodynamic resistance, ra, in the manner described by Chamberlain (1968)
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From page 76... ...
aA indicates atmospheric processes, and S indicates surface processes. SOURCE: Fowler (1 980)
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From page 77... ...
, while the trace metals that are associated with particulate matter tend to be concentrated closer to sources (Figures 5.9 and 5.10~. The short residence times of trace metals result in their being deposited in higher concentration close to urban areas, and of particular concern is the fact that many are present in precipitation at toxic concentrations (Figure 5.11~.
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From page 78... ...
See reference for details_ of measurement procedure. SOURCE: Hodge et al.
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From page 79... ...
79 TABLE 5.3 Average Residence Times in the Atmosphere of Substances Not Given in Table 5.2 Substance Residence Times Reference O3 0.4-90 days Chatfield and Harrison (1977) NO 4-5 days Schlesinger (1979)
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From page 80... ...
0 a) u, 105 103 \~jlO5 __~—— ~ \ \H;1 o 6\ 100 YEARS 10 YEARS 1 YEAR 1 MONTH l 1 DAY 1 HOUR ALKANES HALOALKENES ALKYLNITRATES ALCOHOLS ESTERS KETONES ALKYNES ~10 7 \ \ BENZENE _~N \ I_____ ALKENES TERPENES AMINES AROMATICS SULFIDES MERCAPTANS ALDEHYDES it_ 10 -16 10 -14 k_ ~ -12 -OH, cm~ molecule see ~ 1 be, 10 -10 ORGANIC COMPOUND - OH RADICAL RATE CONSTANT FIGURE 5.7 Atmospheric lifetimes of gaseous organic compounds.
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From page 82... ...
. 50 W~ ~~w FIGURE 5.10 Average zinc deposition by precipitation over the continental United States, September 1966 to March 1967 (in grams per hectare per month)
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From page 83... ...
Each median in the figure is based on the number of data values designated in parentheses. Dashed lines denote threshold of organism toxicity reported by Cough et al.
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From page 84... ...
A national materials accounting system, in conjunction with reasonable estimates of future economic conditions, technological developments, and regulatory initiatives, would provide us with much improved predictions of future emissions of toxic substances to the atmosphere and their subsequent deposition. SUMMARY Transport, diffusion, and deposition processes affecting atmospheric substances are reasonably well understood, at least qualitatively.
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From page 85... ...
NADP (National Atmospheric Deposition Program) maintains 38 sites across the United States for weekly collection and analysis of major ions in wet and dry deposition (Mosaic 1979, Gibson 1979)
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From page 86... ...
As one of the major portions of the biogeochemical cycle of many substances, and certainly for all of interest in this study, the atmosphere is the recipient of emissions from both nature and man, and it is also the source of these substances being delivered back into various ecosystems. The atmosphere and the biosphere are thus inseparably linked by the biogeochemical cycles of substances.
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