Acid Deposition Atmospheric Processes in Eastern North America (1983) / Chapter Skim
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3. Theoretical Models of Regional Air Quality
3. Theoretical Models of Regional Air Quality
Pages 55-86
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From page 55... ...
One purpose of analyzing the relationships between emissions of precursor gases and deposition of acidic or acid-forming substances is to develop means for assessing the potential effectiveness of alternative proposals for mitigating the adverse effects of acid deposition. Uncertainties in the current understanding of the relevant physical and chemical processes are reflected in uncertainties in analytical models of these relationships.
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From page 56... ...
Empirical models consist of analyses of observations in the field; Chapter 4 deals with empirical approaches used to manipulate data and test hypotheses. In the class of theoretical models are both deterministic calculations and estimates of material balance (or budgets)
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From page 57... ...
Wet deposition 2.5 1.5 Dry deposition 3.3 2.5 Outflow to oceans 3.9 3.0 11.7 8.1 aArea east of 92° W (Mississippi River)
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From page 58... ...
Generally, chemical transformations and deposition processes are treated parametrically, whereas transport is calculated using available data on wind fields, for example. The models are based on sets of continuity equations for concentrations of the species of interest; the continuity equations are coupled through terms representing the production and destruction of species in chemical reactions.
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From page 59... ...
However, the SURADS model has not incorporated cloud processes and wet deposition in published applications. Tests of the SURADS model against the data from the Sulfate Regional Experiment yielded promising results for ambient sulfate conditions but less satisfactory results for sulfur dioxide concentrations "Mueller and Hidy 1982a)
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From page 60... ...
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From page 62... ...
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From page 66... ...
The more complicated the singletrajectory models become, the less application they have in day-to-day operational use because of increased requirements for computer time and power. Single forward trajectories are rarely used to investigate precipitation chemistry because many sources usually contribute acidic precursors along the path of flow.
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From page 67... ...
. More recent studies using an improved data base show that single-trajectory analysis at a site in the Northeast suggests a regional acid precipitation phenomenon (Wilson et al.
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From page 68... ...
More fundamentally, trajectory models incorporate simplifying assumptions, such as isobaric or isentropic behavior, that only approximate the true behavior of air masses. Consequently, in situations in which the assumptions are not valid, model calculations may be unreliable.
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From page 69... ...
. Results from such models should not be relied on in the development of control strategies for regional air-quality problems in which chemical phenomena play a central role.
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From page 70... ...
. The parameterization of wet deposition is extremely difficult for episode calculations because of the variability in time and space of cloud cover, cloud depth, and precipitation.
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From page 71... ...
- ~ Recent diagnostic models of scavenging appear to be moderately successful in resolving some of these individual mechanisms, and these techniques can be expected to be applied to regional models in the future. Considering the crudeness of parameterization of wet and dry deposition, it is surprising that the linear models perform better in estimating the long-term average wet deposition of sulfate than in estimating the ambient concentrations.
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From page 72... ...
The continuity equation and its boundary conditions can be mathematically linear or nonlinear, depending on whether nonlinear operators act on the dependent variable ci, the concentration of species i. A linear operator L is one that satisfies the relationship
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From page 73... ...
the relationship is nonlinear. While detailed discussion of the continuity equation and its solutions is beyond our current purpose, we note that solutions to linear and nonlinear forms of the continuity equation have characteristics that are of practical importance in formulating control strategies.
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From page 74... ...
Most regional source-receptor models in current use yield results of this type. Although p is functional in form, this expression is still a linear equation and necessarily stems from a linear form of the continuity equation.
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From page 75... ...
There is substantial support, however, for the argument that currently we simply do not understand the atmospheric interactions sufficiently well to supply the mathematical detail required by nonlinear concepts. Some attempts have recently been made to incorporate chemical nonlinearities into models of acid precipitation.
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From page 76... ...
and Sampson should be treated with caution. The so-called Rodhe-CrutzenVanderpol model used in both studies employed specific sequences of chemical reactions and assumed uniform additions of polluted background air throughout the period of transport and transformation.
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From page 77... ...
The use of reaction (3.1) is equivalent to assuming that the addition of the hydroxy radical to SO2 terminates the chain reactions of the HO radical, and by some undefined process the initial product of reaction (3.3)
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From page 78... ...
does not seriously perturb the concentration of the hydroxy radical. The best available experimental evidence today supports the contention that the HO level in reacting mixtures of hydrocarbons, NOx' and SO2 is relatively insensitive to SO2 concentrations and that the sequence (3.3)
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From page 79... ...
and sulfate background with reaction (3.2) but without sulfate background FIGURE 3.2 Effect of the assumptions of background sulfate and chain termination on the Rodhe-Crutzen-Vanderpol model for chemical transformation.
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From page 80... ...
We conclude from these results that deviations of SO2 conversion rates from linearity with respect to SO2 concentration may be much smaller than has been implied recently from the results of simulations employing the seemingly realistic yet simplified reaction schemes. Generation of nitric acid in gas-phase reactions does involve termination of an HO-radical chain directly via HO + NO2(+M)
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From page 81... ...
Thus a more detailed analysis of the complex homogeneous chemistry of the troposphere predicts that the relationship between changes in ambient concentrations of SO2 and changes in gas-phase formation of sulfate should exhibit only small deviations from linearity. The simple theoretical considerations of Oppenheimer (1983)
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From page 82... ...
For example, if the H2O2 available in cloud water were consistently only 40 percent of the SO2 that is dissolved in the cloud water at a given location, and if oxidation occurred largely through the H2Oz-HSO3 reaction, then a 60 percent reduction of the SON would result in no reduction in the sulfuric acid in cloud water at this location, but subsequent reductions would lead to proportionally lower acid formation and deposition. It is also possible that even with sufficient oxidant in cloud water, other substances that may also be present, such as formaldehyde, may inhibit the H2O2-HSO3 reaction.
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From page 83... ...
The models and their results are useful research tools. However, because of deficiencies in the base of meteorological data required as input and because of the sensitivity of their output to simplifying assumptions regarding both the physical and chemical processes, we do not regard currently available models as sufficiently developed to be used with confidence in predicting responses of the atmospheric system to alternative control strategies.
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From page 84... ...
. Measurements of oxidant concentrations in cloud water, although limited, suggest that concentrations may be sufficient in eastern North America for complete oxidation of SC2, except perhaps in winter.
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From page 85... ...
1980. An atmospheric sulfur budget for eastern North America.
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From page 86... ...
1978. Origins of air masses producing acid precipitation at Ithaca, New York: a preliminary report.
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