Ground-level pollutants

At what time of day and under what meteorological conditions is maximum ground-level pollution likely to occur at locations several kilometers inland from a shoreline industrial complex whose pollutants are released primarily from stacks of moderate height (about 40-130 m) ... 

First order parameters affecting dispersion stem from meteorological conditions. These, as much as any other consideration, determine how a stack is to be designed for air pollution control purposes. Since the operant transport mechanisms are determined by the micro-meteorological conditions, any attempt to predict ground-level pollutant concentrations is dependent on a reasonable estimate of the convective and dispersive potential of the local air. The following are meteorological conditions which need to be determined ... 

Downdraft A natural or mechanical downward airstream, either that may, due to its temperature and/or velocity, cause thermal discomfort. In the case of a stack discharge, the term downwash may be used for the downward air current in the lee of the chimney that takes the smoke and other emissions below the emission discharge level causing ground-level pollution. 

The Clean Air Act of 1967, amended in 1970, called upon the Administrator of the Environmental Protection Agency (EPA) to promulgate national primary and secondary ambient air quality standards for each air pollutant for which air quality criteria have been issued (2). A national primary ambient air quality standard is the maximum ground level pollutant concentration which in the judgment of the Administrator of EPA can be tolerated to protect the public health, based on the published air quality criteria. A national secondary ambient air quality standard is a more stringent concentration level which in the. judgment of the Administrator is required to protect the public welfare from any known or anticipated adverse effect associated with the presence of the air pollutant in the ambient air. The criteria for an air pollutant, to the extent that is practical, shall include those variable factors which may alter the effects on public health or welfare by the air pollutant and any known or anticipated adverse effects on welfare. 

This model has been exercised on a variety of AQCR s for which an accurate emission inventory, meteorological data base, and measured ground level pollutant concentrations are available. The results discussed in this paper represent analyses of the New York, Philadelphia, Niagara Frontier, St. Louis, and Milwaukee AQCRs 13, 14). These AQCRs represent a reasonable cross-section of the eastern urban AQCRs where the primary sulfur dioxide ambient air quality problem exists. The results of these analyses indicate that the level of control required by emission source to achieve ambient air quality standards can be forecast with greatly increased assurance. The following conclusions from this exercise should be considered in the near-term achievement of ambient air quality standards. 

There are virtually no sources of drinking water on Earth that are not contaminated with xenobiotics. Rain water cleanses the atmosphere as it forms and falls. As a result, it contains dissolved acids, organic compounds, and heavy metals such as mercury and selenium in many areas. Surface collection basins from which potable water is drawn—rivers, streams, and lakes—accumulate ground level pollutants in addition to those carried in rain water. Underground water, which is somewhat filtered and generally contains lesser quantities of pollutants than surface water, may itself be contaminated by ground releases of toxicants and by contaminants produced by chemical reactions in the soil and water. 

Reduction in the volatile organic content (VOC) of coatings, contributed by the solvents present, is now required because solvents, principally hydrocarbons, contribute through a reaction chain to an increase in ground level pollution (photochemical ozone). The hole in the ozone layer in the stratosphere concerns chlorofluorocarbons (CFC) and is a very different issue. Also of concern to formulators is the removal of lead, chromium and cadmium, which are long term environmental pollutants in paint residues. 

The NAAQS are expressed ia the form of ground level concentrations (GLC), which are the concentrations of pollutant ia the ambient air as measured at ground level, ia units of either micrograms per cubic meter or ppm. In order to convert a source s emission ia kilograms per hour to a GLC, dispersion modeling must be used. 

Because of the expanded scale and need to describe additional physical and chemical processes, the development of acid deposition and regional oxidant models has lagged behind that of urban-scale photochemical models. An additional step up in scale and complexity, the development of analytical models of pollutant dynamics in the stratosphere is also behind that of ground-level oxidant models, in part because of the central role of heterogeneous chemistry in the stratospheric ozone depletion problem. In general, atmospheric Hquid-phase chemistry and especially heterogeneous chemistry are less well understood than gas-phase reactions such as those that dorninate the formation of ozone in urban areas. Development of three-dimensional models that treat both the dynamics and chemistry of the stratosphere in detail is an ongoing research problem. 

A pollutant is released from an effective height of 50 m and has a ground-level concentra of 300 ng m at a position directly downwind where the ground-level concentration ... 

Fig. 20-1. Concentration of an air pollutant at the point of maximum ground-level concentration as a function of wind speed and Pasquill stability category (A-F).

Air Pollution Dispersion Application of air dispersion modeling principles and EPA tools to assessing environmental impacts from stack and area releases of pollutants Dispersion theory Gaussian plume model Ground-level concentrations Worst case scenarios Air quality impact assessments Stationary source emissions... 

Environmental Fate. A portion of releases of toluene to land and water will evaporate. Toluene may also be degraded by microorganisms. Once volatilized, toluene in the lower atmosphere will react with other atmospheric components contributing to the formation of ground-level ozone and other air pollutants. 

One major item remains before we can apply the dispersion methodology to elevated emission sources, namely plume height elevation or rise. Once the plume rise has been determined, diffusion analyses based on the classical Gaussian diffusion model may be used to determine the ground-level concentration of the pollutant. Comparison with the applicable standards may then be made to demonstrate compliance with a legal discharge standard. 

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