Pollution Prevention and Abatement Handbook WORLD BANK GROUP Effective July 1998 Airshed Models Modeling may be necessary to estimate the changes in ambient air quality—both local and at a distance—caused by a particular set of emissions. Modeling can be appropriate for new plants and for modifications to existing plants. This chapter provides guidance on some mod- els that may be useful in the context of typical World Bank Group projects. Air quality is an issue of increasing concern in utilize different types of models, which are dis- many countries. Projects that introduce new cussed elsewhere in this Handbook. sources of emissions or are designed to reduce Although thermal power plants are often emissions require careful analysis to quantify the singled out as major polluting sources, nearly all effects as far as possible. For many sources, this industrial facilities, especially those with short will typically require mathematical modeling of stacks, have the potential to cause localized the changes in ambient concentrations that result areas of unacceptable air quality. In addition, from the new emissions. The few widely used urban areas can act as diffuse sources of air pol- models are reviewed in this chapter. lution, particularly where poor-quality fuels Air quality modeling can be a complex task, are burned in household stoves. Cases of mul- and the objectives need to be clear. The costs of a tiple point sources or area sources (or both) can study can range from US$10,000 to US$500,000, often be modeled by using simplifying assump- depending on the complexity of the situation and tions or by integrating the impacts of individual the level of detail required; in many cases, costs sources. are at the lower end of this scale. The simplest approach uses a point source dispersion model Use of Near-Field Dispersion Models to estimate the ground-level concentrations of the pollutants of interest at some distance (typically Typically, dispersion models have been used in from hundreds of meters to tens of kilometers) developing countries only in isolated cases where from a point source. More complicated models air pollution had been recognized as a serious allow the examination of multiple sources, in- problem (e.g., Mae Moh, Thailand, and Krakow, cluding area (nonpoint) sources. For an area con- Poland). However, with increasing pollution taining a number of point and nonpoint sources, problems and more emphasis on air quality stan- an air quality model can be constructed that in- dards in developing countries, dispersion mod- cludes all of the sources in the area. In practice, els are expected to be used more extensively in such models are rare because of the costs of de- the future for sector- and project-level environ- velopment and the data required to make the mental assessments, as well as for assistance in model a realistic tool. establishing specific emissions requirements. This chapter examines the application of the As a general guide, it is suggested that a basic most commonly used air quality dispersion mod- analysis of possible impacts on ambient concen- els for assessing the impact on air quality of key trations be carried out on installations that have pollutants—sulfur dioxide (SO2), nitrogen oxides the potential to emit annually more than 500 (NOx), and particulates—emitted from point metric tons of sulfur dioxide or nitrogen oxides, sources.1 Far-field dispersion and acid rain depo- or 50 metric tons of particulate matter or any sition are governed by different principles and hazardous air pollutant. In many cases, simple 82 Airshed Models 83 calculations based on loads and air volumes may Key factors that affect these calculations, and be sufficient to provide an order-of-magnitude therefore the selection of dispersion models, estimate. However, the use of formal models are: should be considered for any project involving • Topography. The area surrounding the plant is large new plant or significant modifications. For characterized either as flat to gently rolling ter- major sources, the modeling should include the rain or complex terrain (having downwind lo- planned source or sources, as well as existing cations with elevations greater than stack sources in the same general area—within a ra- height). dius of 10 to 15 kilometers (km)—so that the • Land use. Whether the surrounding area is ur- cumulative effect of all the facilities on local ban or rural is important because urban areas air quality can be assessed. In some cases, typically have large structures and heat building-wake effects are important (for ex- sources that affect the dispersion of pollutants. ample, where release points such as stacks and In addition, the density of the population af- vents are less than 2.5 times the height of fects the numbers potentially impacted. nearby buildings), and more detailed modeling • Pollutant properties. Physical and chemical may be appropriate. properties of the pollutants influence their The models described in this document per- transport. For modeling sulfur dioxide within tain to “near-field” (less than 50 km from the 5 to 10 km of a source, no chemical transfor- point source) dispersion of sulfur dioxide, nitro- mations are assumed to occur. Beyond this gen oxides, and particulates. Such models esti- distance, an exponential decay function may mate the ground-level concentration of pollutants be useful. Most nitrogen oxide is emitted as in the air, which is then compared with ambient nitric oxide (NO), but in a matter of minutes, air quality standards or guidelines.2 Other mod- depending on the availability of ozone, it be- els that address photochemical smog are not de- comes nitrogen dioxide. The deposition of par- scribed in detail here. ticulates is a function of particle size and travel time. Factors Affecting Dispersion of Pollutants • Source configuration. The height and tempera- ture of the discharge and proximity to struc- The dispersion and ground-level concentration tures affect dispersion. Effective plume height of pollutants are determined by a complex inter- is the physical height of the stack adjusted for action of the physical characteristics of the plant factors that raise the plume (as a result of buoy- stack or other emission points, the physical and ancy or momentum) or lower it (as a result of chemical characteristics of the pollutants, the downwash or deflection). meteorological conditions at or near the site, and • Multiple sources. All dispersion models assume the topographical conditions of the surrounding that the concentrations at any one target site areas. are the arithmetic sum of concentrations from In general, three different calculations are each of the sources being examined. Note that needed to estimate the time-averaged concentra- it is the effects that are summed, not the emis- tion of pollutants at a location downwind from a sions rates or stack parameters. plant: • Time scale of exposure. The recommended mod- els make calculations for the basic time period • The plume rise above the stack must be estab- of one hour. Concentrations for longer time pe- lished (effective stack height). riods, such as 8 hours or 24 hours, are the arith- • The dispersion of the pollutants between the metic averages of the hourly concentrations of source and the downwind locations of inter- those time periods. Annual averages are com- est must be mathematically modeled on the puted by averaging hourly concentrations for basis of atmospheric conditions. a full year or by using models that use a fre- • The time-averaged concentration at ground quency distribution of meteorological events level must be determined. to compute an annual average. The recom- 84 IMPLEMENTING POLICIES: AIR QUALITY MANAGEMENT mended models have the necessary “book- well as at other specified distances, are deter- keeping” incorporated into the processing or mined. No consideration of wind direction is re- available as postprocessor routines. quired because the output represents the concentrations directly downwind. (This model Selecting an Appropriate Model is designed for average North American condi- tions; care should be taken in using it under dif- Model selection requires matching the key char- ferent climatic conditions.) acteristics of the site and the requirements of the Options in the model allow for the effects of evaluation with the capabilities of the model. a single dominant building and for terrain dif- Normally, expert advice is required in making a ferences between the source and the receptors. selection. As a general principle, modeling should To refine the estimates in complex terrain, a always begin with the simplest form possible, more sophisticated screening model is avail- moving to more complex approaches only where able in CTSCREEN, derived from CTDMPLUS their necessity and value can be demonstrated. (see below). At the most basic level, a crude mass balance can Although only a single source (stack) is con- indicate whether a new source is likely to pose a sidered, multiple nearby sources can be screened problem. Alternatively, a simple screening model, by using the sum of the emissions rates from the as described below, can provide a realistic esti- sources as the emissions rate for this single stack. mate of the order of magnitude of the impacts of This will yield an overestimate, since the effects a source. Situations involving multiple sources of geographic separation of the sources or the or varying terrain may require a more sophisti- points of maximum concentration will not have cated effort involving site-specific data collection been included. Scaling factors to estimate con- and more complex models. centrations for longer time averages (3 hours, 8 In some cases, more than one model may be hours, 24 hours, and even one year) are included required. For example, modeling of gaseous in the user’s guide. emissions and particulates in the Mae Moh Val- If the concentrations determined by using a ley, Thailand, required the use of one model for screening model are within the relevant guide- the valley floor, where the terrain is flat, and an- lines, no additional modeling should be neces- other for the mountains that surround the valley sary.