CHAPTER 3: METHODOLOGICAL FRAMEWORK
3.3 Calpuff Model
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3.2.3 Limitations of TAPM
TAPM is time consuming and requires more expertise and resources to run than Gaussian models. It has a limited run length of one year which restricts the use of TAPM for this study which incorporates three years of meteorological data (Hurley et al., 2005). The modelling domain is restricted to 1000 km by 1000 km as TAPM does not consider the curvature of the earth and assumes a uniform horizontal modelling domain. This restriction affects the accuracy of TAPM in representing extreme weather conditions. Meteorological conditions such as wind, temperature and humidity at heights over 5000 m are not represented accurately. It also assumes that cloud processes occur in a single grid spacing of 3 km or less and does not allow for large scale cloud processes linked with extreme weather conditions. Although TAPM can be used for uneven terrain, it cannot represent discontinuities in terrain height, for example cliffs (Hurley, 2005a; 2005b). TAPM has not been used to undertake air pollution dispersion modelling for regulatory purposes in Richards Bay. In addition, TAPM is not on the list of preferred USEPA models that can be used for regulatory purposes (United States Environmental Protection Agency, 2005).
3.3 Calpuff Model
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accurate for distances in the 50 to 300 km range (United States Environmental Protection Agency, 2003).
The Lagrangian principle assumes that emissions travel as a series of puffs. The puffs can be modelled in areas with complex terrain, low and calm wind speeds and for transport over water in coastal areas. Calpuff makes provision for point, area, line, and volume sources which enables cumulative impact assessments to be conducted in areas with multiple emission sources (Earth Tech, 2000a). Calpuff is however limited to the shortest timescale of 1-hour averages due to being influenced by turbulence (Holmes and Morawska, 2006). The puffs emitted from a stack point are modelled separately as each puff changes with the wind direction and speed from hour to hour. The concentration of pollutants is calculated as each puff passes over a receptor point (Zhou et al., 2003). Due to its capability of integrating puffs the model saves time while maintaining its accuracy (Song et al., 2006).
The Calpuff model is an internationally approved model that is used by the USEPA for regulatory purposes (United States Environmental Protection Agency, 2005). The Calpuff model has gained regulatory approval for air dispersion modelling of medium to long- range transport of pollutants (Heydenrych et al., 2005). Due to the capability of Calpuff to evaluate both short and long range pollutant transport, the impacts of pollutants can be measured around an industry’s fence line to the nearest populated areas situated kilometres away.
Calmet is a meteorological model that generates 3-dimensional wind fields. Calmet requires geophysical data including gridded fields of terrain elevation and land use
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categories. The Calmet model uses atmospheric temperature, wind speed, wind direction, cloud cover, relative humidity, and atmospheric pressure as input data while Calpost processes the Calmet output files for plotting on modelling domain maps (Earth Tech, 2000b).
3.3.2 Rationale for using the Calpuff Model
The Calpuff model is one of the most widely used USEPA models that produces satisfactory results for regulatory purposes. It is freely available from the USEPA and operates with a user friendly windows interface with on-line help. There is an input parameter error-checking screen that lists all the errors detected by the Calpuff Graphical User Interface before and after the model runs are undertaken (Earth Tech, 2000a). The validation studies conducted for the Calpuff model are detailed in the USEPA guideline documents (United States Environmental Protection Agency, 2003; 2000b; 2005). Calpuff has been used for regulatory purposes for example, to evaluate the emission reduction measures at an old fossil-fuel power plant in the US state of Illinois due to its capabilities of long range transport and to handle complex 3-dimensional wind fields. Calpuff was also used to estimate primary and secondary particulate matter concentrations from the power plant. Calpuff was selected as the model for the study due to its USEPA regulatory use (Levy et al., 2002). In addition, Calpuff was selected for use in Richards Bay due to its capability to model line sources which are characteristic of the two aluminium smelters in Richards Bay namely, Bayside and Hillside (United States Environmental Protection Agency, 2005).
In recent studies conducted by the CSIR (CSIR, 2004; 2005) and Airshed Planning Professionals study (Airshed, 2006a); the Hazard Area Wiz Kit (HAWK) dispersion model
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was used to simulate SO2 concentrations in the uMhlathuze Municipality. The HAWK model was developed in South Africa by an environmental consultancy company and has similar capabilities to that of Calpuff. The HAWK model is a Lagrangian puff model that can also simulate SO2 emissions from multiple sources, can operate under calm winds and over complex terrain, with a grid resolution varying from 50 m to 500 m. The averaging periods of HAWK includes 10-minutes; 1-hour; 24-hour and annual average periods, which differs from Calpuff which does not produce a 10-minute average concentration (CSIR, 2004; 2005 and Airshed, 2006a). The use of Calpuff will be a valuable comparison with other model runs in the uMhlathuze Municipal area.
Although the HAWK model is not approved for regulatory purposes, it was used in the CSIR (2004), CSIR (2005) and the Airshed (2006) studies due to it being the locally preferred model for all dispersion modelling exercises by the RBCAA. Additionally, the HAWK model has been validated twice by the model developer in the Richards Bay area (CSIR, 2004; 2005). The two validation reports referred to in the CSIR studies were peer reviewed in 2005 by Professor Eugene Cairncross. It was found that there were significant differences between the HAWK modelled results and measured results in Richards Bay.
Further, the validation studies were viewed as biased due to them being carried out by the developers of the HAWK model (Cairncross, 2005a; 2005b). These findings again justify the use of an alternate model, namely Calpuff.
3.3.3 Modelling Options Selected
The simulation period for the study was from 2002 to 2004, with meteorological data processed for the same period. A receptor elevation of 1 m above ground level was selected as this falls within a range acceptable for the measurement of ground level
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pollutant concentrations. Five modelling scenarios were selected for this study. Scenario 1 was the baseline or control scenario comprising current SO2 emissions data; Scenario 2 was the worst case scenario comprising Registration Certificate or permitted emissions data.
The permitted emissions were thereafter reduced by 25%, 50% and 75% across all sources to assess ambient SO2 concentrations around sensitive receptors and are called Scenarios 3 to 5 respectively. The averaging times selected were 1-hour, 24-hour and annual averaging periods.