AIR DISPERSION MODELLING IN COASTAL AREAS WITH ROUGH TERRAIN, USING CALPUFF PRIME

Transcription

1 AIR DISPERSION MODELLING IN COASTAL AREAS WITH ROUGH TERRAIN, USING CALPUFF PRIME Marcia C. Parsons, Fracflow Consultants Inc., 154 Major s Path, St. John s, NL, Canada, A1A 5A1, Faisal Khan, Memorial University of Newfoundland, St. John s, NL, Canada, and John E. Gale, Fracflow Consultants Inc., 154 Major s Path, St. John s, NL, Canada, A1A 5A1. ABSTRACT To study the effects of exit temperature, exit velocity, and stack height on contaminant dispersion and concentration, the revised version of the CALPUFF air dispersion model was used to predict the dispersion of total suspended particulates (TSP) and sulphur dioxide (SO 2 ) emissions released from an industrial source in a coastal region with rough terrain. The CALPUFF model accounts for the influence that large water bodies have on atmospheric pollutant dispersion, accounts for plume rise and flow around building obstacles, and is suitable for use in areas with complex terrain. This study shows that stack heights and exit velocity are controlling factors in pollutant dispersion. An increase in the stack height and exit velocity, resulted in a decrease in maximum pollutant concentrations. When the height of the stack is increased, the effective plume rise is enhanced and this promotes buoyancy induced dispersion. An increase in exit velocity increases plume momentum leading to higher mixing and turbulent dispersion. An increase in exit temperature did not have a significant effect on concentrations near the source, but did result in a decrease in pollutant concentrations at a distance from the source due to the thermal buoyancy effect. INTRODUCTION Industrial air emissions pollute the atmosphere and have adverse effects on the environment including, plant life, human health, water bodies, and soils. As the awareness of the harmful effects of industrial air emissions increases, more resources are being allocated from government and private institutions to study 1) the effects of air emissions, 2) the control of these emissions from the various industrial sources, 3) the prediction of the dispersion patterns and pollutant concentrations using air dispersion models, and 4) the development of regulations governing the release of emissions. Ecology and human settlements located in the vicinity of air polluting industries have been adversely affected in the past. Air pollution is known to cause both short-term and long-term illnesses. Short-term health effects include irritation to the eyes, throat and lungs, allergic reactions, and upper respiratory infections (i.e., bronchitis and pneumonia). Long-term effects may include lung cancer, heart disease, chronic respiratory disease, damage to the brain, nerves, liver, and kidneys. The effects of the air pollutant(s) on human health depend on pollutant concentration and exposure, as well as duration, age and health factors. 1

2 Industry is continually aiming to reduce maximum concentrations at sensitive locations by reducing emission rates and optimizing exit stack conditions. Air dispersion modeling helps relate the emission rates and stack configurations to the corresponding pollutant concentrations in areas surrounding the source. Air dispersion modeling allows industry to determine the maximum emissions that they can release into the environment and remain in compliance with ambient air quality standards. This helps industry control air quality and evaluate the effects of improvements on technology (e.g., increasing exit velocity, external baffles, and others) on the dispersion and maximum concentration of the contaminants. In order to study the effects of exit gas velocity, stack height, and exit gas temperature on the dispersion and the concentration of air emissions at receptor locations, an air dispersion model was used to predict the dispersion of total suspended particulates (TSP) and sulphur dioxide (SO 2 ) emissions released from an industrial source. The air dispersion model used in this modeling study is a revised version of CALPUFF (Earth Tech, 2003). This document presents the findings of long-term TSP and SO 2 dispersion in a coastal region with rough terrain, incorporating CALPUFF PRIME into the model run. AIR DISPERSION MODELS Air dispersion models are a useful tool for assessing environmental air pollutant concentrations. These models are continually advancing and being developed to better predict the dispersion of industrial air emissions and the resulting pollutant concentrations in surrounding areas. By predicting the pattern of pollutant transport, the areas of high pollutant concentration, and pollutant concentrations in sensitive areas surrounding an industrial source, air dispersion models are helping determine environmental exposure to the pollutants. There are a number of air dispersion models available for predicting concentrations of air pollutants including ISC3 (Industrial Source Complex Model), AERMOD, and CALPUFF. The ISC3 model is based on the straight-line Gaussian dispersion principle and can be used in situations with flat to moderately complex terrain (USEPA, 1995). AERMOD is based on a bi-gaussian distribution and can be used in situations with flat to complex terrain. AERMOD incorporates two preprocessors, AERMET and AERMAP that process meteorological data, and terrain and receptor data, respectively. The outputs from these preprocessors are then used in AERMOD (Lakes Environmental, 2003). CALPUFF is a complex Lagrangian Gaussian puff dispersion model that takes into account many factors that enhance the accuracy of this model, relative to other models, in predicting ground level concentrations of contaminants. CALPUFF is suitable for use in study areas with complex terrain (Earth Tech, 2003). The CALPUFF modeling system is essentially a series of three linked computer models, which account for meteorological effects, atmospheric dispersion, and concentration and deposition over time. The computer models used to integrate and process these data are CALMET, CALPUFF, with the output files from CALPUFF being sorted by CALPOST. Since CALPUFF models continuous puffs of pollutants from a source and as the wind direction and velocity change from hour to hour, the 2

3 direction of movement of the individual puffs changes according to the new wind direction (USEPA, 1998). The air dispersion models are continually improving and revisions and updates are being made to current models. In recent studies, the CALPUFF model was further refined and is now able to model groundlevel concentrations of contaminants, particularly in coastal regions, with better accuracy. In a study by Fisher et al. (2003) three air dispersion models (ISC3, AERMOD, and CALPUFF) were compared. That study showed that large water bodies influence atmospheric pollutant dispersion and the CALPUFF model is the only model that is able to account for these effects. The other models have other limitations along with their inability to account for the effects of water bodies. For instance, ISC3 cannot process any terrain elevations that exceed the height of the source and tends to overestimate concentrations when the study area has complex terrain (Fisher et al., 2003). STUDY AREA The industrial emission source in this study is located in a coastal area with rough terrain in Eastern Canada. To the immediate west of the source is a small port and beyond that is a salt water bay. Directly east of the source, is steep terrain with a slope of approximately 0.4. The nearest residential area is located approximately 3 kilometres northwest of the emission source. North of the source and the residential area, the land is forested with an overall planar aspect, but with a moderate relief. An international airport, which has a meteorological station, is located approximately 3 kilometres away. This meteorological station provided the hourly upper air data and surface data required by CALMET. The upper air data include values for pressure, elevation, temperature, wind speed, and wind direction. The surface data include values for wind speed, wind direction, air temperature, ceiling height, cloud cover, surface pressure, and relative humidity. In 2003, the average monthly temperatures ranged from 8.4 degrees Celsius in February to 16.9 degrees Celsius in August (Environment Canada, 2004). Average monthly precipitation ranged from 52.4 mm in May to mm in March. During the months of December, January, February, and March, the ground was snow covered. The wind frequencies for the study area were graphed using WRPLOT View (Lakes Environmental Software). The wind rose for the model period is shown in Figure 1. The most frequent winds for the study period are from the west-southwest, followed closely by east-northeast The model domain selected was 11.2 kilometres in the x-direction (west-east) and 8.4 kilometres in the y-direction (north-south). The entire model grid has a grid spacing of 100 metres. The elevations of all of the grid points were obtained from twenty-four 1:2500 scale digital maps of the study area. In this air dispersion model, the 14 default CALPUFF land use categories were used (10, 20, -20, 30, 40, 51, 54, 55, 60, 61, 62, 70, 80, and 90; see Table 1 for details). Each land use type has six assigned values, one each for surface roughness, albedo, Bowen ratio, soil heat flux parameter, anthropogenic 3

4 heat flux, and leaf area index. The surface roughness is a measurement of the average vertical relief and small-scale irregularities of the terrain surface. The surface roughness is low for the areas in the study area classified as water bodies and is high for the areas classified as urban or built-up land and forest. The albedo is the fraction of light that is reflected by a given surface. For instance, the albedo of a snow-covered area is much higher than that of a paved parking lot. The leaf area index is the ratio of the total area of all leaves on a plant to the area of ground covered by the plant. All areas that have the potential to have plant growth have leaf area indexes assigned to them. A value of zero is assigned to the water bodies. The leaf area index is as high as seven (7) in the areas identified as forest land. MODEL RUNS To study the effects of exit temperature, exit velocity, and stack height on contaminant dispersion and concentration, a total of four model runs were completed. Run 1 is the base case, Run 2 has an increased stack height, Run 3 has an increased exit velocity, and Run 4 has an increased exit temperature (Table 2). The emission rate of SO 2 and TSP remained constant in all of the model runs. Please note that the emission rates and source characteristics, as well as stack and building heights are hypothetical to preserve client confidentiality. Building downwash was considered in each of the model runs. Building downwash effects are a significant factor in modeling the dispersion of air emissions from stacks with surrounding buildings. The approximate height of the emission stack is 55.5 metres above ground surface. The height of the building structure to the south of the stack is approximately 70 metres. The other surrounding building structures are not as high as the stack, but are of significant heights. CALPUFF PRIME was incorporated into all of the model runs. PRIME, an acronym for Plume Rise Model Enhancements, is a module that incorporates calculations into the CALPUFF model that account for plume rise and flow around building obstacles. For each model run, a total of 9,408 gridded receptors and 4 discrete receptors were identified. Discrete Receptor #1 is located in the middle of the port to the southwest of the source. Discrete Receptor #2 is located to the immediate east of the source. Discrete Receptor #3 is located in the residential area northwest of the source. Discrete Receptor #4 is located to the east of the source on elevated terrain, approximately 2 metres above the base case stack height. MODEL RESULTS AND DISCUSSION The CALPUFF model predicted SO 2 and TSP dispersion in two predominant directions: northeast and southwest, reflecting the dominant wind directions apparent in the windrose diagram (Figure 1). The maximum 8-hour average SO 2 and TSP concentrations were located to the immediate northeast of the source in each of the model runs. The CALPUFF model predicted the same general dispersion of SO 2 4

5 and TSP in all runs. The SO 2 and TSP concentration isopleths for Run 1 are plotted in Figures 2 and 3. The isopleths dispersing in the southwest direction are widely spaced (demonstrating high dispersion) and the isopleths dispersing in the northeast direction are very closely spaced (demonstrating low dispersion). The close spacing of the isopleths to the northeast of the source is a result of the steep terrain in this direction. The terrain is acting as a barrier to pollutant dispersion causing the reflection of the pollutant plume and the pollutant accumulation in the area between the source and the steep slope. The base case model run (Run1) predicted higher maximum 8-hour average concentrations of SO 2 and TSP, to the immediate northeast of the source, than the other three model runs predicted. The predicted concentration of SO 2 and TSP in Run 1 was µg/m 3 and µg/m 3, respectively (Figures 2 and 3). The maximum 8-hour average concentrations decreased with increasing stack height and exit velocity. The stack height was increased by 20 metres in Run 2 from the base case, resulting in a µg/m 3 and µg/m 3 reductions in SO 2 and TSP, respectively. An increase in the stack height enhances effective plume rise and promotes buoyancy induced dispersion. Velocity was increased in Run 3 by only 5 m/s from the base case, resulting in a µg/m 3 and a µg/m 3 reduction in SO 2 and TSP, respectively. This is mainly due to increased plume momentum leading to higher mixing and turbulent dispersion. An increase in exit temperature had little effect on the maximum 8-hour average concentrations of the two pollutants. Exit temperature was increased by almost 47 percent, but the maximum concentration of SO 2 and TSP was only reduced by 0.06 µg/m 3. This clearly demonstrates that stack heights and exit velocity are among the primary factors that control dispersion at the source. The maximum concentration of pollutants for the 8-hour averaging period at the 4 discrete receptors in each model run are presented in Table 4. In the port (Discrete Receptor #1), located approximately 1.5 km to the west of the source, the increase in stack height, exit velocity, and exit temperature resulted in decreases of 12 µg/m 3, 4 µg/m 3, and 10 µg/m 3, respectively, in the 8-hour average concentration of SO 2 and TSP. Discrete Receptor #2 is located approximately 0.3 km east of the source. The increase in stack height and exit velocity resulted in decreases of 5 µg/m 3 and 8 µg/m 3, respectively, in the 8-hour average concentration of SO 2 and TSP. There was no change in the pollutant concentrations with an increase in stack temperature at Discrete Receptor #2. The residential area (Discrete Receptor #3) is located approximately 3.5 km northwest of the source. In this area, the increase in stack height and exit velocity resulted in an approximate decrease of 3 µg/m 3 and 2 µg/m 3, respectively, in the 8-hour average concentration of SO 2 and TSP. An increase in exit temperature resulted in an approximate 4 µg/m 3 decrease in SO 2 and a 3 µg/m 3 decrease in TSP. Discrete Receptor #4 located approximately 0.5 km to the east of the source on elevated terrain. The increase in stack height, exit velocity, and exit temperature resulted in decreases of approximately 8 µg/m 3, 6 µg/m 3, and 2 µg/m 3, respectively, decrease in the 8-hour average concentration of SO 2 and TSP. 5

6 From the above analysis, it is evident that an increase in exit temperature does not have a significant effect on concentrations near the source, but does tend to result in a decrease in pollutant concentrations at a distance from the source. This is due to the thermal buoyancy effect whose effect appears at a distance from the source. For instance, when the exit temperature was increased by almost 47 percent, there was an approximate 10 µg/m 3 decrease in the maximum pollutant concentrations 1.5 km away from the source, and no change in pollutant concentrations at a receptor 0.3 km away from the source. APPLICATIONS By relating the emission rates, exit stack characteristics, and site specific conditions to the corresponding pollutant concentrations in areas surrounding the source, air dispersion models can be used for several purposes by regulatory bodies and industry. Some of these purposes are summarized below: Predict the dispersion of industrial air emissions and the resulting pollutant concentrations in areas surrounding an industrial source(s); Identify the areas of high pollutant concentration; Determine pollutant concentrations in sensitive areas surrounding an industrial source; Determine the environmental exposure to the pollutants; Government is able to assess ambient air quality compliance by industry using air dispersion modeling; Industry can use air dispersion models to predict the potential impact of modified stack configurations on air dispersion patterns and concentrations; Industry can use air dispersion models to predict the potential impact of new fuels on air dispersion patterns and concentrations; Allow industry to determine the maximum amounts of contaminants (emission rates) that they can release into the environment and still remain in compliance with ambient air quality standards; and And more importantly, assess ways to reduce overall emission rates in a cost effective manner. CONCLUSIONS Air dispersion modeling is a useful tool for assessing the environmental impact of an industrial source on surrounding areas. While there are several air dispersion models available, the CALPUFF model accounts for the influence that large water bodies have on atmospheric pollutant dispersion, accounts for plume rise and flow around building obstacles, and is suitable for use in areas with complex terrain. The study area has rough terrain and the terrain s influence on pollutant dispersion was evident in the widely spaced isopleths in the southwest direction, demonstrating high dispersion, and the closely spaced isopleths in the northeast direction, demonstrating low dispersion. In the northeast direction, the steep 6

7 terrain is acting as a barrier to pollutant dispersion causing the reflection of the pollutant plume and the pollutant accumulation in the area between the source and the steep slope. Stack height and exit velocity are among the main dispersion controlling parameters. Exit temperature does not have as great an effect on pollutant dispersion as stack height and exit velocity. An increase in the stack height enhances effective plume rise and promotes buoyancy induced dispersion, resulting in a decrease in the maximum pollutant concentration. An increase in exit velocity decreases the maximum pollutant concentration as an increased plume momentum leads to a higher mixing and turbulent dispersion. It is evident in this study that an increase in exit temperature does not have a significant effect on concentrations near the source, but does tend to result in a decrease in pollutant concentrations at a distance from the source. This is due to the thermal buoyancy effect whose effect appears at a distance from the source. Air dispersion models, such as CALPUFF, can be used for several purposes by regulatory bodies and industry by relating the emission rates, exit stack characteristics, and site specific conditions to the corresponding pollutant concentrations in areas surrounding the source. This study demonstrates how industry can use air dispersion models to predict the potential impact of modified stack configurations on air dispersion patterns and concentrations in areas where the maximum concentrations will impact sensitive receptors. 7

8 Table 1 Default CALMET land use categories and associated geophysical parameters used in the present study (Earth Tech 2003). Land Use Type Description Surface Roughness (m) Albedo Bowen Ration Soil Heat Flux Parameter Anthropogenic Heat Flux (W/m 2 ) Leaf Area Index 10 Urban or Built-Up Land 20 Agricultural Land - Unirrigated -20 Agricultural Land - Irrigated Rangeland Forest Land Small Water Body Bays and Estuaries Large Water Body Wetland Forested Wetland Nonforested Wetland Barren Land Tundra Perennial Snow or Ice Note: Negative values indicate irrigated land use. 8

9 Table 2 Emission stack data for the four model runs. Model Run Stack Height (m) Base Elevation (m) Stack Diameter (m) Exit Velocity (m/s) Exit Temp. (K) SO 2 Emission Rate (g/s) TSP Emission Rate (g/s) Run 1 (base case) Run Run Run Table 3 Maximum 8-hour average concentrations of SO 2 and TSP for each model run. Model Run SO 2 TSP Run Run Run Run Table 4 The maximum 8-hour average concentrations of SO 2 and TSP at the discrete receptors for each model run. Discrete Receptor Number SO 2 Run 1 Run 2 Run 3 Run 4 TSP SO 2 TSP SO 2 TSP SO 2 TSP

10 REFERENCES CITED Earth Tech, CALPUFF Modeling System. Earth Tech: Official CALPUFF Web Site. Available online at: (Accessed on May 2, 2004). Environment Canada, Climate Data Online. Available online at: (Accessed on May 2, 2004). Fisher, A. L., M. C. Parsons, S. E. Roberts, P. J. Shea, F. I. Khan, and T. Husain, Long-term SO2 Dispersion Modeling Over a Coastal Region. Environmental Technology, Volume 24, pp Lakes Environmental, AERMOD Tech Guide. Available online at: (Accessed on May 14, 2004). United States Environmental Protection Agency (USEPA), A comparison of CALPUFF with ISC3. United States Environmental Protection Agency, Office of Air Quality Planning and Standards, Research Triangle Park, NC. United States Environmental Protection Agency (USEPA), User guide for the industrial source complex (ISC3) dispersion models. USEPA, Office of Air Quality Planning and Standards, Emissions, Monitoring and Analysis Division, Research Triangle Park, North Carolina. 10

11 Fracflow Consultants Inc.

12 Figure X-Direction(km) Maximum eight-hour average ground-level SO2 concentrations predictedby the CALPUFFairdispersion model for the industrial source and surrounding area (concentrations in grams per cubic metres.1g=1,000,000 micrograms). Map grid is orientednorth-south and East-West.

13 Figure X-Direction (km) Maximum eight-hour average ground-level TSP concentrations predicted by the CALPUFFairdispersion model for the industrial source and surrounding area(concentrations in grams per cubic metres.1g=1,000,000 micrograms). Map grid is oriented North-South and East-West.