GIS-assisted Dispersion of SO₂ in the Industrial Regions

MATERIALS AND METHODS

Locating air pollutant sources in the study area: Zarghan is located 25 km northeast of Shiraz, nearby Shiraz-Tehran highway. The town is also surrounded by many different air pollution sources. Industrial complexes are located about 10 km from Zarghan such as, Shiraz oil refinery, Abbarik industrial park with more than 35 industries, Hafez ceramic industry, Loab Iran industry, Fars and Sina chemical industries. In addition to these industrial complexes, the rocky cliff with a height of 300 m in the east side of the town blocked the air flow through the town.

Determination of SO₂ by diffusive sampler: We used a diffusive sampler to adsorb SO₂ in the air by a tube consisting of adsorbent material. SO₂ entered the adsorbent tube by molecular diffusion. The tube is 20 mm in diameter and made of polypropylene. To reduce the effects of wind and rain, the sampler were fixed into a fiberglass container by a metal clamp as shown in Fig. 1. The adsorbent samplers were installed at the elevation of 3-4 m from the ground. Air sampling was conducted for a period of 17 days in January 2012. After the adsorption period, the sampler tubes were sealed and returned to the laboratory for further analysis.

After collecting the specimens, they were sent to Pasam Company in Switzerland for determination of SO₂. The extraction was done by a mixture of potassium carbonate and glycerin. Ion chromatography was used to analyze SO₂.

SO₂ sample collecting locations: Since many different sources of air pollution are located in the vicinity of the residential area of Zarghan, the boundary conditions sampling points were selected by mesh. The Geographical coordinates, names, and reasons for choosing the sampling points are given in Table 1. Due to the small size of the study area, 10 points were selected and one sample was collected at each point.

Drawing GIS maps: An image of the coverage area was obtained by Google Earth software. Geographical coordinates of 4 points of suitable dispersion were determined by the software and used in an Excel file as the ground reference (Fig. 2).

Using ArcGIS technique, the image was processed as the ground reference and the result saved in TIFF format with a pixel size of 5 m. The data obtained from sampling of SO₂ were interpolated using the passive sampling method with different methods such as inverse weighted distance to the power 2 (IWD)² , natural nearest neighbor.

Fig. 1(a-b):	Clamp and SO2 adsorbent sample

Fig. 2:	Coordinates of the reference points

Table 1:	Sampling locations at 10 different stations in the industrial regions of Zarghan, Universal Transverse Mercator (UTM) coordinates and reasons for the selection of that location

Fig. 3:	Maps of Zarghan and the wind rose at the time of air sampling Nearest neighbor method B-Kriging method C-IWD method

File Format (TIFF) is a raster image format with a pixel size of 5 m for each specimen was prepared (with at least 10 sample points). All interpolated layers were then cut into the size of sample point’s area to perform interpolation for all layers.

RESULTS

Table 2 showed the results of SO₂ measurements in different sampling stations in the industrial regions of Zarghan. According to data, the maximum and minimum concentration of SO₂ are related to Shiraz oil refinery (63.3 μg m^-3) and old cemetery (4 μg m^-3), respectively. Due to the importance of wind direction, the maps of the wind rose of the sampling season with the map of Zarghan are shown in Fig. 3. As shown in Fig. 4, the geographic coverage area of SO₂ concentration has been studied using different methods of Nearest Neighbor (NN), Inverse Distance Weighted IWD, and Kriging. SO₂ pollution maps were prepared using passive sample interpolation.

Fig. 4(a-c):	GIS results using different interpolation methods (a) Nearest neighbor, (b) Kriging and (c) IWD methods Sampling stations in the industrialized Zarghan area

Table 2:	SO2 concentration (μg m-3) at different sampling stations in the industrial regions of Zarghan

DISCUSSION

A comparison of measured SO₂ concentration with the values recommended by the WHO air quality guidelines:

Fig. 5:	SO2 concentration at different sampling stations in the industrialized Zarghan area

Figure 5 shows SO₂ concentration in different sampling stations. According to this Figure, SO₂ concentration in Zarghan residential areas is within the acceptance limit.

GIS data analysis: As the GIS figures showed, the oil refinery and the highway are among the most important sources of SO₂ pollutions. In general, the concentration of SO₂ in the area near the oil refinery was much higher than the area near the highways. The prevailing wind direction was not toward Zarghan at the time of sampling; although, it could affect Dudej area which is located at a distance of about 3 km from the oil refinery. According to GIS figure, the industrial park did not have a significant effect on sulfur dioxide levels in Zarghan. The SO₂ concentration in the industrial park having a distance of 4000 meters from the oil refinery was 24 μg m^-3 which is still much higher than the concentration in Dudej area. Interestingly, SO₂ concentration at the distance of 200 m from the highway close to the industrial park was significantly higher than the one at Kargar station near to the highway. As a result, the industrial park had intensified air pollution caused by the oil refinery by an amount of 5 μg m^-3. Despite the prevailing wind in the highway directed toward Zarghan, it has affected a maximum radius of 150 meters around. The SO₂ concentration in Dudej was in the upper limit of 24-hour guideline value, while it was within the standard limits in Zarghan residential areas. The reason for higher level of SO₂ in Dudej was due to its closer distance to the oil refinery. According to Figures, the nearest neighbor interpolation method showed the linear effects of pollution on the highway clearly. Figures obtained by interpolation method revealed the expansion of pollutions in the area. As shown, Zarghan has been located in the green zone and SO₂ could not be accumulated in the town despite the existence of wind. According to the wind rose map, during December the prevailing direction was not toward Zarghan. It would be possible to record higher level of SO₂ concentration in Zarghan if samples were collected during windy months directed toward the city. On the contrary, the pollution due to the highway was affected by prevailing winds blowing toward the city, and as seen in the GIS images, the highway pollution could not affect the city.

CONCLUSION

The contribution of different sources and dispersion radius of SO₂ in the study area were determined by the passive sampling method and interpolation tools in ArcGIS technique. The concentrations of SO₂ around Shiraz oil refinery and highway were high. The results indicated that SO₂ concentration was not exceeded the standard limit in the residential area and the role of the local highway and industrial park was not significant during December. It is highly recommended to determine SO₂ concentration in the area during other months of the year, especially October, which the prevailing wind directed toward Zarghan.

ACKNOWLEDGMENTS

The authors would like to thank Islamic Azad University- Estahban Branch and Zarghan municipality for financial support. We wish to acknowledge Shiraz University of Medical Sciences for supporting the laboratory analysis. Special thanks are given to municipal and Islamic city council authorities for their close cooperation.