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During the first half of 1985, sulphur dioxide concentrations remained 

 less than both Montana and United States ambient air quality standards. 

 The maximum 1-hour concentration of 12.7 pphm was recorded at the 

 International Boundary site during January. This concentration is 

 approximately one-fourth of the Montana standard. The highest 24-hour 

 concentration of 0.9 pphm also occurred at the International Boundary site 

 during January. The Montana 24-hour standard is 10 pphm. A comparison of 

 sulphur dioxide concentrations with Class II Prevention of Significant 

 Deterioration increments demonstrates that the 3-hour concentration is the 

 averaging time which consumes the most increment. The 6.4 pphm 3-hour 

 average at the International Boundary site would consume 33 percent of the 

 available increment. All of the higher sulphur dioxide concentrations 

 noted above occurred with northwest winds, indicating that the 

 Saskatchewan Power Corporation generating facilities were the possible 

 source. 



The Montana and United States standards for total suspended particulates 

 were not exceeded at any of the three monitoring sites. The highest 

 24-hour concentration was 110 ug/m^ recorded on April 19, 1985, at the 

 Richardson site. The International Boundary site recorded the highest 

 geometric and arithmetic means at 26.5 and 29.6 ug/m^, respectively. The 

 levels of total suspended particulates observed during 1985 are low and 

 representative of rural Montana. 



