"INTRODUCTION Under the Prevention of Significant Deterioration (PSD) provisions of the Clean Air Act regulations1/ the U.S. Environmental Protection Agency (EPA) has established by regulation maximum allowable increases in ambient SO2 and total suspended particulate matter (TSP) concentrations that are applicable to major new facility developments or modifications of existing facilities. These regulations are apt to pose severe constraints on oil and gas development whenever such facilities are in close proximity to Class I areas (areas where it has been deemed desirable to maintain pristine air quality@ such as wilderness areas and national parks) with their associated very small increments (see Table I-l). Even the larger allowable increments of Class II areas@ in which these facilities are most likely to be located@ may prove to be an impediment to development (again see Table I-l). To determine if a proposed development will exceed these allowable increments@ and thus be denied a permit@ an air quality modeling study (and analysis) is typically required. Coal gasification and liquefaction facilities as well as shale oil facilities can be expected to be similarly affected. Additional examples of potentially affected facilities are discussed in footnote/16 on page II-4. As an example@ this report illustrates the siting limitations@ due to PSD requirements@ associated with the development of a hypothetical natural gas plant having a 99.0 percent efficient desulfurization facility which involves sulfur removal with tail-gas clean up and emission of an oxidized product. The emission of sulfur dioxide (SO-) to the atmosphere resulting from the removal of hydrogen sulfide (H^S) from ""sour"" natural gas and the subsequent impact on ambient S0? concentrations must be analyzed before a construction permit can be issued. Two hypothetical facilities have been considered: a large facility with a processing capacity of 250 MMscfd (million standard cubic feet per day) and a medium-sized facility with a 125 MMscfd capacity. 1/ Clean Air Act $$160-169. 42 U.S.C. $$7470-7491 (sUPP. 1978) (40 40 cfr $52.21)"