Recent air quality trends in the Great Smoky Mountains

Air quality trends in the Great Smoky Mountains are reviewed in light of recent nearby trends in utility SO₂ and NO_x emissions. Emission reductions mandated by Title IV of the Clean Air Act were expected to result in significant improvements in atmospheric sulfate levels (both fine particulate and wet deposition), especially in the PSD Class I area that is the Great Smoky Mountains National Park (GSMNP). In addition, there was an expectation that some measurable reductions in nitrate deposition and ozone levels might also be realized. However, data collected since 1988 by the NTN/NADP (National Trends Network/National Acid Deposition Program), IMPROVE (Interagency Monitoring of Protected Visual Environments) and CASTnet long-term monitoring networks have shown mixed results.

Regionally, SO₂ emissions are down since 1994 as Phase I of the Title IV emission cuts took effect, whereas Phase II emission reductions have yet to be implemented. The emissions decline to date has not been homogeneous, however, as utilities were given flexibility in implementing SO₂ controls across entire power generation systems. Conventional wisdom in 1990 was that the relatively long lifetimes associated with sulfate production and removal would minimize the risk that non-homogeneous sulfur emission reductions could lead to major disparities in the resulting benefits.

Title IV benefits from NO_x reductions were expected to be more localized due to the shorter space and time scales associated with NO_x-VOC-O₃ photochemistry and removal processes. Thus, site-specific benefits were not as certain as those expected from SO₂ reductions. In addition, NO_x emissions increases from other sources have the potential to offset reductions from utility fossil fuel boilers. Near the GSMNP, measured annual and seasonal trends in sulfate wet deposition are generally downward since the early 1990s, in line with previous expectations. However, fine particle sulfate (FPS) levels show little in the way of an annual trend, and have actually increased slightly during the summer since the early 1990s. There appears to be little correlation between major SO₂ emission reductions and FPS in the vicinity of the GSMNP. Likewise, nearby utility NO_x reductions are not reflected in either the nitrate wet deposition or O₃ trends for the GSMNP, possibly reflecting NO_x emissions growth from nearby non-utility sources. Taken together, the recent trends in the GSMNP suggest influences on air quality that are not expected to be entirely mitigated by Title IV emission reductions, nor by the recent EPA-mandated state implementation plan (SIP) for multi-state NO_x emission reductions.