Spatial and temporal patterns of ozone at Great Smoky Mountains National Park and implications for plant responses

Abstract

Great Smoky Mountains National Park (GRSM) is the most visited National Park in the United States and has the highest levels of biodiversity of any park unit. It is a relatively small park (∼210,433 ha), but topographically complex, with an elevational range of 1757 m. The Park has historically been subject to elevated levels of pollutants, including sulfur dioxide (SO2), ozone (O3), and nitrogen oxides (NOx). Ozone trends are analyzed from 1989 to 2016 for six monitoring sites in and adjacent to GRSM and ranging in elevation from 564 m to 2030 m. Low-elevation sites have minimum O3 concentrations in early morning and maximum concentrations in mid-to late afternoon. High-elevation sites have flatter profiles, smaller diurnal ranges, and maxima that occur in early evening or at night. The W12624-h exposure index increases with elevation up to 823 m, after which it plateaus. W12624- h exposures increased between the years 1989–∼2002, and have substantially decreased afterwards. The highest 1-h concentration ever recorded in the Park was 135 ppb, which occurred on 25 August 1998. At most sites the maximum 3-month W12624-h exposures have shifted from mid-summer to spring (Apr–Jun). Decreases in exposure result primarily from reduction of hourly averaged O3 concentrations ≥ 60 ppb. Ozone episodes (3 or more consecutive hours when O3 ≥ 60 ppb) have decreased in frequency, magnitude, and duration from 1999 to 2016. Decreases in W126 exposures are correlated with lowered NOx emissions from regional TVA power plants and appear to be a direct result of the State Implementation Plan (SIP) call associated with the Clean Air Act Amendments, resulting in the cleanest air in GRSM over the period of record. Lower cumulative W126 O3 exposures and reduction in high O3 concentrations appear to be having beneficial effects on the vegetation within the Park. Keywords: W126 exposure index, Trends, Elevation, Vegetation