Ozone trends in Great Smoky Mountains National Park over the past two decades: Good news for plants and evidence that the Clean Air Act works
Great Smoky Mountains National Park (GRSM) is the most visited national park in the country, but has a history of extensive ozone pollution. Previous studies have documented adverse effects of tropospheric ozone (O3) on plants in the Park. Recently, reports have documented reductions in ozone concentrations ([O3]) in the eastern United States, yet no comprehensive survey of long-term O3 trends in GRSM has been reported. We report here diurnal and seasonal trends in ozone, obtained from the U.S. EPA for five sampling locations in GRSM and one adjacent to GRSM over the period 1989 to 2012. Sampling locations spanned an elevational range from 564 m at Cades Cove to 2030 m at Clingmans Dome.
Low elevation sites show an early morning minimum for [O3] around 6 am and a peak between 1 and 4 pm. High elevation sites have flatter profiles with minimum [O3] occurring between 8 and 11 am and maxima most often at night. The W126 exposure index increases with elevation up to 800 m, after which it plateaus. There is some indication that sites in NC have lower W126 and maximum [O3] than those in TN. This may reflect scavenging, as winds move primarily west to east, but also dilution when O3 mixes with air in the free troposphere as it moves over GRSM. W126 exposures increased from 1989 to 1999 but dropped substantially after 2002, and were associated with the complete loss of [O3] ≥ 100 ppb and a decrease in frequency of [O3] ≥ 60 ppb, especially at high elevation sites. By 2012, average diurnal profiles had no hourly concentrations ≥ 60 ppb. The maximum 3-month W126 index has shifted to earlier in the season (Apr – Jun) in the last decade, especially at low elevations. Ozone episodes (> 3 consecutive hours with [O3] ≥ 60 ppb) have become less frequent, shorter, and of lower exposure. Decreases in [O3] in the Park since 2002 are highly correlated with concomitant decreases in NOx emissions from regional TVA Power Plants. The elimination of this NOx resulted from implementation of the 1997 8-hour O3 standard (NOx SIP call) as required by the Clean Air Act, a temporary slow-down in the economy, and turnover in the motor fleet to less polluting vehicles. Plants and ecosystems in Great Smoky Mountains National Park today are at much lower risk from tropospheric O3 than they were just a decade ago.