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Atmosphere 2018, 9(4), 125; https://doi.org/10.3390/atmos9040125

Understanding Long-Term Variations in Surface Ozone in United States (U.S.) National Parks

1
Department of Chemistry, State University of New York College of Environmental Science and Forestry, Syracuse, NY 13210, USA
2
Department of Earth, Ocean and Atmospheric Science Florida State University Tallahassee, FL 32306, USA
3
National Park Service, Air Resources Division, Lakewood, CO 80235, USA
4
Clean Air Markets Division, United States Environmental Protection Agency, Washington, DC 20460, USA
*
Author to whom correspondence should be addressed.
Received: 6 February 2018 / Revised: 13 March 2018 / Accepted: 17 March 2018 / Published: 25 March 2018
(This article belongs to the Section Air Quality)
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Abstract

Long-term surface ozone observations at 25 National Park Service sites across the United States were analyzed for processes on varying time scales using a time scale decomposition technique, the Ensemble Empirical Mode Decomposition (EEMD). Time scales of interest include the seasonal cycle, large-scale climate oscillations, and long-term (>10 years) trends. Emission reductions were found to have a greater impact on sites that are nearest major urban areas. Multidecadal trends in surface ozone were increasing at a rate of 0.07 to 0.37 ppbv year−1 before 2004 and decreasing at a rate of −0.08 to −0.60 ppbv year−1 after 2004 for sites in the East, Southern California, and Northwestern Washington. Sites in the Intermountain West did not experience a reversal of trends from positive to negative until the mid- to late 2000s. The magnitude of the annual amplitude (=annual maximum–minimum) decreased at eight sites, two in the West, two in the Intermountain West, and four in the East, by 5–20 ppbv and significantly increased at three sites; one in Alaska, one in the West, and one in the Intermountain West, by 3–4 ppbv. Stronger decreases in the annual amplitude occurred at a greater proportion of sites in the East (4/6 sites) than in the West/Intermountain West (4/19 sites). The date of annual maximums and/or minimums has changed at 12 sites, occurring 10–60 days earlier in the year. There appeared to be a link between the timing of the annual maximum and the decrease in the annual amplitude, which was hypothesized to be related to a decrease in ozone titration resulting from NOx emission reductions. Furthermore, it was found that a phase shift of the Pacific Decadal Oscillation (PDO), from positive to negative, in 1998–1999 resulted in increased occurrences of La Niña-like conditions. This shift had the effect of directing more polluted air masses from East Asia to higher latitudes over the North American continent. The change in the Pacific Decadal Oscillation (PDO)/El Niño Southern Oscillation (ENSO) regime influenced surface ozone at an Alaskan site over its nearly 30-year data record. View Full-Text
Keywords: ozone; trends; Ensemble Empirical Mode Decomposition; annual amplitude; seasonal cycle; El Niño Southern Oscillation; Pacific Decadal Oscillation; National Park Service ozone; trends; Ensemble Empirical Mode Decomposition; annual amplitude; seasonal cycle; El Niño Southern Oscillation; Pacific Decadal Oscillation; National Park Service
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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. (CC BY 4.0).
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McGlynn, D.; Mao, H.; Wu, Z.; Sive, B.; Sharac, T. Understanding Long-Term Variations in Surface Ozone in United States (U.S.) National Parks. Atmosphere 2018, 9, 125.

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