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.
Highlights
High concentrations of tropospheric ozone can have adverse effects on human health and vegetation
The ozone data were decomposed into 12 oscillatory components using Ensemble Empirical Mode Decomposition (EEMD)
25 U.S National Park Service (NPS) datasets were analyzed for spatial and temporal variability in long-term ozone using EEMD. This method has allowed for an analysis of the spatial and temporal variability of long-term ozone data on four time-scales, which include annual, interannual, interdecadal, and multidecadal
Summary
High concentrations of tropospheric ozone can have adverse effects on human health and vegetation. The United States (U.S.) Environmental Protection Agency (EPA) recently lowered the National Ambient Air Quality Standard (NAAQS) for ozone from 75 ppbv to 70 ppbv [1,2], in 2015. Regional emission controls for nitrogen oxides (NOx ) have aided in the decrease of summertime ozone in the eastern U.S as well as parts of California [3,4]. May make ozone attainment difficult for regions of higher elevation in the western U.S [6,8,9,10]. Meeting and maintaining lower ozone standards may be more difficult in a warming climate [11] There are concerns that the influx of East Asian emissions [5,6,7], and the influences by interannual and interdecadal variability in the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO)
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