Measurement, Modeling, and Analysis of Ozone and its Key Precursors in the Southeast United States National Parks

Abstract

Lower tropospheric ozone (O3) continues to be a major air pollution problem in the southeast United States due to its adverse effects on humans and the environment. This study presents measurements, modeling, and interpretation of ozone (O3) and its key precursors based on measurements collected from three Southeast US National Parks: Shenandoah National Park (SHEN), VA, Great Smoky Mountains National Park (GRSM), TN/NC, and Mammoth Cave National Park (MACA), KY. Our analysis shows that whereas O3 concentration at low-elevation rural site rises sharply to a short-lived afternoon maximum and drops to near zero at night similar to most urban or suburban areas, O3 at high-elevation rural sites does not display a significant drop during nighttime. As a result, longer-time averaged O3 levels at high-elevation sites are significantly higher than those at nearby low-elevation sites and high-elevation sites are more likely to exceed the new O3 National Ambient Air Quality Standard (NAAQS) as the standard changes to a smaller concentration averaged over a longer time period. We introduced an indicator, the (NOy-NO)/NOy ratio, to represent air mass age. O3 increases with the increase of photochemical age represented by higher (NOy-NO)/NOy ratio at both high and low elevation locations. The current study also presents a turnover point (0.9 for value of (NOy-NO)/NOy) that separates the NOx-limited and non-NOx-limited ozone production regimes. The regional background O3 concentrations that are not directly influenced by anthropogenic emissions are estimated to be ~35 ppbv at GRSM, and ~43 ppbv at MACA inferred from the linear regression of O3 on (NOy — NO). Under the 1-hour NAAQS, both GRSM and SHEN site were at the edge of O3 exceedance and MACA did not exceed the 1-hour NAAQS for O3. Under the new 8-hour NAAQS, however, all three national parks are in serious nonattainment of O3 NAAQS.

The second section of this study focuses on elucidating source attribution, influence area, and process budget of reactive nitrogen oxides in the Southeast national parks, since nitrogen oxides are considered as the limiting factor to ozone production in these areas characterized with strong reactive biogenic VOCs emission. Multiple linear regression analysis provides that point sources contribute a minimum of 23% and 27% of total NOy at GRSM and MACA, respectively, during the whole measurement period. Another technique, emission inventory analysis based EPA Emission Inventory, provides a similar estimate that a minimum of 26% and 45% of total NOy can be attributed to point source emission at GRSM and MACA sites, respectively. Trajectory-cluster analysis shows that air masses from western (20% out of all air masses) and southwest (17%) sweep over GRSM site most frequently, while pollutants transported from eastern half (i.e., East, Northeast, and Southeast) has limited influence (< 10%) on air quality in the Great Smoky Mountain National Park. Further examination of pollutants associated with these air masses reveals that the highest O3 concentrations are associated with trajectories from the North and Southwest directions, which can be tracked back to Ohio Valley region and coastal region along Gulf of Mexico. Process budget analysis using Multiscale Air Quality SImulation Platform (MAQSIP) model reveals that chemistry contributions of 32% and 84% to NOz correspond to 26% and 80% to O3 at GRSM and MACA, respectively. The similarity between NOz and O3 process budgets serves as further evidences of close association between nitrogen oxides and effective O3 production at these rural locations.

In the last part of this study we examined annual, seasonal, and diurnal distributions, as well as case studies of high ozone episodes observed during a multiple-year enhanced monitoring campaign at the Southeast national parks, namely. Though there are no continuous increases in either annual exceedances or the 4th highest ozone concentration at these sites, a long-term increase trend can be identified from measured data. Most frequent exceedances occur in August and September for GRSM site while the maximum exceedances are found in June or August at MACA site. Maximum exceedances at low-elevation site are seen in the midday, and extend into a few hours after sunset. High ozone episodes can be observed, however, in any hour at high elevation site during all photochemically active seasons, most frequently around sunset, and least frequently in early morning. While the air masses associated with ozone exceedances at GRSM site originated in all directions, those with high O3 at MACA, however, reveal dominant transport originated from southwest with very few exceptions from north. Examining two early September cases shows that high nitrogen oxides concentration is the main driver to elevate ozone concentration to exceedance level at MACA site while ozone transport from polluted area works to form exceedances at GRSM, a mountain top site. Almost all high ozone episodes at MACA site are found developed with clear sky, high temperature, low relative humidity, as well as weak anticyclones traveling in a uniform anti-cyclonal pathway surrounding a high-pressure area. High ozone episodes observed at mountain site, however, are not necessary to be associated with these factors.