John D. Ray

Spatial Distribution of Tropospheric Ozone in National Parks of California: Interpretation of Passive-Sampler Data

The National Park Service (NPS) has tested and used passive ozone samplers for several years to get baseline values for parks and to determine the spatial variability within parks. Experience has shown that the Ogawa passive samplers can provide ±10% accuracy when used with a quality assurance program consisting of blanks, duplicates, collocated instrumentation, and a standard operating procedure that carefully guides site operators. Although the passive device does not meet EPA criteria as a certified method (mainly, that hourly values be measured), it does provide seasonal summed values of ozone. The seasonal ozone concentrations from the passive devices can be compared to other monitoring to determine baseline values, trends, and spatial variations. This point is illustrated with some kriged interpolation maps of ozone statistics. Passive ozone samplers were used to get elevational gradients and spatial distributions of ozone within a park. This was done in varying degrees at Mount Rainier, Olympic, Sequoia–Kings Canyon, Yosemite, Joshua Tree, Rocky Mountain, and Great Smoky Mountains national parks. The ozone has been found to vary by factors of 2 and 3 within a park when average ozone is compared between locations. Specific examples of the spatial distributions of ozone in three parks within California are given using interpolation maps. Positive aspects and limitations of the passive sampling approach are presented.

Nonmethane hydrocarbons in the rural southeast United States national parks

Measurements of volatile organic compounds (VOCs) were made at three rural sites in the southeast U.S. national parks: Mammoth Cave National Park, Kentucky; Cove Mountain, Great Smoky Mountains National Park, Tennessee; and Big Meadows, Shenandoah National Park, Virginia. In 1995 the three locations were sampling sites for the Southern Oxidants Study (SOS) Nashville Intensive, and the measurements of VOCs for Shenandoah were also made under contract with the National Park Service. Starting in 1996, the National Park Service added the other two parks to the monitoring contract. Hydrocarbon measurements made during June through September for the years 1995, 1996, and 1997 were analyzed in this study. Source classification techniques based on correlation coefficient, chemical reactivity, and ratioing were developed and applied to these data. The results show that anthropogenic VOCs from automobile exhaust appeared to be dominant at Mammoth Cave National Park, and at Cove Mountain, Great Smoky Mountains National Park, but other sources were also important at Big Meadows, Shenandoah National Park. Correlation and ratio analysis based on chemical reactivity provides a basis for source-receptor relationship. The most abundant ambient VOCs varied both in concentration and order depending on park and year, but the following VOCs appeared on the top 10 list for all three sites: isoprene (6.3 to 18.4 ppbv), propane (2.1 to 12.9 ppbv), isopentane (1.3 to 5.7 ppbv), and toluene (1.0 to 7.2 ppbv). Isoprene is naturally emitted by vegetation, and the others are produced mainly by fossil fuel combustion and industrial processes. Propylene-equivalent concentrations were calculated to account for differences in reaction rates between the hydroxyl radical and individual hydrocarbons, and to thereby estimate their relative contributions to ozone formation.

Atmospheric sulfur dioxide at Mauna Loa, Hawaii

Measurements of sulfur dioxide (S02) were made at the National Oceanic and Atmospheric Administrations Mauna Loa Observatory in Hawaii, during a 12-month period beginning in December 1988. SO2 concentrations varied from background levels of less than 0.05 ppbv to a maximum of 50 ppbv, during episodes that lasted from 2 to 24 hours. Emissions from the Kilauea crater, approximately 35 km southeast of the observatory at an elevation of about 1000 m above sea level (asl), and the current eruption at Puu O′o 50 km east-southeast, are the most likely sources for the higher concentrations. These episodes occurred 10–25 times each month, mostly during the day; peak concentrations were usually recorded at mid-day. The SO2 concentrations can be grouped into three periods; low (June–September), high (October–January) and intermediate (February–May). A clear diurnal cycle of SO2 concentration exists throughout the year, although day-night changes were greatest during October–January and were barely detectable during the June–September period. The highest SO2 concentrations were recorded when the predominant wind direction was northerly to northwesterly, even though the apparent sources are in the southeastern sector. Nighttime concentrations were usually at background levels; however, many exceptions were observed. A few cases of higher than background SO2 were observed when free tropospheric (FT) conditions were identified. The possibility that long-range transport was the cause for elevated SO2 concentrations under FT conditions was examined using air mass back trajectories analyses. The highest nighttime SO2 concentrations, under FT conditions, were observed during periods with slow easterly trajectories, and the lowest concentrations were found during westerly flows. Twenty-four nighttime free tropospheric events were recorded when the SO2 concentration exceeded 0.2 ppbv. During 18 of these episodes, unusually high CO2 concentrations were observed.

Carbon monoxide in the U.S. mid‐Atlantic troposphere: Evidence for a decreasing trend

Nearly continuous measurements of carbon monoxide (CO) were made at Shenandoah National Park-Big Meadows in rural Virginia, a site considered representative of regional air quality, from December 1994 to November 1997. Similar observations were also made at this location from October 1988 to October 1989. These observations combine to indicate a decreasing trend in CO concentration over the U.S. mid-Atlantic region of about 5.0 ppbv yr -1 , with greater than 95% confidence that the slope is significantly different from zero. The decrease suggests U.S. reductions in anthropogenic CO emissions have been effective in reducing pollutant levels. The observed trend is consistent with the U.S. EPA reported trend in emissions and the decrease in Northern Hemisphere tropospheric background CO mixing ratios observed by other researchers.

The vertical distribution of atmospheric H2O2: A case study

Vertical profiles of H2O2 mixing ratios were obtained for each season from a site in central Arkansas during 1988. Aircraft-based measurements indicated that H2O2 mixing ratios followed an annual cycle, peaking during the summer at >6 parts per billion by volume (ppbv). The minimum occurred in winter when mixing ratios for H2O2 averaged about 0.2 ppbv. The H2O2 mixing ratio generally peaked at an altitude of about 800 mbar (2 km), although there may have been some seasonal dependence. The annual cycle followed variations in solar intensity, water mixing ratio, and temperature. Within a season, strong variations could be related to meteorological events. A daily cycle was inferred in which the H2O2 mixing ratio varied by a factor of 2 to 3; the peak observed values were at night. H2O2 mixing ratios at altitudes higher than 0.7 km were generally greater than local SO2 values above 0.7 km during all but the winter season.

Surface level measurements of ozone and precursors at coastal and offshore locations in the Gulf of Maine

The northeastern United States has episodic high ozone several times each year at locations remote from urban and industrial centers. Extended measurements of ozone and the ozone precursors, volatile organic compounds (VOC) and nitrogen oxides, were made at Acadia National Park, Cape Elizabeth, and other coastal Maine locations during the North Atlantic Regional Experiment (NARE) intensive. In addition, ozone was measured from a commercial ferry, the Scotia Prince, in the Gulf of Maine where ozone concentrations up to 129 ppb were observed. Two high-ozone episodes were observed during late August 1993 when ozone was greater than 90 ppb along much of the Maine coast. NOy concentrations at Acadia averaged 2.0 ppb (±1.97), maximum 12.0 ppb. During the high-ozone episodes, NOy had linear relationships to ozone with slopes of 4–16. The timing of maximum values and extent of the high-ozone air mass suggests that urban plumes transported over the Gulf of Maine are brought inland by sea breezes to the coastal regions but not to the interior areas of Maine.

A comparison of surface and airborne trace gas measurements at a rural Pennsylvania site

A comparison of trace gas concentrations between a ground sampling site and an instrumented aircraft was performed at the Scotia field site in central Pennsylvania. The comparison included four research flights in August 1988. The ground site was operated by the National Oceanic and Atmospheric Administration (NOAA) Aeronomy Laboratory, and the NOAA King Air instrumented aircraft was operated by the NOAA Air Resources Laboratory. The study demonstrates continuity in the values of the state parameters (wind speed and direction, temperature, and dew point) from the surface to the aircraft observations at the times of the four flights. Considered within the context of the differing time-space scales, continuity was also demonstrated in the trace gas measurements. The aircraft and surface measurements of the trace gas concentrations were in better agreement when the aircraft measurements demonstrated uniform concentrations over the length of the flight path. The mean O3 and H2O2 values of the lowest-altitude aircraft readings were usually greater than those measured at the surface, thus indicating higher-altitude sources and removal processes at lower altitudes. The aircraft-measured values for SO2, NOy, and hydrocarbons were usually lower than those measured at the surface, as expected for primary pollutants emitted near ground level. Closer agreement was obtained in the comparisons of secondary (O3, H2O2) than of primary (SO2, NOy, hydrocarbons) pollutants.

Temporal variability and case study of high O 3 episodes in two southeastern US national parks

Despite a decreasing trend nationwide, eight-hour O3 concentrations in 25 of US national parks have increased by 8% during last decade. This study presents a 13-year observation of high O3 at the Great Smoky Mountains (GRSM) and Mammoth Cave (MACA) national parks, both among the 25 impacted parks. Although there is no monotonic increase, the later half witnessed three-fold exceedances than the former. O3 exceedances occurred most frequently in June at MACA, and in August or September at GRSM. High O3 episodes at MACA occurred during daytime or early evening, but exceedances at GRSM can be found in any hour. Air masses with high O3 at GRSM came from all directions, whereas those at MACA are predominantly from the southwest. Case studies show that high O3 episodes at MACA are developed under clear sky, high temperature, low humidity, and weak winds traveling in a uniform anti-cyclonal pathway.

Modeling and Analysis of Ozone and Nitrogen Oxides in the Southeast United States National Parks

High O3 episodes are observed in several eastern US national parks, among which the Great Smoky Mountains National Park by far of the fastest increase in frequency of exceedance days (days when any 8-hour average O3 concentration exceeding 85 ppbv). It has been well established that the southeast US rural areas are characterized with strong biogenic VOCs emissions and that O3 production in this region is mostly NOx-limited during summer time. Thus, understanding the contribution of nitrogen oxides to O3 formation during transport and for local photochemistry is essential to predict what effects the planned reductions in NOx emissions from large point sources might have on observed O3 concentrations at these southeast national parks. Our specific interests in this study are: 1) to quantify the relative importance of point sources and mobile sources to total nitrogen oxides emissions; 2) to identify origins of air masses associated with high levels of nitrogen oxides and O3; 3) to quantify contributions of individual chemical and physical processes, i.e., chemistry, transport, emission, and deposition, to the budget of production and removal of nitrogen oxides and O3 in the southeast national parks.

Measurements and Modeling of Regional Air Quality in three Southeast United States National Parks

Since the passage of the 1970 Clean Air Act (CAA), regulatory efforts to comply with the 0.12-ppmv National Ambient Air Quality Standard (NAAQS) for O3 have proved inadequate [(NRC); Dimitriades, 1989]. O3 nonattainment continues to be a problem, especially in the southeast United States. This is attributed to the oxidation of NOx in the presence of excessive amounts of biogenically emitted VOCs such as isoprene [Trainer et al., 1987; Chameides et al., 1988]. The new 8- hour O3 National Ambient Air Quality Standard (NAAQS) (0.08 ppm) is likely to bring more suburban and rural locations into noncompliance [Chameides et al., 1997]. Biogenic VOCs emitted by vegetation [Fuentes et al., 2000; Fehsenfeid et al., 1992; Lamb et al., 1993] and anthropogenic VOCs emitted by human activities are both widely present in rural aTeas [Kang et al., 2001; Hagerman et al., 1997]. Previous studies indicate that the influence of these VOCs on important aspects of atmospheric chemistry such as O3 production can be significant [Trainer et al., 1987; Chameides et al., 1988]. Clearly, if O3 concentrations are to be successfully controlled by implementation of control on primary pollutant emissions, the roles of both natural and anthropogenic VOCs in these rural areas must be thoroughly understood. However, our understanding of O3 and VOC budgets in rural areas is still very limited. Emissions of biogenic VOCs as well as the roles of both biogenic and anthropogenic VOCs in O3 production in rural areas are largely uncharacterized [Guenther et al., 2000].