Ralph J. Valente

Daytime buildup and nighttime transport of urban ozone in the boundary layer during a stagnation episode

A 3-day period of strong, synoptic-scale stagnation, in which daytime boundary-layer winds were light and variable over the region, occurred in mid July of the 1995 Southern Oxidants Study centered on Nashville, Tennessee. Profiler winds showed light and variable flow throughout the mixed layer during the daytime, but at night in the layer between 100 and 2000 m AGL (which had been occupied by the daytime mixed layer) the winds accelerated to 5-10 m s-1 as a result of nocturnal decoupling from surface friction, which producect inertial oscillations. In the present study, we investigate the effects of these wind changes on the buildup and transport of ozone (03). The primary measurement system used in this study was an airborne differential absorption lidar (DIAL) system that profiled 03 in the boundary layer as the airplane flew along. Vertical cross sections showed that 03 concentrations exceeding 120 ppb extended up to nearly 2 km AGL, but that the 03 hardly moved at all horizontally, instead forming a dome of pollution over or near the city. The analysis concentrates on four meteorological processes that determine the 3-D spatial distribution of 03 and the interaction between urban and rural pollution: (1) daytime buildup of 03 over the urban area, (2) the extent of the drift of pollution cloud during the day as it formed, which controls peak 03 concentrations, (3) nighttime transport by the accelerated winds above the surface, and (4) vertical mixing of pollution layers the next day. Other consequences of very light-wind conditions were intra-regional differences in daytime mixed-layer depth over distances of 50 km or less, and indications of an urban heat-island circulation.

Emissions of NO from soil at a rural site in central Tennessee

Field measurements of soil emissions of NO from a Mountview silt loam soil with three land uses (forest, fertilized pasture, and fertilized corn) were made on a commercial farm during a summer and autumn sampling period. A new automated closed-chamber sampling system was developed to allow simultaneous measurements on five chambers per 100 m2 plot. Individual chambers with hinged tops, covering 0.3 m2 of soil area were pneumatically operated via data logger control to sample soil NO flux every third hour. Spatial variability in emission rates was high. For each land use type the range from the lowest to the highest emitting chamber was approximately threefold. Land use type significantly affected soil NO emissions. The fertilized pasture had the highest mean emission rate (44.1 ng N m−2 s−1), followed by the fertilized corn (27.0 ng N m−2 s−1), and the forest (8.4 ng N m−2 s−1). NO emission rates and soil nitrate levels at the forest plot were considerably higher than at other forest sites in the region, possibly due to runoff from an adjacent fertilized hayfield. The results of this study, when extrapolated to a regional estimate, suggest that emissions of NO from soils could play a significant role in summertime tropospheric ozone photochemistry in the southeastern United States.

Rural ozone in the southeastern United States

Abstract Ozone measurements are reported for five rural sites in the Tennessee Valley region of the southeastern U.S. for periods ranging from 18 to 83 months during the years 1977 through 1984. Rural ozone (O 3 ) levels were found to equal or exceed urban values for the same region. The daily maximum 1-h average concentration was found to peak during the summer months, while the 24-h average concentrations were greatest in the spring. The annual cycle of daily maximum concentrations is related to the seasonal photochemical cycle. The annual cycle in 24-h average concentrations is best explained by the combined effects of the annual cycles in solar intensity and noctural O 3 depletion. There was no indication that stratospheric intrusions exhibited a significant influence on the annual O 3 cycles. Evidence was found for elevated O 3 levels during touchdown of plumes from large power plants. No long-term trend in rural O 3 concentrations, either daily maxima or means, was discernible.

Relative production of ozone and nitrates in urban and rural power plant plumes: 1. Composite results based on data from 10 field measurement days

A rather limited number of large power plants are responsible for about 2/3 and 1/3 of the U.S. anthropogenic emissions of SO2 and NOx, respectively. Considerable uncertainty continues to prevail about the local and regional impact of their potentially harmful secondary products (e.g., ozone, sulfates, nitrates), We have analyzed state-of-the-art data of the Southern Oxidant Study (SOS)-Nashville Field Study (1994, 1995) for 10 days of summer daytime field measurements by instrumented aircraft in the plumes of three large, tall-stack, base-load, Tennessee Valley Authority (TVA) coal-fired power plants in northwestern Tennessee: Gallatin (G), located within the Nashville urban ozone nonattainment area, and Cumberland (C) and Johnsonville (JV) in rural isoprene-rich forested areas about 100 km to the west of Nashville. The average 1995 emissions of NOx from these three sources ranged over more than an order of magnitude. In this paper, we have explored plume chemical evolution and the magnitude, efficiency, and yield of ozone and NOz, (NOx oxidation products, mostly inorganic and organic nitrates) production in a broad variety of plume transport and chemistry scenarios within the convective boundary layer (CBL) in rural and urban settings. The results show that (1) plume chemical maturity and peak production capacities of ozone and NOz were realized quite close to the sources, within 30–40 km and 4 hours of daytime transport for Gallatin (smallest NOx emission rate, QNOx, and suburban environment) and typically within 100 km and 6 hours of CBL transport for Cumberland (highest QNOx and rural environment rich in isoprene); (2) the ozone impact of Gallatin on Nashville can exceed that of Cumberland, and under favorable transport and chemical conditions, both power plants can contribute as much as 50 ppb of excess ozone to the urban area, raising local peak levels well in excess of 100 ppb; (3) an estimated 3.1±0.7 molecules of ozone and more than 0.6 molecules of NOz, may be produced in large isolated rural power plant plumes (PPPs) per molecule of NOx release, and the corresponding peak yields of ozone and NOz may be significantly greater in urban PPPs; (4) the rate of NOz production ≈ 10–15% h−1 in isolated rural PPPs, and higher in urban PPPs; (5) NOz production is favored in all PPPs at first when the chemistry is VOC-limited; later, with increasing VOC ingestion from the background, the chemistry increasingly favors NOx-limited ozone production, starting at plume edges, and ultimately throughout the diluted plume. These results have major implications on outstanding issues related to the environmental impact and regulatory control of electric utility industry NOx emissions.

An overview of the airborne activities during the Southern Oxidants Study (SOS) 1995 Nashville/Middle Tennessee Ozone Study

The cause and extent of elevated ozone levels which are often found during summer in the southeastern United States were the focus of the intensive Southern Oxidants Study (SOS) 1995 Nashville/Middle Tennessee Ozone Study. Six aircraft were extensively instrumented and were deployed in concert during the summer of 1995 from the Nashville Metropolitan Airport. This overview describes the capabilities of the deployed assets, and how their use was managed. Results from the measurements on individual aircraft and their interpretation are contained in the individual papers that follow.

The production of O3 in an urban plume: Airborne sampling of the Atlanta urban plume

Abstract As part of the Southern Oxidant Study, The Tennessee Valley Authoritys instrumented helicopter made a series of air sampling flights over the city of Atlanta. The flights were made during the summer of 1992 to investigate the evolution of the urban O 3 plume. Air samples were taken during morning and afternoon hours; the morning data were used to estimate background O 3 and the afternoon data were used to estimate O 3 production efficiency, i.e. the number of O 3 molecules produced per molecule of NO y emitted. Detailed data on O 3 production were available for five afternoon flights. Within the radius sampled, three zones were identified: the source zone where afternoon levels were comparable with the morning levels, the production zone where 03 increased rapidly within a short distance, and the dilution zone where both O 3 and its precursors were diluted, at the same rate. O 3 peak levels, or the transition from net production to dilution occurred at 20–40 km from the city center. O 3 production efficiency for the five afternoon flights was between 4 and 10, in good agreement with previous surface measurements.

Historic PM2.5/PM10 Concentrations in the Southeastern United States—Potential Implications of the Revised Particulate Matter Standard

This report summarizes a PM2.5/PM10 particulate matter data set consisting of 861 PM2.5/PM10 sample pairs collected with dichotomous samplers by the Tennessee Valley Authority (TVA) from 1982 to 1991. Eight monitoring stations, ranging from urban-industrial to rural-background, were operated across three east-central U.S. states. Annual average PM2.5 concentrations ranged from 12.6 to 21.3 micrograms per cubic meter (μg/m3), with an overall mean of 15.7 μg/m3. Likewise, annual average PM10 concentrations ranged from 17.8 to 33.7 μg/m3, with an overall mean of 23.7 μg/m3. High summer-low winter seasonality was evident, particularly for PM2.5, with the highest monthly PM2.5 and PM10 concentrations in August (26.4 and 37.5 μg/m3, respectively) and the lowest in February (9.9 and 15.3 μg/m3, respectively). A strong association (r2 = 0.84) was found between PM and PM mass with PM mass contributing, on average, 67% of PM10 mass. Applying TVAs PM2 5/PM10 ratio to recent (1993-1995) regional high-volume PM10 Aerometric Information Retrieval System (AIRS) data for the east-central United States suggests that as many as 80% of monitored counties would have equaled or exceeded the level of the new annual PM2.5 metric of 15 μg/m3. A decline in average PM2.5 mass on the order of 3-5 μg/m3 from 1982 through 1991 is also suggested. Daily PM2 5 mass appears to be reasonably well associated (r = 0.47) with maximum hourly ozone during the warmer months (spring through fall). Sulfate compounds comprise a major portion of the measured PM2 5 mass, with that fraction being highest in the summer months. Viewed collectively, these data suggest that although compliance with the annual and 24-hr PM and 24-hr PM metrics should prove readily attainable, the annual PM2.5 metric will present a major regulatory management challenge for much of the east-central United States.

Field comparison of static and flow‐through chamber techniques for measurement of soil NO emission

A field comparison of flow-through and static chamber techniques for measuring soil emissions of NO was performed on fertilized soil at a commercial cotton (Gossypium hirsutum L.) farm near Muscle Shoals, Alabama, during July 1992. The purpose of the study was to compare soil NOx. emissions data taken using two different techniques at a common field site. Emission rates with collocated chambers using the two techniques were compared, and spatial means were also compared for 17 National Oceanic and Atmospheric Administration (NOAA) plots and 10 Tennessee Valley Authority (TVA) plots. Emission rates of NO at the site covered a broad spectrum, ranging from less than 1 to greater than 100 ng N m−2 s−1. Data from collocated TVA static and NOAA flow-through chambers showed a correlation coefficient of 0.98 with a linear regression slope of 0.97. A t test indicated that the mean difference was not statistically different than zero. The plot mean emission rates were 17.7 and 18.0 ng N m−2 s−1 for the TVA and NOAA chambers, respectively, for an 8-day comparison period. These findings indicate that data sets collected with these methods are comparable and may be combined without concern for differences in technique. These results also reveal that the techniques used by each group in attempting to characterize overall site mean emissions are remarkably similar, despite differences in chamber size, plot location, extent of areal coverage, and random error associated with the measurements. This finding is significant in that it means that field data used to characterize emissions estimates by both protocols can be pooled to better estimate regional soil NO emission inventories.

Influence of natural hydrocarbons on ozone formation in an isolated power plant plume

On 4 days during the 1995 Southern Oxidant Study (SOS), air samples were taken in the plume of the Cumberland Power Plant in Tennessee using an instrumented helicopter. On these days a notable difference in excess ozone in the plumes was observed. Excess ozone varied from 20 ppb on July 7, 1995, up to 55 ppb on July 16. While the total amount of non-methane VOC was quite similar, significant differences were observed in the levels of reactive hydrocarbons, mostly isoprene. This study examines the parameters that govern both emission rates of isoprene and its dispersion. These include temperature and wind speed on the surface and aloft, total solar radiation, and the height of the mixed layer. The results revealed and computer model simulations confirmed that although differences were not very large, the combinations of all of these parameters favored lower ambient isoprene levels and, consequently, lower ozone production on July 7 and higher production on the 3 other days.

Measuring inorganic nitrate species with short time resolution from an aircraft platform by dual‐channel ozone chemiluminescence

A measurement technique for determining nitrate (the sum of nitric acid and particulate nitrate) with a few seconds time resolution in plumes is needed to resolve within-plume features. A technique using dual ozone-chemiluminescent NO detectors with a selective nitrate scrubber in one sampling train is promising if used with an appropriate sampling inlet, and if nitrate is the desired analyte. We report the design of, and preliminary results from a dual channel ozone-chemiluminescent system, each channel containing a gold-CO catalyzed converter which reduces all odd nitrogen species (NOy) quantitatively to NO; one channel also contains a nylon filter to remove nitrate from the air stream prior to the converter (this signal is termed NOy*). This system was deployed successfully in a Bell 205 helicopter during the 1995 Southern Oxidants Study Nashville Ozone Study. The converters were mounted forward near the air intake, and zero air and calibration gases admitted simultaneously to both channels during flight operations. The difference signal between the two channels (NOy-NOy*) indicated apparent nitrate levels in the sampled air with a time resolution of <5 s and a limit of detection of about 1 ppbv. Nitrate levels observed with this system in plumes and background air during the Nashville Ozone Study were highly correlated with ozone and varied from below detection limits to ≈20 ppbv. Nitrate levels were also highly correlated with the calculated difference between NOy and the sum of NO and NO2 (NOz). Higher nitrate levels as a fraction of NOz were found in power plant plumes (≥60%) compared with urban plumes (≈50%) and background air, consistent with apparently lower ozone production efficiencies in power plant plumes vis-a-vis urban plumes.