The plume of Tennessee Valley Authority's coal-fired Cumberland power plant was sampled during four different days in the summer of 1998 and 1999 from an instrumented helicopter. The extent of formation of ozone and various secondary NOy species in the plume was measured and the rates of loss processes estimated. The rates of these processes were found to be similar during three of the four sampling days. On the fourth day conversion and removal processes within the plume were significantly slower apparently due to lower ambient temperatures, and poor dispersion conditions. On the three more ‘reactive’ days ozone yield (Y) was found to be in the range of 1.5–2.6 molecules of O3 produced per molecule of NOx emitted. The ozone production efficiency, estimated from Y and the average chemical age of the farthest distance sampled, varied from 2.3 to 5.4.

ABSTRACT The rate of conversion of SO2 to SO4 2- was re-estimated from measurements made in the plume of the Cumberland power plant, located on the Cumberland River in north-central Tennessee, after installation of flue gas desulfurization (FGD) scrubbers for SO2 removal in 1994. The ratio of SO2 to NOy emissions into the plume has been reduced to ~0.1, compared with a prescrubber value of ~2. To determine whether the SO2 emissions reduction has correspondingly reduced plume-generated particulate SO4 2- production, we have compared the rates of conversion before and after scrubber installation. The prescrubber estimates were developed from measurements made during the Tennessee Plume Study conducted in the late 1970s. The post-scrubber estimates are based upon two series of research flights in the summers of 1998 and 1999. During two of these flights, the Cumberland plume did not mix with adjacent power plant plumes, enabling rate constants for conversion to be estimated from samples taken in the plume at three downwind distances. Dry deposition losses and the fact the fact that SO2 is no longer in large excess compared with SO4 2- have been taken into account, and an upper limit for the conversion rate constant was re-estimated based on plume excess aerosol volume. The estimated upper limit values are 0.069 hr-1 and 0.034 hr-1 for the 1998 and 1999 data, respectively. The 1999 rate is comparable with earlier values for nonscrubbed plumes, and although the 1998 upper limit value is higher than expected, these estimates do not provide strong evidence for deviation from a linear relationship between SO2 emissions and SO4 2- formation.

Airborne measurements were performed in the plume of the Cumberland Power Plant during August 1998 using a highly sensitive SO2 instrument. The measurements confirmed previous suggestions that NOy species are removed from the plume at a faster rate than SO2. The differential removal rate (the difference between loss rate of NOy and that of SO2) was estimated to be 0.06h−1. This value implies that the NOy loss rate is in the range of 0.09–0.14h−1. The application of a mathematical argument, based on the convolution integral, enabled improved synchronization of the data from the SO2 and NOy instruments. Examination of the synchronized data revealed that the concentration ratio of SO2 and NOy varies across the plume. Near the source it is higher at the wings of the plume, while in the core of the plume it is similar to the ratio at the release point. Two possible explanations of the observations are discussed: conversion to non-measurable NOy species, and in-plume loss of NOy (as HNO3) via dry deposition.

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.

O3 production in the Nashville urban plume during the O3 episode that occurred on July 11–July 13 1995, is examined to characterize the factors that control the ozone production rate and efficiency, and to examine the relative importance of natural and anthropogenic sources of hydrocarbons to ozone production in the urban center and outlying areas. The analysis focuses on data collected during aircraft flights on July 11 when the Nashville area was sampled more or less continuously from about 1000 to 1800 LT. The instantaneous ozone production rate P(O3) in the downtown area from late morning through midafternoon on July 11 ranged between 10 and greater than 30 ppbv/h depending on location. After 1700 local time, production rates dropped to a few ppbv/h owing to the diminished solar intensity. Instantaneous production efficiencies with respect to NOx in the downtown area ranged between 2.5 and 8, linearly depending on the ratio of the hydrocarbon to NOx, OH reactivity. Integral O3 production efficiencies corrected for NOz losses ranged between 1.5 and 4. The lowest efficiency was observed in the downtown area in the morning where NOx concentrations were high and hydrocarbon to NOx reactivity ratios were the lowest. Throughout the day, P(O3) in the downtown area was limited by the availability of hydrocarbons. Anthropogenic hydrocarbons and CO contributed about 66% of the total hydrocarbon OH reactivity in the downtown area. In the mature urban plume downwind of Nashville, P(O3) dropped to 6–9 ppbv/h at midafternoon and was controlled by the availability of NOx. Integral O3 production efficiencies in the mature urban plume ranged between 3.5 and 4. When present in large quantities (1–3 ppbv), isoprene significantly increased both the rate and efficiency of ozone production as long as the photochemical system was not strongly NOx‐limited.

Airborne measurements were made of gaseous and particulate species in the plume of a large coal-fired power plant after flue gas desulfurization (FGD) controls were installed. These measurements were compared with measurements made before the controls were installed. The light scattering and number and volume distributions of plume excess particles were determined by nephelometry and optical particle counting techniques. The plume impact based on optical techniques was much lower than that observed in earlier measurements. Indeed, plume excess volumes as a function of particle size were of the same magnitude as the variability of the background volume distribution. In situ excess plume scattering actually decreased with distance from the source, in contrast to pre-FGD conditions. The upper limit for the dry rate of SO2-to-SO4(2-) conversion was estimated from plume excess volume measurements to be about 4% hr-1. This is slightly greater than the upper limit, 3.5% hr-1, estimated by earlier researchers, but the same as that estimated using the present technique with the earlier data. The cross-plume profile of volume suggests SO2-to-SO4(2-) conversion is highest at the plume edges. The greatest benefit of SO2 reduction on plume excess volume and visibility appears to occur far down-wind of the source.

The evolution of photochemical smog in a plant plume was investigated with the aid of an instrumented helicopter. Air samples were taken in the plume of the Cumberland Power Plant, located in central Tennessee, during the afternoon of 16 July 1995 as part of the Southern Oxidants Study – Nashville Middle Tennessee Ozone Study. Twelve cross-wind air sampling traverses were made at six distance groups from 35 to 116km from the source. During the sampling period the winds were from the west–northwest and the plume drifted towards the city of Nashville TN. Ten of the traverses were made upwind of the city, where the power plant plume was isolated, and two traverses downwind of the city when the plumes were possibly mixed. The results revealed that even six hours after the release, excess ozone production was limited to the edges of the plume. Only when the plume was sufficiently dispersed, but still upwind of Nashville, was excess ozone (up to 109 ppbv, 50–60ppbv above background levels) produced in the center of the plume. The concentrations image of the plume and a Lagrangian particle model suggests that portions of the power plant plume mixed with the urban plume. The mixed urban power plant plume began to regenerate O3 that peaked at 120ppbv at a short distance (15–25km) downwind of Nashville. Ozone productivity (the ratio of excess O3 to NOy and NOz) in the isolated plume was significantly lower compared with that found in the city plume. The production of nitrate, a chain termination product, was significantly higher in the power plant plume compared to the mixed plume, indicating shorter chain length of the photochemical smog chain reaction mechanism.

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.

This study was motivated by the recent work of Buhr et al. [1996] which reported losses of NOy from large power plant plumes as high as 0.25 hour−1, much higher than generally accepted values. If true, conclusions pertaining to the efficiency of ozone and nitrate production in the lower troposphere would need major revisions. The results of Buhr et al. were based on aircraft measurements in four TVA (Tennessee Valley Authority) power plant plumes on July 7, 1995, as part of the Nashville/Middle Tennessee Ozone Study, a measurement program of the Southern Oxidants Study (SOS), whereas the results reported in this paper are also based on measurements made in the same SOS study aboard another instrumented aircraft (the TVA helicopter), in plumes of one of these power plants (the Cumberland Steam Plant in northwestern Tennessee) during five different days in 1994 and 1995. Between the 1994 and 1995 sampling periods, emissions of SO2 at the Cumberland plant were reduced by nearly 95% by installation of scrubbers. Our data from the one 1994 day show that the ratio of excess SO2 to NOy, in the plume core increased significantly with plume age, indicating a potentially high differential loss rate of NOy (excess loss of NOy relative to SO2) of about 0.12 hour−1. However, results based on the larger 1995 data set indicate a low differential NOy loss rate of only 0.00±0.03 hour−1, consistent with accepted low loss rates. Because the SOS‐Nashville/Middle Tennessee Ozone Study was not specifically designed to explore the NOy loss issue, the question of NOy loss rates in plumes is not currently resolved and additional focused field studies are needed.

The highest O-3 levels observed during the 1995 Southern Oxidants Study in middle Tennessee occurred during a period of air stagnation from July 11 through July 15. Extensive airborne (two fixed wing and one helicopter) and ground-based measurements of the chemistry and meteorology of this episode near Nashville, Tennessee, are presented. In situ airborne measurements include O-3, NOy, NO, NO2, SO2, CO, nitrate, hydrocarbons, and aldehydes. Airborne LIDAR O-3 measurements are also utilized to map the vertical and horizontal extent of the urban plume. The use of multiple instrumented research aircraft permitted highly detailed mapping of the plume chemistry in the vertical and horizontal dimensions. Interactions between the urban Nashville plume (primarily a NOx and hydrocarbon source) and the Gallatin coal-fired power plant plume (primarily a NOx and SO2 source) are also documented, and comparisons of ozone formation in the isolated and mixed urban and power plant plume are presented. The data suggest that during this episode the background air and the edges of the urban plume are NOx sensitive and the core of the urban plume is hydrocarbon sensitive. Under these worst case meteorological conditions, ambient O-3 levels well over the level of the new National Ambient Air Quality Standard (NAAQS) for ozone (80 ppb) were observed over and just downwind of Nashville. For example, on July 12, the boundary layer air upwind of Nashville showed 60 to 70 ppb O-3, while just downwind of the city the urban plume maximum was over 140 ppb O-3. With a revised ozone standard set at 80 ppb (8 hour average) and upwind levels already within 10 or 20 ppb of the standard, only a slight increase in ozone from the urban area will cause difficulty in attaining the standard at monitors near the care of the urban plume during this type of episode. The helicopter mapping and LIDAR aircraft data clearly illustrate that high O-3 levels can occur during stagnation episodes within a few kilometers of and even within the urban area. The extremely light boundary layer winds (1-3 m s(-1)) contributed to the creation of an ozone dome or blob which stayed very near to the city rather than an elongated plume. The small spatial scale of the zone of high O-3 concentrations is mapped in detail demonstrating that the regulatory monitoring network failed to document the maximum O-3 concentrations. Modelers using such regulatory data to test photochemical algorithms need to bear in mind that magnitude and frequency of urban ozone may be underestimated by monitoring networks, especially in medium-sized urban areas under slow transport conditions. Finally, this effort shows the value of collaborative field measurements from multiple platforms in developing a more complete picture of the chemistry and transport of photochemical O-3.

This study examines the use of ambient measurements of a number of "photochemical indicators" as a basis for determining ozone-NOx-hydrocarbon sensitivity and for evaluating the performance of ozone models. The successful photochemical indicators are: 03/NO , 03/NOz (where NOz = NOy-NOx), 03/HN03, H202/HN03, and H202/NOz. Results of Urban Airshed Model (UAM-IV) simulations for Atlanta, GA, New York, NY, and Los Angeles, CA, show that high values of these species ratios are correlated with NOx-sensitive chemistry and low values are associated with reactive organic gases (ROG)-sen-sitive chemistry. Correlations between measured 03 and NO in Atlanta and between 03 and NOz in Los Angeles are consistent with theory and reflect the difference between likely NOx-sensitive chemistry in Atlanta and hydrocarbon-sensitive chemistry in Los Angeles. Measured 03, NOx and NO are used to evaluate model performance during two air pollution events in Atlanta and Los Angeles. The performance evaluation includes model scenarios for each city with different anthropogenic and biogenic emission rates and different NOx-ROG sensitivity predictions. Simulations with different NOx-ROG chemistry are found to give similar predictions for peak ozone but different values for photochemical indicators. Comparison with measured values of photochemical indicators provides a more stringent test of model performance than evaluation versus observed ozone.

The mercury mass balance for a coal fired power plant was prepared from measurements at all inputs and outputs. The fate of mercury was determined from samples taken simultaneously in the ducts, stack, and plume. Scrubber solution analysis, and samples taken across the scrubber, show that oxidized mercury is scrubbed almost completely, with less than 0.1% of the total as methyl mercury. Detecting the plume with a rapid NO{sub x} detector, helicopter-mounted samplers collected separate samples of gaseous and particulate mercury. Analysis showed that elemental mercury continues to oxidize following the scrubber, at temperatures below 60{degrees}C, and oxidized mercury attaches to plume particulates.

As part of the Southern Oxidant Study, The Tennessee Valley Authority's 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 O3 plume. Air samples were taken during morning and afternoon hours; the morning data were used to estimate background O3 and the afternoon data were used to estimate O3 production efficiency, i.e. the number of O3 molecules produced per molecule of NOy emitted. Detailed data on O3 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 O3 and its precursors were diluted, at the same rate. O3 peak levels, or the transition from net production to dilution occurred at 20–40 km from the city center. O3 production efficiency for the five afternoon flights was between 4 and 10, in good agreement with previous surface measurements.

A widely used passive cloudwater collector was operated continuously along with an automated canopy water throughfall measurement system at a high elevation spruce forest site. Cloudwater collection rate (Rc) and throughfall (TF) rate were examined to determine their interrelationship. In addition, the use of the cloudwater collector for inferring cloud liquid water content was examined. The degree of direct hourly correlation was fairly good between Rc and canopy TF rate for non‐rain periods when the canopy was saturated (the former explained 58% of the variance in the latter). The correlation was even better (R² = 0.84) when TF rate was time‐lagged one hour. Estimated liquid water content, using collector data, was characterized by a large degree of uncertainty. This uncertainty appears to be caused, to a large extent, by the inability of the estimation method to account for the variation in cloudwater collection efficiency as a function of wind speed and droplet size.

A high elevation atmospheric monitoring station has been in operation at the summit (1689) of Whitetop Mountain, Virginia (N 36{degrees}38', W 81 {degrees}36') since the fall of 1985. The primary focus of measurements made at this station is to characterize acidic input to the surrounding spruce fir forests of Whitetop Mountain and neighboring Mount Rogers. The purpose of this paper is to describe the measurement results of a subset of the existing data base-those cloud water measurements made during the first two summers of the project (1986 and 1987). Analysis of cloud water concentration, cloud type, and air mass origin are made for each cloud event occurring during three to four week long measurement intensives. Also, both qualitative and quantitative analyses are made of regional source/receptor relationships.