Robert E. Imhoff

PHOTOCHEMISTRY OF OZONE FORMATION IN ATLANTA, GA-MODELS AND MEASUREMENTS*

Chemical measurements made during an air pollution event in Atlanta, GA have been compared with results from several photochemical simulations. Measurements included Os, primary reactive organic gases (ROG), aldehydes, PAN, total reactive nitrogen (NO,,) and HzOz, with vertical profiles for primary ROG. Photochemical models using two different chemical representations and a range of assumptions about winds, vertical mixing and emissions were used to simulate the event. Results show that assumptions about vertical mixing can cause a variation in simulated surface concentrations of primary hydrocarbons of a factor of two or more. A tendency to underestimate isoprene was found in comparison with measured vertical profiles. The models tend to overestimate concentrations of HCHO, H,02 and PAN in comparison with measurements. Peak 0s and concurrent NO, from helicopter measurements was used as a basis for evaluating individual model scenarios. Scenarios were developed with different Op-NO,-ROG sensitivity, but only the NO,-sensitive scenarios are consistent with measured 09, NO, and isoprene. Key word index: Ozone, nitrogen oxides, hydrocarbons, photochemical smog, hydrogen, peroxide.

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.

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.

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.

Loss rate of NO y from a power plant plume based on aircraft measurements

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.

Ozone yields and production efficiencies in a large power plant plume

Abstract The plume of Tennessee Valley Authoritys 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 NO y 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 O 3 produced per molecule of NO x 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.

Estimates of particle formation and growth in coal-fired boiler exhaust—I. Observations

Abstract Data collected in plumes from coal-fired cyclone boilers are examined to determine particle formation and growth. The emphasis is on fine particles (diameters of 0.2–2 m) most likely to influence plume opacity. For the boilers examined, these particles consist primarily of water-sulfuric acid droplets formed from emitted SO 3 . Observations in plumes from SO 2 -scrubbed and non-scrubbed boilers, under various operating conditions (with different coal types and SO 3 control methods), are used to interpret the influence of operating conditions on plume opacity. Results suggest that the plume particle size distribution is a complex function of boiler operating conditions. Particle concentrations in the critical size range affecting opacity do vary with the magnitude of SO 3 emissions, but absolute concentrations are generally less than expected. These data provide the basis for testing, as described in a companion paper, the performance of a plume particle model.

ANALYSIS OF SUMMERTIME CLOUD WATER MEASUREMENTS MADE IN A SOUTHERN APPALACHIAN SPRUCE FOREST

This paper presents an analysis of cloud water measurements made during the summers of 1986 and 1987 at Whitetop Mountain, Virginia (36.639° N, 81.605° W). Analysis of cloud water chemistry, cloud type, and air mass origin are made for each cloud event occurring during one 3 to 4 week measurement ‘intensive’ per year. Regional source/receptor relationships are also investigated. Cloud water concentrations of major ions (i.e., H+, SO42−, NO3 −, and NH4+) are consistently higher during orographically formed ‘cap’ cloud events. Differences in cloud liquid water content between cap and frontal cloud events explains most, but not all, of the cloud water ion concentration differences. The remaining difference can be explained by greater rainfall associated with frontal cloud events. Most of the cloud water sulfate measured at Whitetop Mountain is apparently due to nucleation of aerosol sulfate within cloud droplets and not to local in-cloud aqueous phase SO, oxidation. No strong source/ receptor relationships were evident from this analysis. Most 72 hr air trajectories arriving at Whitetop Mountain during the cloud events described in this paper originated in the southeastern United States. Few came from the Ohio River Valley or the northeastern United States.

Rates of Conversion of Sulfur Dioxide to Sulfate in a Scrubbed Power Plant Plume

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.

Estimates of particle formation and growth in coal-fired boiler exhaust—II. Theory and model simulations

Abstract A pre-existing model of plume particles is examined to determine its usefulness for simulating particle formation and growth in coal-fired, cyclone boiler exhaust. Of greatest interest are fine particles (diameters of 0.2–2 μm) most likely to influence plume opacity. For the boiler examined, these particles consist primarily of water-sulfuric acid droplets formed from emitted SO 3 . Theoretical consideration is given to nucleation in both homo- and bimolecular systems to allow comparison with the nucleation and growth formulations used in the plume particle model. Observations in plumes from a non-scrubbed boiler, under various operating conditions, are used to evaluate model performance. Model simulations were found to overestimate particle formation and growth rates and inaccurately reproduced observed particle size distributions. Some of this bias is likely due to the particle nucleation formulation in the model, but modeled growth processes also appear to be too fast. Current binary nucleation theory offers little reason to be optimistic that substantial model improvements can be made at this time.