M. Luria

The correlation of temperature and rural ozone levels in southeastern U.S.A.

Abstract As a part of the southern oxidant study, a comprehensive air quality study was performed at a rural site in southern Tennessee. The observations performed between 25 July and 2 September 1991 included measurements of primary and secondary pollutants as well as meteorological measurements. Relatively high levels of NOy were observed when CO and SO2 levels were at estimated regional background indicating a significant non-combustion source. A multivariate linear regression analysis suggested that nearly 2 3 of the NOy above the estimated regional background level relates to SO2 emitting sources with the remaining 1 3 to CO emitting sources. The association between primary pollutants and temperature was found to be weak or insignificant. However, most secondary pollutants (except PAN) positively correlated with temperature. In the case of O3, a better association was found with a combination of temperature and NOy. Comparison between the observation and model simulation suggested that approximately 50% of the increase in O3 levels observed at the Giles site may relate to the intrinsic dependence of rate constants on temperature. The number of O3 molecules produced per NOy molecule present increases with temperature (between 22 and 33°C) as did the chemical air mass age ( NO x NO y ). On the other hand, the number of O3 molecules produced per molecule of NOx consumed remained nearly constant. The difference between NOz and the sum of the individual NOz species measured separately (PAN, HNO3 and nitrate aerosol) suggests the presence of an additional NOz species that increased with temperature.

Atmospheric oxidation of flue gases from coal-fired power plants—A comparison between conventional and scrubbed plumes

Abstract A series of plume samples was taken by instrumented aircraft to determine atmospheric oxidation rates of flue gases from coal-fired power plants. This study was carried out at two of TVAs large power plants: Colbert Steam Plant, located in Pride, Alabama, a conventional power plant that burns high (3.7%) sulfur coal; and Widows Creek Steam Plant, located at Stevenson, Alabama, which generates 29% of its electrical power from a boiler that burns high (3.9%) sulfur coal but that is equipped with a wet limestone SO 2 scrubber. The average atmospheric oxidation rates at both sites were almost identical, indicating that the atmospheric oxidation rate is not significantly affected by the presence of a scrubber. The average morning rates for SO 4 = and NO 3 − formation for both the scrubbed and unscrubbed plumes were found to be 0.012 and 0.029 h −1 respectively. Rates larger by a factor of two were found for the afternoon measurements. For both parts of this study, the average rate for NO conversion to NO 2 was found to be 0.51 h −1 , and the rate for NO x removal was estimated to be 0.12 h −1 . On one day during the second part of this study (August 23, 1978), net O 3 production in the plume was observed. Net production of O 3 was attributed to the mixing of the power plant plume with a polluted airmass transported from Chattanooga, Tennessee.

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.

Dynamics of sulfate particle production and growth in smog chamber experiments

The dynamics of sulfate particle formation from flue gases of a coal-fired power plant were studied in a smog chamber, to simulate atmospheric conversion of sulfur dioxide to particulate sulfate. A very good agreement between the experimental data and the model predictions was obtained. The photochemical smog chamber used was a of cylindrical shape, mounted on transportable chassis. 27 refs.

Cross-sectional studies of plumes from a partially SO2-scrubbed power plant☆

An instrumented helicopter was used at the Tennessee Valley Authority (TVA) Widows Creek Steam Plant to collect a series of samples at several elevations within the plume at fixed distances downwind. During one segment of this study, much higher oxidation rates were observed in the lower part of the plume than in the upper part. These rates occurred on a day when the plume could be clearly separated into two parts. The upper part could be traced to the 305-m stack, which emits flue gases from combustion of low-sulfur coal. The lower part could be traced to two remaining sources, one of which is equipped with a wet limestone scrubber for flue gas desulfurization. A detailed analysis of the plume structure ruled out the possibility that the higher oxidation rate was associated with the scrubbed plume. The authors believe that the higher rate was a result of greater dispersion of the lower plume caused by a cross-wind shear and a mechanically-induced turbulence resulting from the special topography of the area and by a developing thermal boundary layer. Two edge-of-plume effects—maximum NO2 concentration at the border of the plume and a high concentration of condensation nuclei in the vicinity of the plume-are also discussed.

Model calculations of the chemical processes occurring in the plume of a coal-fired power plant

Abstract Computer simulations of the homogeneous, gas phase chemical reactions which occur in the plume of a coal-fired power plant were conducted in an effort to understand the influence of various environmental parameters on the production of secondary pollutants. Input data for the model were selected to reproduce the dilution of a plume from a medium-sized power plant. The environmental conditions chosen were characteristic of those found during mid-August in the south-eastern United States. Under most conditions examined, it was found that hydroxyl radicals were the most important species in the homogeneous conversion of stack gases into secondary pollutants. Other free radicals, such as HO2 and CH3O2, exceeded the contribution of HO radicals only when high background hydrocarbon concentrations are used. The conversion rates calculated for the oxidation of SO2 to SO42− in these plumes were consistent with those determined experimentally. The concentrations and relative proportions of NOx (from the power plant) and reactive hydrocarbons (from the background air) determine, to a large extent, the plume reactivity. Free radical production is suppressed during the initial stages of dilution due to the high NOx levels. Significant dilution is required before a suitable mix is attained which can sustain the free radical chain processes common to smog chemistry. In most cases, the free radical concentrations were found to pass through maxima and return to background levels. Under typical summertime conditions, the hydroxyl radical concentration was found to reach a maximum at a HC NO x ratio of approximately 20. The meteorological conditions, as well as the ambient and stack gas concentrations, determine the plume travel time at which the hydroxyl and other free radical concentration maximum occurs. The model also predicts the presence of ozone ‘bulges’ under a wide variety of environmental and plant operational conditions.

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.