Elizabeth M. Bailey

O3 and NO y relationships at a rural site

Measurements of O3, NO, NO2, peroxyacetyl nitrate (PAN), HNO3, and NOy were made during a 6-week period in the summer of 1991 in Giles County, Tennessee. These data were analyzed to determine the factors controlling the relationship between O3 and NOy at this rural site. A strong association was observed between the O3 and NOx oxidation product (NOz = NOy - NOx) levels. The higher O3 levels were associated with air masses impacted by higher NOx emissions that had been photochemically processed. An analysis of the data indicates that the ultimate O3 production is about 10 molecules of O3 produced for each molecule of NOx emitted. The analysis results also suggest that O3 net production continues until about 70% of the NOx has been converted into NOz. The PAN/HNO3 ratios observed suggest that the air masses in Giles County are composed of higher volatile organic carbon/NOx ratios than the air masses observed at other rural sites in eastern North America. A comparison of the data analysis results to model simulations and smog chamber experiments suggests that most of the time, Giles County is in an NOx-limited regime for O3 production.

Gas-phase, cloud and rain-water measurements of hydrogen peroxide at a high-elevation site

Abstract Gas-phase hydrogen peroxide (H 2 O 2 ) measurements were made at the summit of Whitetop Mountain (1689 m), VA, during the summer and fall of 1986. Aqueous-phase (clouds and rain) H 2 O 2 measurements were made at the same location during the spring, summer and fall of 1986. Measurements indicate a strong seasonal dependence for H 2 O 2 , with highest levels in the summer and lower concentrations in spring and fall. The mean gas-phase H 2 O 2 concentration measured during the summer study was 0.8 ppb while the fall mean was 0.15 ppb. Gas-phase concentrations were strongly correlated with O 3 , ambient temperature and dew-point and only weakly correlated with light intensity. Hydrogen peroxide exhibited a slight diurnal variation with daytime values exceeding night-time levels by 26%. Cloud-water H 2 O 2 showed no significant correlation with any of the major ions present in cloud-water. The cloud-water H 2 O 2 levels reported include the highest value (247 μM) thus far reported in the literature. The H 2 O 2 concentrations in cloud samples were usually, but not always higher than concentrations measured in rain samples. However, samples collected during a simultaneous cloud-rain event yielded higher H 2 O 2 concentrations in the rain, indicating that H 2 O 2 levels aloft exceeded those measured near the ground.

Relationships between PAN and Ozone at sites in Eastern North America

Measurements of ozone and PAN (peroxyacetic nitric anhydride) were made at four sites in eastern North America; Bondville, Illinois, Egbert, Ontario, Scotia, Pennsylvania, and Whitetop Mountain, Virginia., in July and August of 1988 as part of a study of regional oxidant photochemistry. The concentrations of PAN ranged from <0.010 to 9.2 parts-per-billion by volume (ppbv) and those of 03 ranged from <2 to 139 ppbv. Diurnal concentration profiles showed PAN and 03 to be removed within nocturnal boundary layers, especially if nitric oxide was present, and that 03 was, for most sites, more rapidly removed than PAN. The only mountain top site at which PAN was measured showed distinctly different diurnal profiles in which 03 was actually higher at night, suggesting that convective flow driven by surface cooling served to transport air down from higher in the mixed layer at night. The afternoon production of PAN and 03 was also apparent and led to an overall linear correlation of 03 and PAN between the hours of 1300 and 1800. The comparison of this result with other measurements, estimates of PAN formation rates, and model calculations indicates that the production of PAN relative to ozone is far greater (x3) than can be accounted for by acetaldehyde reactions with OH, there is a wide disparity in model descriptions of PAN production, and confirms the non-linear dependence of 03 production on NOx.

The oxidation of sulfur dioxide to sulfate aerosols in the plume of a coal-fired power plant

Abstract Instrumented aircraft have been used to determine the rate of oxidation of sulfur dioxide to sulfate aerosols within the dispersing plume of a coal-fired power plant. Measurements have been made in the stack and from 10 to 105 km downwind of a 2600-MW power plant. Sampling experiments were conducted during the winter of 1975–1976 and in the fall of 1976. Average temperatures and relative humidities varied from −5 to 17°C and 22 to 71% respectively. Data were also collected on sulfate production at night. An average of 1.1% and a maximum of 4.3% of the plume sulfur was in the form of sulfate. Most of the atmospheric oxidation that was observed appeared to occur in the immediate vicinity of the power plant. The average oxidation rate, beyond 10km, was found to be 0.2% h −1 . Analyses of individual particles showed that the sulfur found in plume particles is usually associated with submicron fly ash particles.

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.

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.

Evaluation of the condensed carbon bond (CB-IV) mechanism against smog chamber data at low VOC and NOx concentrations

Abstract The condensed Carbon Bond mechanism (CB-IV) has been evaluated against smog chamber data obtained under conditions representative of urban air. A synthetic urban VOC mixture was studied at NOx concentrations ranging from 25 to 167 ppb and VOC/NOx ratios ranging from 2.7 to 10. The results indicate that the mechanism underpredicts the rates of O3 formation and NO oxidation by peroxy radicals by 15–30%. The maximum ozone concentration for VOC/NOx ratios of 8–10 was underpredicted by only about 10%. At lower VOC/NOx ratios the maximum amount of ozone formed is underpredicted by about 30%; however, an ozone concentration maximum is not always obtained. PAN is severely underpredicted (60–90%). Measured formaldehyde concentrations are in good agreement with the calculated concentrations. The individual hydrocarbons ethene and propene were also studied under similar conditions and the performance of the CB-IV mechanism for these compounds was tested.

Modeling the effect of ethanol/gasoline blend usage on ozone formation in urban areas

The potential effects of ozone formation which would result if the 1985 vehicle fleet were converted from gasoline to ethanol/gasoline blends were investigated using the EKMA approach. Changes in vehicular emissions were used to calculate changes in ambient HC reactivities and ambient levels of HC, NO/sub x/ and CO for a number of scenarios. Ozone formation was then modeled for each scenario. Effects on ozone formation were not significant unless the usage of ethanol/gasoline blends resulted in deterioration of the emissions control systems. Ambient NO/sub x/HC ratios were shown to have a significant effect on ozone formation.