Peter K. Mueller

Seasonal variations in elemental carbon aerosol, carbon monoxide and sulfur dioxide: Implications for sources

As part of Maryland Aerosol Research and CHaracterization (MARCH-Atlantic) study, measurements of 24-hr average elemental carbon (EC) aerosol concentration were made at Fort Meade, Maryland, USA, a suburban site within the Baltimore-Washington corridor during July 1999, October 1999, January 2000, April 2000 and July 2000. Carbon monoxide (CO) and sulfur dioxide (SO2) were also measured nearly continuously over the period. Tight correlation between EC and CO in every month suggests common or proximate sources, likely traffic emissions. The EC versus CO slope varies in different seasons and generally increases with ambient temperature. The temperature dependence of EC/CO ratios suggests that EC source strength peaks in summer. By using the well established emission inventory for CO, and EC/CO ratio found in this study, EC emission over North America is estimated at 0.31±0.12 Tg yr−1, on the low end but in reasonable agreement with prior inventories based on emission factors and fuel consumption.

Analysis of a summertime PM2.5 and haze episode in the mid-Atlantic region.

Abstract Observations of the mass and chemical composition of particles less than 2.5 μm in aerodynamic diameter (PM2.5), light extinction, and meteorology in the urban Baltimore-Washington corridor during July 1999 and July 2000 are presented and analyzed to study summertime haze formation in the mid-Atlantic region. The mass fraction of ammoniated sulfate (SO4 2-) and carbonaceous material in PM2.5 were each ∼50% for cleaner air (PM2.5 < 10 μg/m3) but changed to ∼60% and ∼20%, respectively, for more polluted air (PM2.5 > 30 μg/m3). This signifies the role of SO4 2- in haze formation. Comparisons of data from this study with the Interagency Monitoring of Protected Visual Environments network suggest that SO4 2− is more regional than carbonaceous material and originates in part from upwind source regions. The light extinction coefficient is well correlated to PM2.5 mass plus water associated with inorganic salt, leading to a mass extinction efficiency of 7.6 ± 1.7 m2/g for hydrated aerosol. The most serious haze episode occurring between July 15 and 19, 1999, was characterized by westerly transport and recirculation slowing removal of pollutants. At the peak of this episode, 1-hr PM2.5 concentration reached ∼45 μg/m3, visual range dropped to ∼5 km, and aerosol water likely contributed to ∼40% of the light extinction coefficient.

Coupling Thermodynamic Theory with Measurements to Characterize Acidity of Atmospheric Particles

The reference method currently being used in community health studies to characterize the acidity of atmospheric particles is based upon measuring the pH of aqueous extracts of atmospheric particles collected on filter media. These measurements represent total extractable acidity, i.e., hydrogen ion concentration at sufficient dilution of the sample, rather than the actual acidity of airborne particles. In contrast, models based upon chemical equilibrium theory can be used to estimate the hydrogen ion concentration and water content of atmospheric particles from the observed concentration of major solutes (sulfate, nitrate, chloride, ammonium, and sodium). Using about 100 data points from an intensive measurement study in Uniontown, Pennsylvania during 1990, we examined the nature and the causes of the deviations between measured and estimated hydrogen ion concentrations. We found that the measured hydrogen ion concentrations were substantially higher than the estimated concentrations because dilution of ...

Study of the relationship of distant SO2 emissions to Dallas-Fort Worth winter haze.

ABSTRACT A study was conducted to estimate the changes in wintertime visual air quality in Dallas-Fort Worth (DFW) that might occur due to proposed reductions in SO2 emissions at two steam electric generating plants in eastern Texas, each over 100 km from the city. To provide information for designing subsequent investigations, the haze was characterized broadly during the first year of the study. Meteorological data acquired then demonstrated that, during haze episodes, emissions from only one of the two plants were likely to be transported directly to DFW. Therefore, the second year of the study was centered on just one of the power plants. Air quality was then characterized within the urban area and at rural locations that would be upwind and downwind of the plant during transport to DFW. An instrumented aircraft measured plume dispersion and the air surrounding the plume on selected days. A mathematical model was used to predict the change that would occur in airborne particulate matter concentrations in DFW if SO2 emissions were reduced to reflect the proposed limitations. The contribution of particles in the atmosphere to light extinction was estimated, and simulated photographs were produced to illustrate the visibility changes. The study concluded that the proposed emission reductions would, at most, subtly change perceived wintertime visibility.

Modeling of potential power plant plume impacts on Dallas-Fort Worth visibility.

ABSTRACT During wintertime, haze episodes occur in the Dallas-Ft. Worth (DFW) urban area. Such episodes are characterized by substantial light scattering by particles and relatively low absorption, leading to so-called “white haze.” The objective of this work was to assess whether reductions in the emissions of SO2 from specific coal-fired power plants located over 100 km from DFW could lead to a discernible change in the DFW white haze. To that end, the transport, dispersion, deposition, and chemistry of the plume of a major power plant were simulated using a reactive plume model (ROME). The realism of the plume model simulations was tested by comparing model calculations of plume concentrations with aircraft data of SF6 tracer concentrations and ozone concentrations. A second-order closure dispersion algorithm was shown to perform better than a first-order closure algorithm and the empirical Pasquill-Gifford-Turner algorithm. For plume impact assessment, three actual scenarios were simulated, two with clear-sky conditions and one with the presence of fog prior to the haze. The largest amount of sulfate formation was obtained for the fog episode. Therefore, a hypothetical scenario was constructed using the meteorological conditions of the fog episode with input data values adjusted to be more conducive to sulfate formation. The results of the simulations suggest that reductions in the power plant emissions lead to less than proportional reductions in sulfate concentrations in DFW for the fog scenario. Calculations of the associated effects on light scattering using Mie theory suggest that reduction in total (plume + ambient) light extinction of less than 13% would be obtained with a 44% reduction in emissions of SO2 from the modeled power plant.

The modification of a semi-empirical long-range transport model to allow estimation of ambient sulfate concentrations

Abstract This paper describes the modification of a semi-empirical long-range transport model (STATMOD) to allow the estimation of ambient SO2 and sulfate concentrations, and sulfur concentrations in rain. These substances are relevant to the chemistry of deposition and the optical properties of aerosols. The improved model incorporates a treatment of SO2 oxidation to sulfate in non-precipitating clouds. Model parameters were determined by fitting model estimates of ambient SO2 and sulfate concentrations, and sulfate concentrations in rain, to corresponding observations from the Eulerian Model Evaluation and Field Study (EMEFS) and the National Dry Deposition Network (NDDN) databases for the period July-August-September 1988. The derived value of the parameter corresponding to the in-cloud conversion of SO2 suggests that sulfate formed in non-precipitating clouds made a substantial contribution to ambient sulfate concentrations belonging to the 1988 data set used in this study. To evaluate the general applicability of the parameter value, model estimates were compared with observations obtained from the Sulfate Regional Experiment (SURE), the Eastern Regional Air Quality Study (ERAQS), and the Acid Deposition System (ADS) databases for a similar time period in 1978. The results showed that the estimated concentrations were correlated with the observed concentrations of sulfur species for a similar period of a different year. This provides a test of the realism of the model as a first-cut engineering tool to examine the relationship between SO2 emissions and ambient sulfate levels.