C. Fred Rogers

A laboratory resuspension chamber to measure fugitive dust size distributions and chemical compositions

Abstract A laboratory resuspension chamber is described that segregates dried and sieved geological material into size fractions less than 0, 1.0, 2.5, 10 and ∼30 gmm aerodynamic diameters onto filter media suitable for chemical analysis. Eight impactor inlets located in a rectangular array within a 0.4 m 3 chamber provide the size segregation. Tests show that mass concentrations across the array vary by no more than ± 10%. Filter samples are normally submitted to gravimetric, X-ray fluorescence, ion chromatography, automated colorimetric, atomic absorption spectrophotometric, and thermal/optical reflection analysis for ∼ 50 chemical species. Chemical profiles and size distribution have been measured on more than 200 dust samples throughout the world.

Sensitivity of estimated light extinction coefficients to model assumptions and measurement errors

Abstract The optical properties of aerosol particles, expressed in terms of scattering and absorption coefficients, determine radiation transfer and visibility in the atmosphere. In principle, given sufficiently detailed input data regarding particle concentration, morphology, size, and index of refraction, particulate scattering and absorption coefficients can be estimated. In reality, the estimation of light extinction is constrained by our limited ability to measure the physical and chemical properties of aerosol particles. To evaluate the reliability of light-extinction estimates under such constraints, we applied impactor size distribution inversion and Mie scattering models to several urban and rural U.S. aerosol data sets. The scattering algorithm includes five chemical components, nitrate, sulfate, organic carbon, elemental carbon, and geological dust in internally mixed particles. Particle composition may be treated as homogeneous or distributed between an insoluble core and an aqueous shell. Liquid water is added to dry aerosol mass in discrete size bins and a distribution number is estimated. Extinction is calculated with Mie theory. For the data sets examined, light scattering estimated with this model agreed with measured scattering to within 26% on average. We describe the sensitivity of the method to input assumptions about particle composition and morphology, liquid water as a function of relative humidity, and particle size distribution. Apportioning estimated scattering to chemical components of an ambient aerosol using species extinction efficiencies has no clear theoretical basis. The merits of several approaches for doing so are examined.

Long-Term Efficiencies of Dust Suppressants to Reduce PM10 Emissions from Unpaved Roads

A 14-month study was undertaken to assess the long-term efficiencies of four dust suppressants (i.e., biocatalyst stabilizer, polymer emulsion, petroleum emulsion with polymer, and nonhazardous crude-oil-containing materials) to reduce the emission of PM10 from public unpaved roads. PM10 emission rates were calculated for each test section and for an untreated section for comparison purposes. Emission rates were determined from PM10 concentrations measured from 1.25 m to 9 m upwind and downwind of the road and above its surface. Calculated emission factors ranged between zero and 1,361 g-PM10/vehicle kilometer traveled (VKT) (average uncertainty = ±35 g-PM10/ VKT) for the four types applied. One week after application, suppressant efficiencies ranged between 33% and 100% for the four types applied. After 8-12 months of exposure to weathering and 4,900-6,400 vehicle passes, the suppressant efficiencies ranged from zero to 95%. Roadway surface properties associated with low-emitting, well-suppressed surfaces are (1) surface silt loading and (2) strength and flexibility of suppressant material as a surface layer or cover. Suppressants that create surface conditions resistant to brittle failure are less prone to deterioration and more likely to increase long-term reduction efficiency for PM10 emissions on unpaved roads.

Measurements of mobile source particulate emissions in a highway tunnel

Recent studies have linked atmospheric fine particulate matter (PM2.5) with human health effects. In many urban areas, mobile sources are the dominant source of PM2.5). Dynamometer studies have also implicated diesel engines as being a significant source of ultrafine particles. In order to characterise particulate emissions from in-use vehicles, we performed an on-road study of emissions from vehicles operating in the Tuscarora Mountain Tunnel along the Pennsylvania Turnpike. As part of this study, we obtained chemically speciated, size-segregated PM emission rates, particle size distributions, chemically speciated profiles of diesel emissions for use in source apportionment studies, a comparison with years past how much improvement there has actually been in diesel particulate emission rates, and measurements of particulate emission rates from light-duty gasoline vehicles to evaluate the relative significance of this source. This paper describes the experimental methods and presents the preliminary results of the on-road particulate emissions measurements.

Measurements of Aerosol Particle Size: Improved Precision by Simultaneous Use of Optical Particle Counter and Nephelometer

Improved accuracy and precision in size measurements of submicrometer volatile particles are achieved by reconciling simultaneous measurements by optical particle counter (OPC) and nephelometer. Reconciliation of data obtained from the two instruments required a careful calibration and measurement of relative humidity in both instruments. Correspondence between the nephelometer response and that calculated from the OPC size distribution provides a standard that limits uncertainty and inaccuracy in the OPC size measurement. This approach is demonstrated for size measurements of volatile submicrometer aqueous NaCl droplets, for which a least-squares regression of the data provided a 0.95 correlation coefficient. The quantitative reconciliation of the two optical measurements provides an experimental test of their accuracy.

Characterization of Exhaust Particles from Military Vehicles Fueled with Diesel, Gasoline, and JP-8

Abstract Diluted exhaust from selected military aircraft ground-support equipment (AGE) was analyzed for particulate mass, elemental carbon (EC) and organic carbon (OC), SO4 2−, and size distributions. The experiments occurred at idle and load conditions and utilized a chassis dynamometer. The selected AGE vehicles operated on gasoline, diesel, and JP-8. These military vehicles exhibited concentrations, size distributions, and emission factors in the same range as those reported for nonmilitary vehicles. The diesel and JP-8 emission rates for PM ranged from 0.092 to 1.1 g/kg fuel. The EC contributed less and the OC contributed more to the particulate mass than reported in recent studies of vehicle emissions. Overall, the particle size distribution varied significantly with engine condition, with the number of accumulation mode particles and the count median diameter (CMD) increasing as engine load increased. The SO4 2− analyses showed that the distribution of SO4 2− mass mirrored the distribution of particle mass.

A Quantitative Description of Vehicle Exhaust Particle Size Distributions in a Highway Tunnel

Abstract During the period May 18-May 22, 1999, a comprehensive study was conducted in the Tuscarora Mountain Tunnel on the Pennsylvania Turnpike to measure real-world motor-vehicle emissions. As part of this study, size distributions of particle emissions were determined using a scanning mobility particle sizer. Each measured size distribution consisted of two modes: a nucleation mode with midpoint diameter less than 20 nm and an accumulation mode with midpoint diameter less than 100 nm. The nucleation and accumulation components in some distributions also exhibited second maxima, which implies that such particle size distributions are superpositions of two particle size distributions. This hypothesis was utilized in fitting the particle size distributions that exhibited second maxima with four lognormal distributions, two for the nucleation mode and two for the accumulation mode. The fitting assumed that the observed particle size distribution was a combination of two bimodal log-normal distributions, one attributed to the heavy-duty diesel (HDD) vehicles and another attributed either to a different class of HDD vehicles or to the light-duty spark ignition vehicles. Based on this method, estimated particle production rates were 1.8 × 1013 and 2.8 × 1014 particles/vehicle-km for light-duty spark ignition and HDD vehicles, respectively, which agreed with independently obtained estimates.

Resuspension of Particles by Aerodynamic Deagglomeration

A deagglomerator system was developed, characterized by laboratory tests, and flown under low-gravity (low-g) microgravity conditions. Requirements for a dry powder deagglomeration system were generated by university and National Aeronautics and Space Administration (NASA) scientists from diverse fields of interest including exobiology, planetary sciences, and atmospheric sciences. Existing deagglomeration methods and devices are reviewed. An aerosol generation method suitable for dry powders over a large range of particle sizes and types at high concentrations with consistent deagglomeration efficiency are evaluated. Development of a pulsed-flow laboratory device and experimental approaches to meet the requirements without being g-dependent are described. Results of laboratory one-g quantitative characterization on one type of dry powder particle generator is discussed. Data from NASA low-g tests are summarized.

A fine‐particle sodium tracer for long‐range transport of the Kuwaiti oil‐fire smoke

Evidence for long-range transport of the Kuwaiti oil-fire smoke during the months following the Persian Gulf War has been more or less indirect. For example, high concentrations of aerosol particles containing soot and oil-combustion tracers such as vanadium observed at great distances from the Middle East may have come from sources other than the oil fires. However, more-recent data on the aerosol chemistry of Kuwaiti oil-fire plumes provides a direct link between those fires and aerosols collected at the Mauna Loa Observatory (MLO) during the late spring and summer of 1991. By itself, temporal covariation of fine-particle concentrations of elemental carbon, sulfur, and the noncrustal V / Zn ratio in MLO aerosols suggested a link to large-scale oil-combustion sources, but not necessarily to Kuwait. However, high concentrations of fine-particle (0.1–1.0 µm diameter) NaCl were observed in the “white” oil-fire plumes over Kuwait during the summer of 1991. Further analysis of the Mauna Loa data indicates strong temporal correspondence between the noncrustal V / Zn and noncrustal Na / Zn ratios and strong consistency between the noncrustal Na to noncrustal V ratios found at Mauna Loa and in the Kuwaiti oil-fire plume. In the absence of other demonstrable sources of fine-particle Na, these relationships provide a direct link between the Kuwaiti oil fires and aerosol composition observed at MLO.

Field comparison of optical particle counters

Abstract Particle size distribution measurements were obtained simultaneously at a coastal field site with six different commercial optical particle counters (OPCs) in clear-air and in fog. Systematic differences between the measurements from the various instruments were documented. The differences were smallest (factor of 1.5–2.5) for sub-micron particles and largest (factor of 8–15) for super-micron particles. These discrepancies were due to inaccuracies in the instruments and to sampling errors. The high size resolution of one OPC enabled the sub- and super-micron droplet populations in fog to be measured. Significant differences occurred between the concentrations of equivalent size particles in clear-air and in fog demonstrating the sensitivity of the measurements to the free-air relative humidity.