Robert A. Eldred

Spatial and seasonal trends in particle concentration and optical extinction in the United States

In the spring of 1988 an interagency consortium of Federal Land Managers and the Environmental Protection Agency initiated a national visibility and aerosol monitoring network to track spatial and temporal trends of visibility and visibility-reducing particles. The monitoring network consists of 36 stations located mostly in the western United States. The major visibility-reducing aerosol species, sulfates, nitrates, organics, light-absorbing carbon, and wind-blown dust are monitored as well as light scattering and extinction. Sulfates and organics are responsible for most of the extinction at most locations throughout the United States, while at sites in southern California nitrates are dominant. In the eastern United States, sulfates contribute to about two thirds of the extinction. In almost all cases, extinction and the major aerosol types are highest in the summer and lowest during the winter months.

Long-range transport of North African dust to the eastern United States

The long-range transport of North African dust to the Middle East, Europe, South America, and the Caribbean has been well documented during the past 25 years. With the advent of routine collection and analysis of fine aerosols at national parks, monuments, and wilderness areas in the continental United States, these North African dust incursions can now be tracked, characterized, and quantified across much of the eastern half of the United States. Identification of the North African source of these dust episodes is confirmed by mass distribution measurements, a characteristic Al/Ca ratio, isentropic backward air mass trajectories, and sequential plots of the spatial distribution of the dust plumes. North African dust incursions into the continental United States persist for ∼10 days and occurred, on average, 3 times per year from 1992 to 1995. Fine soil mass usually exceeds 10 μg m−3 during these dust episodes and dominates local fine soil dust by an order of magnitude or more, even in the so-called “dust bowl” states of the central United States. Size-resolved measurements of elemental composition taken during July 1995 indicate that the mass mean diameter of the transported North African dust is <1 μm. The high mass scattering efficiency and abundant particle surface area associated with these submicron soil aerosols could have important consequences for both the radiative balance of the region and the chemistry of the local aerosols during summer when the long-range transport of North African dust to the United States is most common.

Examining the relationship among atmospheric aerosols and light scattering and extinction in the Grand Canyon area

During the winter and summer months of 1990 a special study called Project MOHAVE (measurement of haze and visual effects) was carried out with the principle objective of attributing aerosol species to extinction and scattering and the aerosol species to sources and/or source regions. The study area included much of southern California and Nevada, Arizona, and Utah; however, the intensive monitoring sites and primary focus of the study was on the Colorado Plateau of northern Arizona, southern Nevada, and Utah. This paper reports on the apportionment of various aerosol species to measured fine and coarse mass concentrations and these species to scattering and extinction. The study is unique in that a number of “ambient” integrating nephelometers were operated to measure the ambient scattering coefficient, while transmissometers were used to measure atmospheric extinction. Comparison of measured scattering, extinction, and aerosol species concentration, both statistically and theoretically, allows for an estimate of scattering and absorption efficiencies. Analysis suggests that using elemental carbon, derived from thermal optical techniques, to estimate absorption may significantly underestimate absorption. Using elemental carbon, absorption is estimated to be 5% of extinction, while direct measurements of absorption suggest that it is about 30% of measured extinction. Furthermore, because light absorption by soil is usually not accounted for, soil extinction is underestimated by about 30%.

Analysis of respirable fractions in atmospheric particulates via sequential filtration

It has long been recognized that information on particle size distributions in atmospheric aerosals is necessary for meaningful evaluations of potential health hazards. Such information is also important in establishing particulate sources, transport, transformations, and sinks, especially in combination with elemental and chemical data. While instruments exist to collect size segregated samples of particulates, they are too complex and expensive to encourage use of multiple units in field situations.

Examining the relationship between atmospheric aerosols and light extinction at Mount Rainier and North Cascades National Parks

Abstract During the summer of 1990, the National Park Service carried out a study in the state of Washington called the Pacific Northwest Regional Visibility Experiment using Natural Tracers (PREVENT). The goal of the study was to apportion atmospheric aerosols to scattering and extinction and to source types at Mount Rainier and North Cascades National Parks. The study was designed to collect all necessary emissions, meteorology, ambient concentrations, and atmospheric optical data necessary to support a variety of source attribution techniques. This paper will report on the apportionment of various aerosol species to measured fine mass concentrations and ambient scattering coefficients. One highlight of this study was the near-ambient measurement of atmospheric scattering with a modified integrating nephelometer. It is therefore possible to explore the relationship between hygroscopic aerosols and scattering in the ambient atmosphere.

Composition of PM2.5 and PM10 Aerosols in the IMPROVE Network

Abstract PM2.5 and PM10 particulate concentrations measured at 42 sites in the Interagency Monitoring of Protected Visual Environments(IMPROVE) network at Class I visibility areas throughout the United States over the 1993 seasonal year showed well-defined regional patterns. The PM2.5 concentration had a large gradient from West to East, averaging 3 μg/m3 in most of the West compared to 13 μg/m3 in the Appalachian region. Coarse particle concentration (2.5-10 μm) exhibited a small gradient from North to South, but no gradient from West to East. No change in PM2.5 or PM10 was observed from 1988 to 1993 for sites with complete records. The elemental concentration (H, Na-Pb) was determined from all PM2.5 Teflon filters and from the PM10 filters from four sites: an eastern and western urban site (Washington, D.C. and South Lake Tahoe) and the nearest Class I site (Shenandoah and Bliss). Soil was the largest component of the coarse particle concentration. Sulfur accounted for 3% of the coarse particle concentr...

Characterization of particles in the arid west

Abstract This paper describes spatial and temporal variations of airborne paniculate matter in the eight western states included in the western fine particle network (WFPN). The samples were generated by a 40-site monitoring network of remotely sited dichotomous samplers of the stacked filter and virtual design, with a coarse fraction between 15 μm and 2.5 μm dia. and a fine fraction below 2.5 μm dia. The units operate for two 72 h period each week, delivering samples analyzed gravimetrically for mass and by particle induced X-ray emission (PIXE) for elements sodium and heavier. Results are presented for the period October 1979 to May 1980, showing regional patterns of particles especially in the fine modes. A sulfur episode which occurred in the southwest is examined via trajectory analysis, while factor analysis is applied to the entire data set to generate information on particulate sources.

Dating Rock Varnishes by the Cation Ratio Method with PIXE, ICP, and the Electron Microprobe

The measurement of rock varnish cation-ratios [(K+Ca)/Ti] and barium is evaluated by analyzing the same varnish scrapings with PIXE, inductively coupled plasma, neutron activation and wavelength dispersive electron microprobe. Results among these different methods are generally similar for ratios, but absolute concentrations differ in part due to uncertainties associated with weighing small samples. Barium concentrations are typically less than 1 % by weight; higher concentrations can be found in varnishes with depressions eroded into the varnish by fungi and lichens, and later infilled with aeolian detritus often including barium sulfate. Since these infilled hollows are known to produce anomalous varnish cation ratios, high barium values can, therefore, be used as an indicator that a sample is inappropriate for cation-ratio dating. The glacial chronology at Pine Creek, California, is revised in light of new data and a better understanding of variables influencing varnish chemistry.

Trends in elemental concentrations of fine particles at remote sites in the United States of America

Abstract The University of California at Davis has been monitoring particulate concentrations in remote sites throughout the United States of America since 1979 in networks operated for the National Park Service and the Environmental Protection Agency. Twelve sites in remote class I visibility areas have almost complete records from June 1982 to August 1992, seven in the southwest, three in the northwest, and two in the east. During this period, two samples of fine particles (0–2.5 μm) on Teflon filters were collected every week and analysed for elemental concentration by Particle Induced X-ray Emission (PIXE). This paper will examine the historical trends for sulfur, zinc, lead, and the soil elements. Measurements during the past four years have verified that the sulfur is present as sulfate. In the southwest, 80% of the sulfur trends in spring, summer and fall decreased, while most of the winter trends increased. The annual trends decreased at six of the seven sites. The trends in the northwest increased slightly. The two eastern sites had the most important trends, with significant increases of almost 4% per year in summer, 1–3% increases in spring and fall, and 2% decreases in winter. The annual increases were between 2 and 3%. Generally there were no significant trends for zinc and the soil elements. Lead at all sites decreased sharply through 1986, corresponding to the shift to unleaded gasoline, but has since leveled off at around 18% of the 1982 means. The most important conclusion of this study is that through the use of stable sampling and analytical protocols, we have been able to determine statistically significant historical trends of as small as 1–2% per year for sites with 10-year records.

Indirect Measurement of Hydrocarbon Aerosols Across the United States by Nonsulfate Hydrogen-Remaining Gravimetric Mass Correlations

In order to determine the levels of fine organic aerosols at remote sites, two methods were devised to calculate organic concentration from data collected on chemically inert teflon filters by the 34-site National Park Service (NPS) Fine Particle Network from 1982 to 1986. The remaining mass method is based on the difference between the measured gravimetric mass and the sum of the ammonium sulfate, soil, and soot carbon components, derived from measured parameters. This remaining mass is between 53% and 67% organic matter, based on two studies measuring organic carbon directly in the arid west. The nonsulfate hydrogen method is based on the concentration of particulate hydrogen, which has been measured on NPS samples since 1984, using techniques developed at Davis, CA. After removing the contribution associated with ammonium sulfate, the remaining nonsulfate hydrogen is predominantly due to organic matter. The two methods are in good agreement, with a sample to sample correlation of 0.76. For each western...