Robert J. Farber

Determination of the particle size distribution and chemical composition of semi-volatile organic compounds in atmospheric fine particles with a diffusion denuder sampling system

Abstract Correct assessment of the factors associated with visibility impairment is dependent on accurate determination of the chemical composition as a function of size of particles present in the atmosphere. Collection of particles on a filter results in underestimation of particulate organic compounds due to losses from the semi-volatile organic fraction during sample collection, i.e. a “negative sampling artifact”. These semi-volatile organic compounds lost from particles collected on a filter can be correctly measured using a diffusion denuder sampling system. This paper describes a multi-system, multi-channel, high-volume diffusion denuder sampler for the accurate determination of the particle size distribution and chemical composition of semi-volatile organic compounds in atmospheric fine particles. Data obtained in Provo, UT and Los Angeles, CA on the chemical composition and particle size distribution of semi-volatile organic compounds in atmospheric fine particles indicate that the majority of the semi-volatile organic compounds lost from particles during sampling were present in particles in the 0.4–0.8 μm size range in the atmospheres sampled. Organic compounds lost from the particles during sampling include paraffinic compounds, aromatic compounds and organic acids and esters.

Determination of semivolatile organic compounds in particles in the Grand Canyon area

Several diffusion denuder-filter-sorbent bed and filter-sorbent bed sampling systems with varying selectivity for the collection of different classes of organic compounds have been used for the collection and determination of atmospheric organic material present at Hopi Point in the Grand Canyon. The data show that the amount of semivolatile organic compounds in particles in the atmosphere in the Southwest United States has been underestimated by sampling with only a filter. The collection of gas-phase organic compounds by the filter causes a small positive artifact. However, a much larger, negative error results from the loss of 40%–80% of the organic material in the particles collected by the filter. The concentration of organic particulate matter present at the Hopi Point site is probably about 2 μg/m3 larger than inferred from previous data.

Improving plume rise prediction accuracy for stable atmospheres with complex vertical structure

Accurately predicting the rise of a buoyant exhaust plume is difficult when there are large vertical variations in atmospheric stability or wind velocity. Such conditions are particularly common near shoreline power plants. Simple plume rise formulas, which employ only a mean temperature gradient and a mean wind speed, cannot be expected to adequately treat an atmosphere whose lapse rate and wind velocity vary markedly with height. This paper tests the accuracy of a plume rise model which is capable of treating complex atmospheric structure because it integrates along the plume trajectory. The model consists of a set of ordinary differential equations, derived from the fluid equations of motion, with an entrainment parameterization to specify the mixing of ambient air into the plume. Comparing model predictions of final plume rise to field observations yields a root mean square difference of 24 m, which is 9% of the average plume rise of 267 m. These predictions are more accurate than predictions given by three simpler models which utilize variants of a standard plume rise formula, the most accurate of the simpler models having a 12% error.

Characterization of regional transport and dispersion using Project MOHAVE tracer data.

ABSTRACT Perfluorocarbon tracers were released continuously from several surface locations and one power plant stack location during the winter (30 days) and summer (50 days) intensive studies as part of Project MOHAVE. Tracers were released in winter from the Mohave Power Plant (MPP) and Dangling Rope, UT, located on the shore of Lake Powell near Page, AZ; and in summer from MPP, the Tehachapi Pass between the Mojave Desert and the Central Valley in California, and El Centro, CA, on the California-Mexico border. At the Tehachapi tracer release site six-hour pulses of a separately identifiable perfluorocarbon tracer were released every four days in order to assess the time for the tracer to clear the monitoring network. Daily 24-hr integrated samples were collected at about 30 sites in four states. Limited tracer concentration data with higher time resolution is also available. Graphical displays and analyses identify several regional transport paths, including a convergence zone in the Mojave Desert, the importance of terrain channeling, especially in winter, and a relationship between 24-hr maximum influence function and distance that may prove useful as a scoping tool and to test regional scale air quality models. In winter, Dangling Rope tracer was routinely transported through the entire length of the Grand Canyon, while in summer, MPP tracer was routinely transported over most of Lake Mead.

Second Generation Chemical Mass Balance Source Apportionment of Sulfur Oxides and Sulfate at the Grand Canyon during the Project MOHAVE Summer Intensive

ABSTRACT Receptor-based chemical mass balance (CMB) analysis techniques are designed to apportion species that are conserved during pollutant transport using conserved source profiles. The techniques will fail if non-conservative species (or profiles) are not properly accounted for in the CMB model. The straightforward application of the CMB model developed for Project MOHAVE using regional profiles resulted in a significant under-prediction of total sulfate oxides (SOx, SO2 plus fine particulate sulfate) for many samples at Meadview, AZ. In addition, for these samples the concentration of the inert tracer emitted from the MOHAVE Power Project (MPP), ocPDCH, was also under-predicted. A second-generation model has been developed which assumes that separation of particles and SO2 can occur in the MPP plume during nighttime stable plume conditions. This second-generation CMB model accounts for all SOx present at the various receptor sites. In addition, the concentrations of ocPDCH and the presence of other inert tracers of emission from regional sources are accurately predicted. The major source of SOx at Meadview was the MPP, but the major source of sulfate at this site was the Las Vegas urban area. At Hopi Point in the Grand Canyon, the Baja California region (Imperial Valley and northwestern Mexico) was the major source of both SOx and sulfate.

A case study of visibility as related to regional transport

Abstract The study of the potential effect of long-range transport from the Los Angeles Basin to the Mohave Desert-Grand Canyon area on visual range is one of the principal goals of the visibility program in the Mohave network. One episode was investigated during an intensive study period in July-August, 1980. Conclusive evidence of transport episodes was obtained by measurements of anthropogenic chlorofluoro-methanes in the impacted areas, but the quantitative significance in terms of visibility could not be assessed for two reasons. First, the maximum plume impact occurred at night and, second, the changes in visibility as observed by three different methods were not large enough to state that a change had occurred that could be associated with a change in the synoptic flow patterns.

Regional Source Profiles of Sources of SOx at the Grand Canyon During Project Mohave

Total fluoride (gas plus fine particle), spherical aluminosilicate particles, particulate selenium, arsenic, lead, bromine, and absorption of light by fine particles have been used to characterize chemical profiles for sources of sulfur oxides impacting the Grand Canyon National Park Class I Visibility Region. During the Project MOHAVE (Measurement of Haze and Visual Effects) winter and summer intensive studies in 1992, these various species were determined at seven sampling sites in and around the Grand Canyon. Extensive upper air and surface-based meteorological measurements were examined to determine probable geographical origins of the air mass present during a given sampling period for each sampling site. Samples corresponding to air masses dominated by transport from a single major source region were used to determine a source profile for each region. Source regions which have been characterized by this analysis include the San Joaquin Valley area, the southern California coastal urban area, the Baja, California-Imperial Valley area, the Arizona and Mexico area (including major smelters) south of the Grand Canyon, the area southeast of the Grand Canyon, the Colorado Plateau area, the Wasatch Front in Utah, and the area in Nevada to the west and northwest of the Grand Canyon. Source profiles giving the ratio of each endemic marker to SO x for each identified regional source in these areas have been determined. The source profiles for the various regional sources are all statistically different and distinguishable from those for other geographically adjoining sources.

The effects of scrubber installation at the Navajo Generating Station on particulate sulfur and visibility levels in the Grand Canyon.

Abstract Grand Canyon National Park (GCNP) is a mandatory Class I federal area that is afforded visibility protection under the Federal Clean Air Act. In this paper, we have examined the effects on visibility and particulate sulfur (Sp) at GCNP as a result of reducing sulfur dioxide (SO2) emissions by 90% from the Navajo Generating Station (NGS). Scrubbers were retrofitted to each of the three units at NGS during 1997, 1998, and 1999. The Inter-agency Monitoring of Protected Visual Environments aerosol network database affords us an opportunity to examine trends in Sp and extinction both prescrubber and postscrubber. The NGS impacts GCNP primarily during the winter (December to February). During winter, at times, there are fogs, stratus, and high-relative humidity in the Grand Canyon. When the NGS plume interacts with these fogs and stratus, rapid conversion of SO2 to Sp can occur. A variety of analytical techniques were used, including cumulative frequency plots of Sp and extinction, and chemical mass balance and tracer source apportionment analysis. We also deployed P value statistical analysis of “extreme” Sp values. Before scrubbers were installed, values of Sp approaching 2 µg/m3 were occasionally observed. Because scrubbers have been installed, high levels of Sp have been markedly reduced. Statistical P value analysis suggests that these reductions were significant. Furthermore, we have also observed that Sp has been reduced throughout the cumulative frequency curve during winter by ~33% since scrubbers were installed. By contrast, during summer when the NGS impact on the Canyon is minimal, there has been only a relatively small decrease in Sp.

Potential particulate impacts at the Grand Canyon from northwestern Mexico

Project MOHAVE was a major air quality and visibility research program conducted from 1990 to 1999 to investigate the causes of visibility impairment in the Grand Canyon National Park region. At Meadview, a remote monitoring site just west of the Grand Canyon National Park, on September 1 and 2, 1992, the concentrations of sulfate (3.1 and 4.3 microg sulfate/m3) were the highest seen in 6 years of monitoring at this site. During this period, the concentrations of SO2 at Meadview were also abnormally high and approximately three times the sulfate concentrations, on a nmol/m3 basis. High concentrations of sulfate and SO2 extended south into southern Arizona and northwestern Mexico. Based on ambient atmospheric conditions, emissions from the Mohave Power Project (MPP) 110 km upwind of Meadview could not have been responsible for the majority of the regionally observed sulfur oxides. The geographical distribution of SO2 and sulfate, and available source information suggest that northwestern Mexico was a significant source of the unusually high observed sulfur oxides. A CMB model developed during Project MOHAVE was used to apportion sulfur oxides at Meadview and other sampling sites throughout the study region for August 31-September 2, 1992. The results indicate that the contribution of MPP to sulfate at Meadview was typical. However, the transport of SOx from northwestern Mexico was elevated throughout much of the region during this time period. This led to the large increase in sulfate concentrations at Meadview on September 1 and 2. These results indicate that emissions from Mexico can be a significant source of particulate material in the Grand Canyon.

Evaluation of wind fields used in Grand Canyon Visibility Transport Commission analyses.

ABSTRACT The Grand Canyon Visibility Transport Commission (GCVTC) was established by the U.S. Congress to assess the potential impacts of projected growth on atmospheric visibility at Grand Canyon National Park and to make recommendations to the U.S. Environmental Protection Agency on what measures could be taken to avoid such adverse impacts. A critical input to the assessment tool used by the commission was three-dimensional model-derived wind fields used to transport the emissions. This paper describes the evaluation of the wind fields used at various stages in the assessment. Wind fields evaluated included those obtained from the Colorado State University Regional Atmospheric Modeling System (RAMS), the National Meteorological Centers Nested Grid Model (NGM), and the National Oceanic and Atmospheric Administrations Atmospheric Transport and Dispersion (ATAD) trajectory model. The model-derived wind fields were evaluated at multiple vertical levels at several locations in the southwestern United States by determining differences between model predicted winds and winds that were measured using radiosonde and radar wind profiler data. Model-derived winds were also evaluated by determining the percent of time that they were within acceptable differences from measured winds. All models had difficulties, generally meeting the acceptable criteria for less than 50% of the predictions. The RAMS model had a persistent bias toward southwesterly winds at the expense of other directions, especially failing to represent channeling by north-south mountain ranges in the lower levels. The NGM model exhibited a substantial bias in the summer months by extending northwesterly winds in the eastern Pacific Ocean well inland, in contrast to the observed southwesterlies at inland locations. The simpler ATAD trajectory model performed somewhat better than the other models, probably because of its use of more upper air sites. The results of the evaluation indicated that these wind fields could not be used to reliably predict source-receptor impacts on a particular day; thus, seasonally averaged impacts were used in the GCVTC assessment.