The Sixty-Sixth Annual Meeting of the Air Pollution Control Association

Abstract

The U.S. Clean Air Act, amended in 1990, mandated the establishment of the Grand Canyon Visibility Transport Commission (GCVTC). The commission is required to submit a report to the U.S. Environmental Protection Agency addressing visibility issues in the region, including "the establishment of clean air corridors, in which additional restrictions on increases in emissions may be appropriate to protect visibility in affected Class I areas." This paper presents a methodology to identify candidate geographic areas for consideration for Clean Air Corridor (CAC) status for Colorado Plateau Class I areas. The methodology uses thousands of model determined trajectories over a five year period (1988 to 1992) to indicate the paths taken by air that arrives during clean air conditions at Class I areas. These clean air back-trajectories identify upwind areas where pollution emissions could jeopardize currently pristine visibility. Using this methodology, six candidate areas are identified, ranging in size from 75,000 to 506,000 square miles, and permitting varying levels of visibility protection for clean air days at Grand Canyon, Canyonlands, and Petrified Forest National Parks. Assuming effective emissions management of the CAC, the larger the CAC, the greater the visibility protection during clean air conditions.     

Meteorological case studies of regional high sulfur episodes in the western United States

Meteorological conditions associated with regional scale episodes of elevated atmospheric sulfur concentrations over the regions of Arizona and New Mexico, and North and South Dakota were studied. Episodes were defined as the period when the atmospheric concentration of fine sulfur (diameters less than 2.5 μm) at each station in the region exceeded one standard deviation above the station's annual geometric mean.For the Arizona and New Mexico region, the most extraordinary episode occurred during September 1979 when an anticyclonic circulation aloft covered the entire region, while a planetary boundary layer (PBL) convergence zone existed over western Arizona and southeastern California. Convergence of pollutants in the PBL along with slow transport and dispersion by the anticyclonic flow took place during the episode. A similar flow pattern was responsible for an August 1981 episode. Copper smelters in southern Arizona and New Mexico apparently were the major cause of these episodes. Southerly or southeasterly flow during September 1982 again transported sulfur from the smelter region, causing another episode.A statistical study of backward air parcel trajectories from Grand Canyon National Park showed that southern Arizona and New Mexico, as well as the San Joaquin Valley of California, were potential source regions of atmospheric sulfur at the Grand Canyon.Long-range transport of pollutants from states east and/or southeast of North and South Dakota was the major cause of episodes in this region. These episodes occurred when high pressure systems were located over southern Canada or the northeastern United States.A statistical study of backward air parcel trajectories from Theodore Roosevelt National Park (North Dakota) showed that long-range transport from states east and/or southeast of the park was primarily responsible for high concentrations of fine sulfur. No clean air parcels arrived at the park from states to the east.     

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

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 Center's Nested Grid Model (NGM), and the National Oceanic and Atmospheric Administration's 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.     

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 Center's Nested Grid Model (NGM), and the National Oceanic and Atmospheric Administration's 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.     

Assessment of the nested grid model estimates for driving regional visibility models in the southwestern United States

The Nested Grid Model (NGM) is a primitive-equation meteorological model that is routinely exercised over North America for forecasting purposes by the National Meteorological Center. While prognostic meteorological models are being increasingly used to drive air quality models, their use in conducting annual simulations requires significant resources. NGM estimates of wind fields and other meteorological variables provide an attractive alternative since they are typically archived and readily available for an entire year. Preliminary evaluation of NGM winds during the summer of 1992 for application to the region surrounding the Grand Canyon National Park showed serious shortcomings. The NGM winds along the borders between California, Arizona and Mexico tend to be northwesterly with a speed of about 6 m/sec, while the observed flow is predominantly southerly at about 2-5 m/sec. The mesoscale effect of a thermal low pressure area over the highly heated Southern California and western Arizona deserts does not appear to be represented by the NGM because of its coarse resolution and the use of sparse observations in that region. Tracer simulations and statistical evaluation against special high resolution observations of winds in the southwest United States clearly demonstrate the northwest bias in NGM winds and its adverse effect on predictions of an air quality model. The "enhanced" NGM winds, in which selected wind observations are incorporated in the NGM winds using a diagnostic meteorological model provide additional confirmation on the primary cause of the northwest bias. This study has demonstrated that in situations where limited resources prevent the use of prognostic meteorological models, previously archived coarse resolution wind fields in which additional observations are incorporated to correct known biases provide an attractive option.     

The Relationship Between Atmospheric Transport and the Particle Scattering Coefficient at the Grand Canyon

Air trajectory and particle scattering data (bsp) for the period 1984-1989 are used to determine the relationship between atmospheric transport and visual air quality at the Grand Canyon National Park. Using cluster analysis, 72-hour back-trajectories arriving four times per day were grouped into distinct transport patterns. Northwesterly and southerly/southwesterly flow dominate in the winter and summer seasons, respectively. Comparisons of bsp values accompanying different transport patterns showed a clear relationship between air flow pathway and light scattering due to small particles during the non-summer months only. An index is defined which describes the percentage of annual trajectories belonging to specific transport routes delivering predominantly clear air to the GCNP.     

Assessment of the Nested Grid Model Estimates for Driving Regional Visibility Models in the Southwestern United States

ABSTRACT

The Nested Grid Model (NGM) is a primitive-equation meteorological model that is routinely exercised over North America for forecasting purposes by the National Meteorological Center. While prognostic meteorological models are being increasingly used to drive air quality models, their use in conducting annual simulations requires significant resources. NGM estimates of wind fields and other meteorological variables provide an attractive alternative since they are typically archived and readily available for an entire year. Preliminary evaluation of NGM winds during the summer of 1992 for application to the region surrounding the Grand Canyon National Park showed serious shortcomings. The NGM winds along the borders between California, Arizona and Mexico tend to be northwesterly with a speed of about 6 m/sec, while the observed flow is predominantly southerly at about 2-5 m/sec. The mesoscale effect of a thermal low pressure area over the highly heated Southern California and western Arizona deserts does not appear to be represented by the NGM because of its coarse resolution and the use of sparse observations in that region. Tracer simulations and statistical evaluation against special high resolution observations of winds in the southwest United States clearly demonstrate the northwest bias in NGM winds and its adverse effect on predictions of an air quality model. The “enhanced” NGM winds, in which selected wind observations are incorporated in the NGM winds using a diagnostic meteorological model provide additional confirmation on the primary cause of the northwest bias. This study has demonstrated that in situations where limited resources prevent the use of prognostic meteorological models, previously archived coarse resolution wind fields in which additional observations are incorporated to correct known biases provide an attractive option.     

Summer Transport Patterns Affecting the Mohave Power Project Emissions

The Mohave Power Project (MPP) is an isolated 1580-MW coal-fired electric generating plant located in Laughlin, NV. Laughlin is a small desert gambling town situated in the lower Colorado River Valley near the junction of three states: Nevada, California, and Arizona. The location of the MPP is approximately 115 km southwest of the western end of the Grand Canyon National Park and about 240 km southwest from the Grand Canyon Village. This paper describes the summer transport patterns of the MPP emittants using illustrated examples from the Project MOHAVE (Measurements of Haze and Visual Effects) 1992 summer intensive study. The intensive study lasted 50 days from mid-July through August and encompassed the major meteorological patterns associated with southwestern U.S. summer meteorology. The MPP emittants were transported toward the Grand Canyon (north to the northeast) during more than 80% of the total hours. Airflow was from the south most of the time due to a combination of the semi-permanent thermal low, differential heating between the Gulf of California and lower Colorado River Valley, and upslope heating of the southern and western slopes of the nearby Colorado Plateau.     

A residence time probability analysis of sulfur concentrations at grand Canyon National Park

A statistical method is developed to determine the locations of major pollutant sources affecting a distant downwind receptor, provided that air trajectories can be estimated. Probability density functions are estimated which indicate

• 1.(1) the overall residence time of air parcels over a given geographic region as they travel toward a receptor and
• 2.(2) residence time for the case of high pollutant concentrations at the receptor.

These functions are used to estimate a conditional probability function which indicates the potential for a source region to contribute to high air pollution concentrations. Finally, a source contribution function is formulated to indicate the relative contribution of different source regions to high concentrations at the receptor. The method is tested using data collected at Grand Canyon National Park in 1980. The dominant pathway for air masses arriving at Grand Canyon during this period was southern California. A lesser pathway was from the border of Utah and Colorado and a minor pathway was from southern New Mexico. The potential source regions of sulfur particles were (in descending order) southern New Mexico, southern California and eastern Nevada. The source contribution function, however, shows that southern California contributed the most high sulfur concentration episodes in 1980, followed by eastern Nevada and southern New Mexico.     

The project MOHAVE tracer study: study design,data quality,and overview of results

In the winter and summer of 1992, atmospheric tracer studies were conducted in support of project MOHAVE, a visibility study in the southwestern United States. The primary goal of project MOHAVE is to determine the effects of the Mohave power plant and other sources upon visibility at Grand Canyon National Park. Perfluorocarbon tracers (PFTs) were released from the Mohave power plant and other locations and monitored at about 30 sites. The tracer data are being used for source attribution analysis and for evaluation of transport and dispersion models and receptor models. Collocated measurements showed the tracer data to be of high quality and suitable for source attribution analysis and model evaluation. The results showed strong influences of channeling by the Colorado River canyon during both winter and summer. Flow from the Mohave power plant was usually to the south, away from the Grand Canyon in winter and to the northeast, toward the Grand Canyon in summer. Tracer released at Lake Powell in winter was found to often travel downstream through the entire length of the Grand Canyon. Data from summer tracer releases in southern California demonstrated the existence of a convergence zone in the western Mohave Desert.     

Tracking Regional Background in a Haze Attribution Experiment

ABSTRACT

Emissions from distant source areas are often imagined to provide a steady background to the emissions of whatever local sources are being studied. As part of Project MOHAVE in summer 1992, several air mass markers and an injected stack tracer were measured hourly near the Grand Canyon. Observed haze events generally coincided with transients in methylchloroform and water vapor, which we interpret as endemic tags for air from southern California and the subtropics. The results depict a dynamic regional background.     

Trajectory analysis of wet deposition at Whiteface Mountain: A sensitivity study

Meteorological trajectory analysis, MAP3S precipitation chemistry data, and hourly precipitation data at Whiteface Mountain (WFM) were used to determine the source direction of sulfate deposited in precipitation for 1978. A sensitivity study was done on source direction from different trajectory models, using different transport layers, and different precipitation information to determine the time deposition occurred. Results obtained from the different precipitation information indicate that precipitation data in 6-h increments or less are essential to correctly identify the trajectories associated with the wet deposition. Bias resulted when trajectories were not only associated with wet deposition. The direction of source regions, defined by 30° sectors, was much less dependent on the trajectory model or transport layer. Results suggest that over one-third of the wet sulfate deposition at WFM in 1978 arrived from the west-southwest, passing through the eastern Great Lakes region.     

An analysis of the impact of the Sudbury smelters on wet and dry deposition in OntariO

Meteorological analyses of acidic deposition data were undertaken to quantify the effect of Sudbury emissions on precipitation quality and air quality in Ontario during the period 1980–1983 with particular emphasis on the Sudbury smelter shutdown period (July 1982–March 1983). The techniques used included air parcel trajectories and detailed meteorological analysis.Results indicate that the contribution of the Sudbury smelters to sulphate wet deposition in central and southern Ontario was small—less than 12 % of the wet deposition at the study sites. The smelter contribution to sulphur dry deposition was greater—possibly as high as 47 % of the total at Kapuskasing, and up to about 20–30% of the total in central Ontario, with a smaller contribution (less than 5 %) to the southwest and west of Sudbury. These percentages apply to the smelter emission rate in the early 1980s, i.e. about 0.9 million metric tons of SO₂ per year.     

An online coupled meteorological and air quality modeling study of the effect of complex terrain on the regional transport and transformation of air pollutants over the Western United States

One of the most prominent of characteristics of the western United States that affects its meteorology is the complexity of its mountainous terrain. The meteorological Mesoscale Model, version 5 with Chemistry (MM5-Chem), an online-coupled atmospheric chemistry model, was used to investigate the effect of this terrain on a high air pollution event in the free troposphere. The simulations were evaluated by comparisons with data from the North American Regional Reanalysis (NARR). Complex terrain was shown to have an important influence on the vertical transport of air pollutants on the regional scale; emissions from ground level were vertically mixed as high as 5 km above sea surface level for the wintertime conditions simulated. The simulations showed that the vertical transport of emissions from the Earth's surface could have a more significant effect on mid and upper level chemical concentrations than chemical production. The vertical transport was caused predominately by terrain forced flow over the mountains’ ridge-line and the terrain forced flow was affected by the mountain peak height and the complexity of the terrain downwind.     

On the use of the synoptic vertical wind component in a transport trajectory model

The study of the transport of polluted air masses has acquired great importance in the last years and as a consequence computed air mass trajectories are now currently associated with the analysis of atmospheric samples. Schematically, we can say that the pollution emitted in the boundary layer is partly transported within the boundary layer itself and partly within the free troposphere. By the latter route pollution can travel long distances. The main problem is now how to simulate the transport of pollutants when such a statification of the air is considered. Different techniques have been used to model the long-range transport of air masses (geostrophic, isobaric, isentropic trajectories…), but, presently, none can give a complete and satisfying response to the problem. The purpose of this study is to explore the use of the analyzed vertical wind component as a tool to compute air mass trajectories in five case studies. This parameter, resulting from the mass balance, is available from the data set of the European Center of Meteorological Weather Forecasts (ECMWF, Reading, U.K.). We shall discuss in which cases the computed trajectory is sensitive to this parameter. Our study suggests that this parameter should be used to compute air mass transport within the free troposphere.     

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

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 microg/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 approximately 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.     

VOC source identification from personal and residential indoor,outdoor and workplace microenvironment samples in EXPOLIS-Helsinki,Finland

Principal component analyses (varimax rotation) were used to identify common sources of 30 target volatile organic compounds (VOCs) in residential outdoor, residential indoor and workplace microenvironment and personal 48-h exposure samples, as a component of the EXPOLIS-Helsinki study. Variability in VOC concentrations in residential outdoor microenvironments was dominated by compounds associated with long-range transport of pollutants, followed by traffic emissions, emissions from trees and product emissions. Variability in VOC concentrations in environmental tobacco smoke (ETS) free residential indoor environments was dominated by compounds associated with indoor cleaning products, followed by compounds associated with traffic emissions, long-range transport of pollutants and product emissions. Median indoor/outdoor ratios for compounds typically associated with traffic emissions and long-range transport of pollutants exceeded 1, in some cases quite considerably, indicating substantial indoor source contributions. Changes in the median indoor/outdoor ratios during different seasons reflected different seasonal ventilation patterns as increased ventilation led to dilution of those VOC compounds in the indoor environment that had indoor sources. Variability in workplace VOC concentrations was dominated by compounds associated with traffic emissions followed by product emissions, long-range transport and air fresheners. Variability in VOC concentrations in ETS free personal exposure samples was dominated by compounds associated with traffic emissions, followed by long-range transport, cleaning products and product emissions. VOC sources in personal exposure samples reflected the times spent in different microenvironments, and personal exposure samples were not adequately represented by any one microenvironment, demonstrating the need for personal exposure sampling.     

Aerosol light scattering measurements as a function of relative humidity: a comparison between measurements made at three different sites

The water uptake by fine aerosol particles in the atmosphere has been investigated at three rural National Parks in the United States (Great Smoky Mountains, Grand Canyon and Big Bend National Parks). The relative humidity (RH) of sample aerosols was varied from less than 20% to greater than 90% using Perma Pure drying tubes as the scattering coefficient of the aerosol was measured with a Radiance Research M903 nephelometer. Data from these studies show that growth curves at all the three sites are similar in shape but the magnitude of growth can vary considerably from day to day. The growth curves from Great Smoky Mountains show smooth continuous growth over the entire range of RH, while the growth curves from the Grand Canyon and Big Bend show smooth and continuous growth on some days and deliquescence on other days. Comparing 12-h filter samples of chemical composition data with the aerosol growth curves, we find that higher fractions of soluble inorganic compounds (sulfate and nitrate) produce growth curves of greater magnitude than do higher concentrations of either organic carbon or soil material.     

On a Strategy for Reducing Short-range Daytime Ground Level Concentrations Due to Emissions from Very Tall Stacks

Abstract

Airborne fine particle sulfur data from the summer intensive of Project MOHAVE (Measurement of Haze and Visual Effects) was analyzed by the Receptor Model Applied to Patterns in Space (RMAPS) model, a novel multivariate receptor-oriented model that applies to secondary and primary species. The sulfur data from 17 sites were found to be well predicted by three spatial patterns interpreted as sources along the valley of the Colorado River; transport from sources located to the southwest; and transport from sources located to the southeast. The model was tested by using parameters derived from the 17-site data set to apportion sulfur for six sites that were not part of the original data set. The sulfur apportionment for these six sites was in agreement with the original apportionment and the physical interpretation of the spatial patterns given above. The effects of systematic and random error on the sulfur apportionment were estimated. The amount of sulfur associated with the Colorado River valley sources was rather insensitive to both types of error. For the two sites in the Grand Canyon National Park, the fraction of total particulate sulfur from the Colorado River valley source is estimated to be in the range of 27-65% at Meadview and 11-28% at Hopi Point.     

A technique for determining regional scale flow and precipitation patterns upwind of a receptor site

A method of analyzing spatial flow and precipitation patterns associated with long-range transport is presented. This technique uses hourly precipitation data and model-calculated mixed layer trajectories to determine these patterns upwind of a receptor site. Precipitation is determined at hourly points along trajectories in a statistical sense using estimates of error in long-range trajectory calculations. This technique is used in seasonal analyses of fine particle concentrations at a remote northern Great Plains sampling site during summer 1980. The analyses show that there are distinctly different flow and precipitation patterns during high and low fine particle concentration periods. Highest fine sulfur particle concentrations occurred when flow was from the south with a higher frequency of precipitation. Highest fine soil particle concentrations occurred when precipitation frequencies were lower.