Measurements and simulations of the visual effects of particulate plumes

In this paper we present field measurements of the visual effects of particulate plumes from two power plants and a copper smelter. The measurements were conducted at downwind distances ranging from 7 to 34 km and for sun-observer angles ranging from 40 to 160°. The visual effects of the power plant plumes were relatively small due to atmospheric dispersion (Kincaid power plant plume in February 1981) or hazy background (Labadie power plant plume in August 1981). The plume from the San Manuel smelter was more visible because of the clean environment. The measurements of plume contrasts range from − 0.15 to + 0.15. Further development of the EPA plume visibility model to improve the treatment of multiple scattering of light and incorporate light absorption by carbonaceous aerosols is described. Teleradiometer measurements and model simulations are in reasonable agreement for cases in which experimental uncertainties are small. The model appears to underpredict forward scattering of light by plume particles.     

Human Visual Sensitivity to Plumes with a Gaussian

This paper discusses results of a research project designed to develop an empirical model that could be used as a tool to predict human visual sensitivity to plumes. The resultant probability of detection algorithm (PROBDET) allows one to estimate the probability of a plume of known size, shape and contrast being detected visually. As a basis for the algorithm, a series of laboratory experiments using a high threshold signal detection procedure and computer generated images of plumes with Gaussian luminance distributions was conducted to measure human visual sensitivity to plumes. Results of the laboratory experiments are compared with results of contrast sensitivity experiments that examined visual sensitivity to stimuli with square and sine wave luminance distributions. An example of the PROBDET algorithm is presented to demonstrate its potential

usefulness for assessing how probability of detection estimates change as plume size and contrast parameters vary. Since this research was designed to build on existing knowledge, a discussion of that knowledge and how it relates to the research conducted is also presented. The focus of this discussion is on the human visual system (HVS) and on how visual sensitivity is affected by factors such as the luminance of the stimulus and the surround, the luminance distribution of the stimulus, the size of the surround, and the size and spatial frequency characteristics of the stimulus.     

Evaluation of the reactive and optics model of emissions (ROME)

The reactive and optics model of emissions (ROME) is a reactive plume visibility model that simulates the potential atmospheric impacts of stack emissions. We present here an evaluation of the ability of ROME to simulate several plume physical and chemical variables, using an experimental data base that consists of a total of 40 case studies from four field programs. The evaluation variables include plume height, horizontal width, NO_x and SO₂ maximum concentrations, NO₂/NO_x concentration ratio at the plume centerline, and plume-to-sky radiance ratios. Three algorithms used to simulate plume dispersion in ROME were compared: (1) the empirical Pasquill–Gifford–Turner (PGT) scheme, (2) a first-order closure (FOC) algorithm and (3) a second-order closure (SOC) algorithm that simulates the instantaneous plume dimensions.The plume height results show a correlation of 0.82 between simulated and measured values and a gross error that is 13% of the mean measured value. For plume horizontal dispersion, the second-order closure algorithm produces a moderate correlation (0.54) and a small bias (5% of the mean measured value) in comparison with the field data. Although the PGT scheme also demonstrates moderate correlation with the measurements, it produces a negative bias by significantly underestimating plume horizontal dispersion. The first-order closure algorithm overestimates plume width and shows the least correlation (with the measurements) of the three dispersion algorithms.For the NYSEG data set where coordinated measurements of stack emissions, meteorology at plume height and plume characteristics were available, the SOC algorithm provides better correlations for NO_x concentrations, NO₂/NO_x ratios and plume visibility than the FOC and PGT algorithms. For plume visibility, the SOC algorithm shows a correlation of 0.96 at 405 nm, the wavelength where the plume was visible, and it simulates no visible plume at the other wavelengths (550 and 700 nm).A comparison of ROME simulations with those of the plume visibility model PLUVUE II shows that ROME, with the SOC algorithm, performs better for all variables.     

Radiative transfer budgets for scattering and absorbing plumes: Measurements and model predictions

The 1981 VISTTA field study characterized the composition and appearance of particle-rich plumes from three different sources. This paper compares the VISTTA observations with the predictions of two plume visibility models. Observations and predictions are analyzed from the perspective of exact solutions to the equations of radiative transfer for a somewhat idealized atmosphere. These solutions, which explicity relate plume/sky contrast to the composition of plume and background and the geometry of sun, plume and observer, are shown to be consistent with the VISTA observations. The simplified relationships are used as the basis for budgeting radiative transfer by the plume and background, and for analyses of the sensitivity of plume appearance to individual variables.The optics predictions of the two models are less accurate for plumes dominated by particle scattering than they are for plumes dominated by NO₂ absorption. Inaccurate prediction of plume particle size distributions can be identified as an important source of error. Inaccurate prediction of background sky radiance is suspected as another.     

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

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.     

An advanced integral model for cooling tower plume dispersion

An advanced integral model is developed for predicting cooling tower plume rise from single natural draft cooling towers. The theoretical formulation of the model is aimed at avoiding many of the pitfalls and unnecessary assumptions of existing models. The model is based on a careful integration of the three-dimensional partial differential equations of conservation across the plume cross-section; radial profiles of temperature, velocity, and total water are assumed to be Gaussian in shape. The model includes a treatment of plume thermodynamics and tower downwash effects. The model has been calibrated with a wide range of laboratory data. Verification of the model with single-tower field data from Chalk Point, Paradise, Lünen, Gardanne and Philippsburg reveals good results.     

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.     

Statistical Comparisons between Teleradiometer-Derived and Slide-Derived Visibility Parameters

An automated scanning densitometer system developed to estimate visibility-related parameters from photographic slides was tested in comparison to similar teleradiometric measurements. Parameters such as target-sky contrast, visual range, atmospheric light extinction coefficient, and plume and layered contrast can be estimated using this system; however, only target-sky radiance ratios and standard visual ranges were compared. More than 1600 concurrent densitometer and teleradiometer data pairs from eight National Park Service air quality and visibility monitoring locations in the western United States were analyzed using the techniques of correlation, linear regression, average bias and difference calculation, and cumulative frequency distribution generation. Correlation coefficients were good, especially with middle-range camera vistas around 50 km distant. Regression slopes approached unity with intercepts near zero. Average bias introduced into the determination of radiance ratios from slides ranged from near zero to 6 percent, depending on target distance. Standard visual range data distributions compared favorably at the low end with some minor differences at the high end.     

The evolution of particles in the plume from a large coal-fired boiler with flue gas desulfurization

Airborne measurements were made of gaseous and particulate species in the plume of a large coal-fired power plant after flue gas desulfurization (FGD) controls were installed. These measurements were compared with measurements made before the controls were installed. The light scattering and number and volume distributions of plume excess particles were determined by nephelometry and optical particle counting techniques. The plume impact based on optical techniques was much lower than that observed in earlier measurements. Indeed, plume excess volumes as a function of particle size were of the same magnitude as the variability of the background volume distribution. In situ excess plume scattering actually decreased with distance from the source, in contrast to pre-FGD conditions. The upper limit for the dry rate of SO2-to-SO4(2-) conversion was estimated from plume excess volume measurements to be about 4% hr-1. This is slightly greater than the upper limit, 3.5% hr-1, estimated by earlier researchers, but the same as that estimated using the present technique with the earlier data. The cross-plume profile of volume suggests SO2-to-SO4(2-) conversion is highest at the plume edges. The greatest benefit of SO2 reduction on plume excess volume and visibility appears to occur far down-wind of the source.     

Simulation of the evolution of particle size distributions in a vehicle exhaust plume with unconfined dilution by ambient air

Over the past several years, numerous studies have linked ambient concentrations of particulate matter (PM) to adverse health effects, and more recent studies have identified PM size and surface area as important factors in determining the health effects of PM. This study contributes to a better understanding of the evolution of particle size distributions in exhaust plumes with unconfined dilution by ambient air. It combines computational fluid dynamics (CFD) with an aerosol dynamics model to examine the effects of different streamlines in an exhaust plume, ambient particle size distributions, and vehicle and wind speed on the particle size distribution in an exhaust plume. CFD was used to calculate the flow field and gas mixing for unconfined dilution of a vehicle exhaust plume, and the calculated dilution ratios were then used as input to the aerosol dynamics simulation. The results of the study show that vehicle speed affected the particle size distribution of an exhaust plume because increasing vehicle speed caused more rapid dilution and inhibited coagulation. Ambient particle size distributions had an effect on the smaller sized particles (approximately 10 nm range under some conditions) and larger sized particles (>2 microm) of the particle size distribution. The ambient air particle size distribution affects the larger sizes of the exhaust plume because vehicle exhaust typically contains few particles larger than 2 microm. Finally, the location of a streamline in the exhaust plume had little effect on the particle size distribution; the particle size distribution along any streamline at a distance x differed by less than 5% from the particle size distributions along any other streamline at distance x.     

Intermittency and conditionally-averaged concentration fluctuation statistics in plumes

A semi-empirical Gaussian plume model is developed which predicts the intermittency factor, and the mean and variance of the non-zero time varying concentration in the plume from a point source. Wind tunnel data are used to verify the theory and to set the empirical constants. Conditionally averaged concentration fluctuation variance, which has zero concentrations removed, is only weakly dependent on source size, while intermittency caused by plume meandering is shown to be strongly dependent on source size. The resulting closed form predictions are presented in a form suitable for estimating risk of exposure to peak concentrations.     

Dispersion of Traffic and Space Heating Emissions In Urban Settings

A long-term dispersion model is presented for traffic and space heating emissions in urban areas, allowing fast assessment of the spatial-averaged and center-maximum pollutant concentrations.

The assumption of study areas with circular shape and normal emissions density profiles is made for the purpose of streamlining model inputs with the inventory data normally available. In addition, the rather typical assumptions of Gaussian dispersion, narrow plume, flat or gently rolling terrain, homogeneous wind field and nonreactive pollutants are made. Values of σz from Briggs correlation are used with an initial value of 30 to account for building effects.

Meterological data inputs are reduced to six parameters, inventory data inputs to two, while computations are simplified to a degree that use of a digital computer is not required.

The model is well suited to yield separate assessments for individual types of sources and control measures, as well as to reveal sensitivities from parameters such as city size, or emission density levels and distribution patterns. Its predictions are virtually identical to those of the CDM-2 UNAPMAR model for study areas with circular shape and normal emissions density profiles, and as results do not appear overly sensitive to shape and distribution patterns, the model is believed to be valid for most urban areas.     

The Evolution of Particles in the Plume from a Large Coal-Fired Boiler with Flue Gas Desulfurization

ABSTRACT

Airborne measurements were made of gaseous and particulate species in the plume of a large coal-fired power plant after flue gas desulfurization (FGD) controls were installed. These measurements were compared with measurements made before the controls were installed. The light scattering and number and volume distributions of plume excess particles were determined by nephelometry and optical particle counting techniques. The plume impact based on optical techniques was much lower than that observed in earlier measurements. Indeed, plume excess volumes as a function of particle size were of the same magnitude as the variability of the background volume distribution. In situ excess plume scattering actually decreased with distance from the source, in contrast to pre-FGD conditions. The upper limit for the dry rate of SO₂-to-SO₄ ^2- conversion was estimated from plume excess volume measurements to be about 4% lir¹. This is slightly greater than the upper limit, 3.5% lir¹, estimated by earlier researchers, but the same as that estimated using the present technique with the earlier data. The cross-plume profile of volume suggests SO₂-to-SO₄ ^2- conversion is highest at the plume edges. The greatest benefit of SO₂ reduction on plume excess volume and visibility appears to occur far downwind of the source.     

Visibility and Mass Concentration in a Nonurban Environment

Considerable interest is currently directed toward atmospheric visibility and its relationship to particle size and mass concentration. Previous work has been limited to heavily polluted urban areas, and visibility studies have not included particle size characterization. An air sampling program was carried out in a nonurban, low pollution area to relate: (a) total particulate mass concentration measured with a high-volume sampler, (b) particulate mass size distribution measured with aerodynamic size selective samplers, and (c) visual range measured by the integrating nephelometer. For low suspended particulate mass concentrations, the following relationship was defined between visual range (L_v) and mass concentration (M ? μg/m³):     

Expanding box models for the long-range transport of chemically reacting airborne material

If a power station plume significantly perturbs the levels of chemically active species in the atmosphere, then the rates of chemical reactions become non-uniform across the plume. This results in different effective plume widths for the different chemical species, which in turn influence the reaction rates. Here coupled equations are derived which for a reaction involving a single oxidant accurately model the total amount of a species in a plume and the associated plume width. The conventional box model slightly underestimates the amount of oxidized material produced. It is therefore suggested that the sensitivity of plume models to assumptions regarding lateral mixing should be tested, using the system of coupled equations derived in the paper.     

Field studies of a power plant plume in the arid southwestern United States

A study is presented of the physics and chemistry relevant to the visual impact of the plume from an electric power generating plant located in the Mojave Desert. The amount of light absorption by particles in the plume did not differ significantly from that by particles in the ambient air. While sulfate and nitrate occasionally contributed substantially to the total particle mass in the plume, generally they contributed < 10% to that mass, with the contribution of sulfate substantially greater than that of nitrate. Mean rates of gas-to-particle conversion in the plume were ∼ 0.6% h⁻¹ for sulfate and ∼ 0.08% h⁻¹ for nitrate. Light scattering by the plume was more dependent on the total mass of particles in the sub-μm size range than on sulfate mass alone. At a wavelength of 550 nm, NO₂ absorption frequently contributed about equally with light scattering particles to the optical depth of the plume.     

Mathematical modeling of turbulent reacting plumes—I. General theory and model formulation

A new, comprehensive model for a chemically reacting plume is presented that accounts for the effects of incomplete turbulent macro- and micromixing on chemical reactions between plume and atmospheric constituents. The model is modular in nature, allowing for the use of different levels of approximation of the phenomena involved. The core of the model consists of the evolution equations for reaction progress variables appropriate for evolving spatially varying systems. These equations estimate the interaction of mixing and chemical reaction and require input parameters characterizing internal plume behavior, such as relative dispersion and fine scale plume segregation. The model addresses deficiencies in previous reactive plume models. Part II is devoted to atmospheric application of the model.     

Analysis of regional visibility in the southwest using principal component and back trajectory techniques

Approximately 3 years of visibility data from a 13-station teleradiometer network in the southwest desert is the basis for the analysis presented. Principal component analysis is employed to identify regions of similarly varying visibility for the enitre data set and by seasons. ‘North’, ‘Center’ and ‘South’ regions are identified in each of the four seasons. These regions change their size, shape and location somewhat through the seasons and thus are referred to as groups with each group containing four seasonal regions. Distinctive mean visibility levels and variations characterize the three groups. Back trajectoryanalysis techniques are developed to infer the nature and extent of influence of upwind areas on the three visibility groups. Two years of four daily back trajectories indicate primary detrimental influence from the southwest for the ‘North’ group and from the southeast for the ‘South’. Areas influencing the ‘Center’-group visibility are a combination of those affecting the other two groups. A method to calculate transport extinction budgets is demonstrated for the three visibility groups.     

Monte Carlo simulation of nitrogen oxides dispersion from a vehicular exhaust plume and its sensitivity studies

The pollutant dispersion behavior from the vehicular exhaust plume has a direct impact on human health, particularly to the drivers, bicyclists, motorcyclists, pedestrians, people working nearby and vehicle passengers. A two-dimensional pollutant dispersion numerical model was developed based on the joint-scalar probability density function (PDF) approach coupled with a k–ε turbulence model to simulate the initial dispersion process of nitrogen oxides, temperature and flow velocity distributions from a vehicular exhaust plume. A Monte Carlo algorithm was used to solve the PDF transport equations in order to obtain the dispersion distribution of nitrogen oxides concentration. The model was then validated by a series of sensitivity experimental studies in order to assess the effects of vehicular exhaust tailpipe velocities, wind speeds and chemistry on the initial dispersion of NO and NO₂ mass concentrations from the vehicular exhaust plume. The results show that the mass concentrations of nitrogen oxides decrease along the centerline of the vehicular exhaust plume in the downstream distance. The dispersion process can be enhanced when the vehicular exhaust tailpipe velocity is much larger than the wind speed. The oxidation reaction of NO plays an important role when the wind speed is large and the vehicular exhaust exit velocity is small, which leads to chemical reduction of NO, and the formation and accumulation of NO₂ in the exhaust plume. It is also found that the effect of vehicular exhaust-induced turbulence in the vicinity of the exhaust tailpipe exit is more dominant than the effect of wind turbulence, while the wind turbulence gradually shows a significant role for the dispersion of nitrogen oxides along with the development of exhaust plume. The range of dispersion of nitrogen oxides in the radial direction is increased along with the development of vehicular exhaust plume.     

Which visibility indicators best represent a population’s preference for a level of visual air quality?

Several studies have been carried out over the past 20 or so years to assess the level of visual air quality that is judged to be acceptable in urban settings. Groups of individuals were shown slides or computer-projected scenes under a variety of haze conditions and asked to judge whether each image represented acceptable visual air quality. The goal was to assess the level of haziness found to be acceptable for purposes of setting an urban visibility regulatory standard. More recently, similar studies were carried out in Beijing, China, and the more pristine Grand Canyon National Park and Great Gulf Wilderness. The studies clearly showed that when preference ratings were compared to measures of atmospheric haze such as atmospheric extinction, visual range, or deciview (dv), there was not a single indicator that represented acceptable levels of visual air quality for the varied urban or more remote settings. For instance, using a Washington, D.C., setting, 50% of the observers rated the landscape feature as not having acceptable visual air quality at an extinction of 0.19 km^?1 (21 km visual range, 29 dv), while the 50% acceptability point for a Denver, Colorado, setting was 0.075 km^?1 (52 km visual range, 20 dv) and for the Grand Canyon it was 0.023 km^?1 (170 km visual range, 7 dv). Over the past three or four decades, many scene-specific visibility indices have been put forth as potential indicators of visibility levels as perceived by human observers. They include, but are not limited to, color and achromatic contrast of single landscape features, average and equivalent contrast of the entire image, edge detection algorithms such as the Sobel index, and just-noticeable difference or change indexes. This paper explores various scene-specific visual air quality indices and examines their applicability for use in quantifying visibility preference levels and judgments of visual air quality.

Implications: Visibility acceptability studies clearly show that visibility become more unacceptable as haze increases. However, there are large variations in the preference levels for different scenes when universal haze indicators, such as atmospheric extinction, are used. This variability is significantly reduced when the sky–landscape contrast of the more distant landscape features in the observed scene is used. Analysis suggest that about 50% of individuals would find the visibility unacceptable if at any time the more distant landscape features nearly disappear, that is, they are at the visual range. This common metric could form the basis for setting an urban visibility standard.