Developing and evaluating techniques for localizing pollutant emission sources with open-path Fourier transform infrared measurements and wind data

This paper presents the simulation and field evaluation results of two approaches to localize pollutant emission sources with open-path Fourier transform infrared (OP-FTIR) spectroscopy. The first approach combined the plume's peak location information reconstructed from the Smooth Basis Function Minimization (SBFM) algorithm and the wind direction data to calculate source projection lines. In the second approach, the plume's peak location was determined with the Monte Carlo methodology by randomly sampling within the beam segment having the largest path-integrated concentration. We first conducted a series of simulation studies to investigate the sensitivity of using different basis functions in the SBFM algorithm. It was found that fitting with the beta and Weibull basis functions generally gave better estimates of the peak locations than with the normal basis function when the plumes were mainly within the OP-FTIR's monitoring line. However, for plumes that were symmetric to the peak position or spread over the OP-FTIR, fitting with the normal basis function gave better performance. In the field experiment, two tracer gases were released simultaneously from two locations and the OP-FTIR collected data downwind from the sources with a maximum beam path length of 97 m. For the first approach, the release locations were within the 0.25- to 0.5-probability area only after the uncertainty of the peak locations was included in the calculation process. The second approach was easy to implement and still performed as satisfactorily as the first approach. The distances from the sources to the best-fit lines (i.e., the regression lines) of the estimated locations were smaller than 10 m.     

Localizing Gaseous Fugitive Emission Sources by Combining Real-Time Optical Remote Sensing and Wind Data

ABSTRACT

This paper presents a new approach to localize point emissions from ground-level fugitive gaseous air pollution sources. We estimate the crosswind plume's ground-level peak location downwind from the source by combining smooth basis functions minimization (SBFM) with path-integrated optical remote sensing concentration data acquired along the crosswind direction in alternating beam path lengths. Peak location estimates, in conjunction with real-time measured wind direction data, are used to reconstruct the fugitive source location. We conducted a synthetic data study to evaluate the proposed peak location SBFM reconstruction. Furthermore, the methodology was validated with open-path Fourier transform infrared concentration data collected with wind direction data downwind from a controlled point source. This approach was found to provide reasonable estimates of point source location. The field study reconstructed source location was within several meters of the real source location.     

Localizing gaseous fugitive emission sources by combining real-time optical remote sensing and wind data

This paper presents a new approach to localize point emissions from ground-level fugitive gaseous air pollution sources. We estimate the crosswind plume's ground-level peak location downwind from the source by combining smooth basis functions minimization (SBFM) with pathintegrated optical remote sensing concentration data acquired along the crosswind direction in alternating beam path lengths. Peak location estimates, in conjunction with real-time measured wind direction data, are used to reconstruct the fugitive source location. We conducted a synthetic data study to evaluate the proposed peak location SBFM reconstruction. Furthermore, the methodology was validated with open-path Fourier transform infrared concentration data collected with wind direction data downwind from a controlled point source. This approach was found to provide reasonable estimates of point source location. The field study reconstructed source location was within several meters of the real source location.     

Evaluating the performance of the horizontal radial plume mapping technique for locating multiple plumes

This paper evaluated the feasibility of using the horizontal radial plume mapping (HRPM) technique to locate multiple emission sources via computational simulation. Seventy-two test maps, each having two Gaussian distributions, were generated in a two-dimensional domain. The HRPM technique with the non-negative least square (NNLS) algorithm was then applied to reconstruct the plumes, assuming a nine-beam scanning beam geometry. The NNLS algorithm successfully reconstructed the source locations of 68 of the 72 test maps. However, when one of the plumes was near the origin, the NNLS did not always identify the peak locations correctly. Furthermore, when the two plumes were spaced closely, the NNLS tended to reconstruct a wide plume covering both plumes instead of separating them due to the resolution limitation of the current nine-beam geometry. In the sensitivity analysis, five sets of random error (1%, 5%, 10%, 20%, and 30%) were added in the path-integrated concentration (PIC) from the 72 test maps, and thus, an additional 360 reconstructions were implemented. Robust results were obtained when the noise added was less than 20%. The results generally support the implementation of the NNLS algorithm in the HRPM technique as described in the U.S. Environmental Agency (EPA) Other Test Method 10 (OTM-10).

Implications: The methodology evaluated in this paper provides near-real-time estimates about the locations of multiple emission sources. The involved optical remote sensing instruments can monitor large spatial areas (e.g., landfills) in a cost-effective way.     

Source characteristics of hazardous Chihuahuan Desert dust outbreaks

The Chihuahuan Desert region is an important contributor to atmospheric dust loading and transport in North America; however, specific dust sources in this region are poorly characterized. Major dust events frequently are characterized by multiple dust plumes developing nearly simultaneously over a large region. Remote sensing data were used to identify the source locations and associated land cover for the most extreme dust events in the Chihuahuan Desert since 2002. Analysis of infrared channels utilizing brightness temperature differences was used to analyze data from geostationary and polar-orbiting satellites, from which dust sources were determined and located. This methodology was applied to the five dust events in the region that resulted in “hazardous” PM₁₀ levels in Texas per the USEPA’s Air Quality Index. Source locations determined from satellite images were used in conjunction with LANDSAT data and Google Earth? images to determine the corresponding land-surface features. Agricultural lands, playas, and their edges are pointed out as focus areas for dust emission, at least during the most intense events. The 130 dust plume initiation sites were relatively uniformly spaced over the landscape, not clumped into a few “hotspots,” suggesting the role of spatiotemporally random meteorological factors in determining major points of emission within and between dust storms. These findings provide an initial characterization of Chihuahuan Desert dust source locations and establish a baseline for continued research in determining potential locations for future dust outbreaks in the southwestern U.S. and northwestern Mexico.     

Experimental evaluation of a radial beam geometry for mapping air pollutants using optical remote sensing and computed tomography

We describe the first experimental evaluation of a non-overlapping radial beam geometry to map air pollutants using computed tomography (CT) and optical remote sensing (ORS) instruments. Nitrous oxide was released from a point source inside a 11 m long×5.4 m wide ventilation chamber. An open path Fourier transform infrared (OP-FTIR) spectrometer gathered path integrated concentration data. The smooth basis function minimization (SBMF) CT algorithm was applied to a radial geometry with 19 rays. Two-dimensional maps were reconstructed from the OP-FTIR measurements and compared with kriged maps calculated from 13-point samples collected simultaneously during the experiments. The CT reconstructions showed good agreement compared to the kriged maps obtained from point samples (concordance correlation factor >0.55). The CT reconstructions also located the peak concentration within 1.2 m compared to the point samplers. In contrast to the complex CT beam geometries proposed in the past, the development of this radial scanning configuration could broaden the application of CT to many optical remote sensing instruments.     

Wind-tunnel study of entrainment in two-dimensional dense-gas plumes at the EPA's fluid modeling facility

This wind-tunnel study has been conducted as part of a collaborative effort to investigate the effect of large surface roughness on the entrainment of air from a neutrally stable simulated atmospheric boundary layer into a continuous dense-gas plume. The present study examined the entrainment rates of dense-gas plumes as they were transported over two surfaces with similar geometry but significantly different roughness lengths (factor of 6). Extensive measurements of the flow and plume structures over a wide range of source Richardson numbers (Ri*) are reported. Carbon dioxide was released from a two-dimensional source in order to obtain a plume with virtually constant Ri*. Over the small roughness, the plume depths were generally large compared with the element heights, whereas over the large roughness, plume depths were comparable with the element heights. Retardation of mean velocities in the lower levels of the dense plumes (with compensating increases in the upper levels) was observed, as well as strong suppression of turbulence over quite large fractions of the boundary-layer depth. These effects increased as Ri* increased. Propagation of dense gas was observed upstream of the source due to gravity spreading. The flow within the plumes was observed to become laminar at the larger Ri*. The primary measurements comprised longitudinal surface concentration profiles. Where the plumes were fully turbulent, the plots of inverse concentration versus downwind distance formed reasonably straight lines. The sought-after entrainment velocities are proportional to the slopes of these lines and were found to diminish quite rapidly with Ri*. More in-depth analyses and intercomparisons with the results of the other laboratories are contained in a companion paper in this same volume (Briggs et al., 2001, Atmospheric Environment 35, 2265–2284).     

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.     

Uncertainty assessment of contaminant plume length estimates in heterogeneous aquifers

The Virtual Aquifer approach is used in this study to assess the uncertainty involved in the estimation of contaminant plume lengths in heterogeneous aquifers. Contaminant plumes in heterogeneous two-dimensional conductivity fields and subject to first order and Michaelis-Menten (MM) degradation kinetics are investigated by the center line method. First order degradation rates and plume lengths are estimated from point information obtained along the plume center line. Results from a Monte-Carlo investigation show that the estimated rate constant is highly uncertain and biased towards overly high values. Uncertainty and bias amplify with increasing heterogeneity up to maximum values of one order of magnitude. Calculated plume lengths reflect this uncertainty and bias. On average, plume lengths are estimated to about 50% of the true plume length. When plumes subject to MM degradation kinetics are investigated by using a first order rate law, an additional error is introduced and uncertainty as well as bias increase, causing plume length estimates to be less than 40% of the true length. For plumes with MM degradation kinetics, therefore, a regression approach is used which allows the determination of the MM parameters from center line data. Rate parameters are overestimated by a factor of two on average, while plume length estimates are about 80% of the true length. Plume lengths calculated using the MM parameters are thus closer to the correct length, as compared to the first order approximation. This approach is therefore recommended if field data collected along the center line of a plume give evidence of MM kinetics.     

Fate of organic contaminants in the redox zones of a landfill leachate pollution plume (Vejen, Denmark)

Samples from 75 sample locations in a landfill leachate pollution plume reveal a significant disappearance of specific organic compounds (SOC's) within the first 100 m of the plume. Only the herbicide Mecoprop® (MCPP) migrates further. Since sorption and dilution cannot account for the decreasing concentrations, degradation is considered to be the governing process. Non-volatile organic carbon shows a corresponding fate probably acting as a substrate for the microbial processes. The first 20 m of the plume are methanogenic/sulfidogenic, judged on the chemistry of the groundwater, followed by a significant ferrogenic zone exhibiting a substantial capacity to degrade the SOC's. The presence of intermediary products (here an oxidized camphor compound) supports the concept of degradation within the ferrogenic zone. This investigation draws the attention to the significant natural attenuation of organic contaminants and to the so far neglected ferrogenic zone in controlling the fate of organic contaminants in leachate plumes.     

Natural attenuation of xenobiotic organic compounds in a landfill leachate plume (Vejen,Denmark)

Demonstration of natural attenuation of xenobiotic organic compounds (XOCs) in landfill leachate plumes is a difficult task and still an emerging discipline within groundwater remediation. One of the early studies was made at the Vejen Landfill in Denmark in the late 1980s, which suggested that natural attenuation of XOCs took place under strongly anaerobic conditions within the first 150 m of the leachate plume. This paper reports on a revisit to the same plume 10 years later. Within the strongly anaerobic part of the plume, 49 groundwater samples were characterized with respect to redox-sensitive species and XOCs. The analytical procedures have been developed further and more compounds and lower detection limits were observed this time. In addition, the samples were screened for degradation intermediates and for toxicity. The plume showed fairly stationary features over the 10-year period except that the XOC level as well as the level of chloride and nonvolatile organic carbon (NVOC) in the plume had decreased somewhat. Most of the compounds studied were subject to degradation in addition to dilution. Exceptions were benzene, the herbicide Mecoprop (MCPP), and NVOC. In the early study, NVOC seemed to degrade in the first part of the plume, but this was no longer the case. Benzyl succinic acid (BSA) was for the first time identified in a leachate plume as a direct indicator, and as the only intermediate of toluene degradation. Toxicity measurements on solid phase-extracted (SPE) samples revealed that toxic compounds not analytically identified were still present in the plume, suggesting that toxicity measurements could be helpful in assessing natural attenuation in leachate plumes.     

Heuristic space–time design of monitoring wells for contaminant plume characterization in stochastic flow fields

An optimization methodology for designing groundwater quality monitoring networks applicable to stochastic flow fields is presented and evaluated. The approach sets itself apart from previous techniques by incorporating the time dimension directly into the objective function. This function is extremized using a directed partial enumeration strategy guided by physical considerations related to transport processes. The result is a set of monitoring well locations and a sampling schedule that minimizes plume characterization error while satisfying constraints on the maximum number of wells and allowable number of active wells. The method is evaluated using hypothetical plumes with varying degrees of heterogeneity. Results indicate that the proposed approach is successful in generating near-optimal sampling networks that satisfy all imposed constraints. Monitoring networks with as little as three active wells and a total of 12 wells are found to provide adequate plume characterization for low toxicity contaminants.     

Remote monitoring of air pollutant emissions from point sources by a mobile lidar/sodar system

This paper describes remote monitoring of air pollutant emissions by a mobile lidar (light detection and ranging)/ sodar (sound detection and ranging) system. First, measurements are carried out in the flue gas plume of a public power plant. The investigations focus mainly on quantifying SO2 emissions, but the uncertainties of such measurements are also emphasized. Furthermore, an example providing valuable data sets for the development and validation of plume dispersion models is outlined with measurements of the dilution of SO2 along the plume axis. Series of repeated determinations of SO2 emissions show a large variation in the obtained flux values, with moderate margins of error. Incomplete recording of the plume within the individual lidar scans, induced by strong looping movements of the flue gas plume, predominantly causes the variations of flux values. Therefore, the highest flux values determined are considered to be the most exact. This is verified by a comparison of measured fluxes with in situ measurements made by the plant operators. The results further indicate that lidar measurements illustrate the location and dimension of aerosol plumes better than the location and dimension of the plumes of gaseous compounds. The wind direction affecting the plume at any moment can be determined faster by lidar than by sodar because the latter requires much longer time intervals of signal averaging. Measurements show higher concentrations of SO2 compared with results from a Gaussian plume model for periods of less than 5 min after dispersion. The findings emphasize the suitability of remote sensing for detecting emissions and for investigating the propagation and dilution of air pollutant plumes.     

Lidar Observations of the Diurnal Behavior of the Cumberland Power Plant Plume

Mobile lidar observations were made downwind of TVA’s Cumberland (Tennessee) power plant as part of the STATE (Sulfur Transport and Transformation in the Environment) program. Vertical profiles of aerosol backscatter have been processed and displayed to show plume structure as an intensity-modulated TV presentation. Available meteorological data, especially the pilot balloon and radiosonde measurements collected during the STATE experiment, have been used to aid in the interpretation of the lidar display. The data show: ? Well defined nighttime plumes, which often tilt or display a layered structure in the shape of a “>”.

? Late morning convective breakup of the plume.

? Well mixed convective plumes during the day.

? Reformation of the layered nighttime plume during the late afternoon.

It appears that the nighttime plume behavior can be related qualitatively to the strong directional shear of the wind with height that often accompanies the stable nighttime atmosphere. The nighttime plume shapes frequently differ markedly from the oval shape one expects of a gaussian plume. Daytime plumes are in better conformance to the expected shape except when constricted by the surface or the top of the mixing layer     

Modelling plume rise and dispersion from pool fires

This paper describes an investigation into the behaviour of smoke plumes from pool fires, and the subsequent generation of empirical models to predict plume rise and dispersion from such a combustion source. Synchronous video records of plumes were taken from a series of small-scale (0.06–0.25m²) outdoor methanol/toluene pool fire experiments, and used to produce sets of images from which plume dimensions could be derived. Three models were used as a basis for the multiple regression analysis of the data set, in order to produce new equations for improved prediction. Actual plume observations from a large (20.7 m×14.2 m) aviation fuel pool fire were also used to test the predictions. The two theoretically based models were found to give a better representation of plume rise and dispersion than the empirical model based on measurements of small-scale fires. It is concluded that theoretical models tested on small-scale fires (heat output ≈70 kW) can be used to predict plume behaviour from much larger combustion sources (heat output ≈70 MW) under near neutral atmospheric conditions.     

Effect of source variability and transport processes on carbon isotope ratios of TCE and PCE in two sandy aquifers

Chlorinated ethenes often migrate over extended distances in aquifers and may originate from different sources. The aim of this study was to determine whether stable carbon isotope ratios remain constant during dissolution and transport of chlorinated ethenes and whether the ratios can be used to link plumes to their sources. Detailed depth-discrete delineation of the carbon isotope ratio in a tetrachloroethene (PCE) plume and in a trichloroethene (TCE) plume was done along cross-sections orthogonal to groundwater flow in two sandy aquifers in the Province of Ontario, Canada. At the TCE site, TCE concentrations up to solubility were measured in one high concentration zone close to the bottom of the aquifer from where dense non-aqueous phase liquid (DNAPL) was collected. A laboratory experiment using the DNAPL indicated that only very small carbon isotope fractionation occurs during dissolution of TCE (0.26 per thousand), which is consistent with field observations. At most sampling points, the delta(13)C of dissolved TCE was similar to that of the DNAPL except for a few sampling points at the bottom of the aquifer close to the underlying aquitard. At these points, a (13)C enrichment of up to 2.4 per thousand was observed, which was likely due to biodegradation and possibly preferential diffusion of TCE with (12)C into the aquitard. In contrast to the TCE site, several distinct zones of high concentration were observed at the PCE site and from zones to zone, the delta(13)C values varied substantially from -24.3 per thousand to -33.6 per thousand. Comparison of the delta(13)C values in the high concentration zones made it possible to divide the plume in the three different domains, each probably representing a different episode and location of DNAPL release. The three different zones could still be distinguished 220 m from the DNAPL sources. This demonstrates that carbon isotope ratios can be used to differentiate between different zones in chlorinated ethene plumes and to link plume zones to their sources. In addition, subtle variations in delta(13)C at plume fringes provided insight into mechanisms of plume spreading in transverse vertical direction. These variations were identified because of the high-resolution provided by the monitoring network.     

Tomographic reconstruction of tracer gas concentration profiles in a room with the use of a single OP-FTIR and two iterative algorithms: ART and PWLS

Computed tomographic (CT) reconstructions of air contaminant concentration fields were conducted in a room-sized chamber employing a single open-path Fourier transform infrared (OP-FTIR) instrument and a combination of 52 flat mirrors and 4 retroreflectors. A total of 56 beam path data were repeatedly collected for around 1 hr while maintaining a stable concentration gradient. The plane of the room was divided into 195 pixels (13 x 15) for reconstruction. The algebraic reconstruction technique (ART) failed to reconstruct the original concentration gradient patterns for most cases. These poor results were caused by the "highly underdetermined condition" in which the number of unknown values (156 pixels) exceeds that of known data (56 path integral concentrations) in the experimental setting. A new CT algorithm, called the penalized weighted least-squares (PWLS), was applied to remedy this condition. The peak locations were correctly positioned in the PWLS-CT reconstructions. A notable feature of the PWLS-CT reconstructions was a significant reduction of highly irregular noise peaks found in the ART-CT reconstructions. However, the peak heights were slightly reduced in the PWLS-CT reconstructions due to the nature of the PWLS algorithm. PWLS could converge on the original concentration gradient even when a fairly high error was embedded into some experimentally measured path integral concentrations. It was also found in the simulation tests that the PWLS algorithm was very robust with respect to random errors in the path integral concentrations. This beam geometry and the use of a single OP-FTIR scanning system, in combination with the PWLS algorithm, is a system applicable to both environmental and industrial settings.     

Water tank measurements of buoyant plume rise and structure in neutral crossflows

In this paper, an experimental study of the rise and development of a single buoyant plume and a pair of in-line buoyant plumes is presented. The investigations were carried out at small scale in a water filled towing tank using both quantitative flow visualisation and local concentration measurements. The measured plume trajectories for a single plume were compared with the Briggs plume rise equation and predictions from a numerical integral model. Plume trajectories were studied for twin in-line plumes, with particular attention to changes in the plume trajectory, especially any additional rise that resulted from the interaction between the two plumes. Concentration field distributions in cross-sections through both single and interacting twin plumes were obtained from the local concentration measurement system. These showed how the interaction affected the plume structure, notably the double vortex system that occurs in a fully developed plume.     

Elements Influencing Cost Allocation in the Pinal Creek Aquifer,Arizona, USA. Part I: Geochemical Fingerprinting and Source Delineation

This is the first in a three-article series in this volume of Environmental Forensics describing the framework that was developed to quantitatively allocate mass loading of metals to the Pinal Creek alluvial aquifer. This article describes geochemical fingerprinting, followed by spatial and temporal analysis of Pinal Creek monitoring well data (more than 600,000 records from hundreds of locations), which identified three distinct source areas and plumes on the basis of facility-specific differences in process geochemistry, ore mineralogy, and solution handling. The primary Webster Lake/Gulch source originated in 1926 (700 mg/L chlorine, 200 mg/L copper, 3000–9000 mg/L iron). Secondary contributions from the Bluebird/Oxhide/Live Oak Gulch/Davis Canyon leach complex in Upper Bloody Tanks Wash began circa 1962 (30–200 mg/L chlorine, 1500–3500 mg/L copper, <500 mg/L iron), and recent (1992–2001) contributions came from Miami Unit hydromining releases to Bloody Tanks Wash (30–70 mg/L chlorine, ～1000 mg/L copper, ～2000 mg/L iron). In Kiser Basin, where the plumes commingle, temporal evolution of the plume chemistry is reflected in a shift from a predominantly iron:copper-rich solution (10:1) to an increasingly copper-dominated signature (1:1) with the arrival of the secondary plume. Based on historical records and geochemical fingerprints, the original acid front migrated through the alluvium at ～0.5 m/day; however, later acidic releases traveled at ～2.3 m/day due to exhaustion of the available buffering capacity. The divisible nature of the plume supports quantitative allocation of contributions to the aquifer.     

BTEX exposures in an area impacted by industrial and mobile sources: Source attribution and impact of averaging time

The impacts of emissions plumes from major industrial sources on black carbon (BC) and BTEX (benzene, toluene, ethyl benzene, xylene isomers) exposures in communities located >10 km from the industrial source areas were identified with a combination of stationary measurements, source identification using positive matrix factorization (PMF), and dispersion modeling. The industrial emissions create multihour plume events of BC and BTEX at the measurement sites. PMF source apportionment, along with wind patterns, indicates that the observed pollutant plumes are the result of transport of industrial emissions under conditions of low boundary layer height. PMF indicates that industrial emissions contribute >50% of outdoor exposures of BC and BTEX species at the receptor sites. Dispersion modeling of BTEX emissions from known industrial sources predicts numerous overnight plumes and overall qualitative agreement with PMF analysis, but predicts industrial impacts at the measurement sites a factor of 10 lower than PMF. Nonetheless, exposures associated with pollutant plumes occur mostly at night, when residents are expected to be home but are perhaps unaware of the elevated exposure. Averaging data samples over long times typical of public health interventions (e.g., weekly or biweekly passive sampling) misapportions the exposure, reducing the impact of industrial plumes at the expense of traffic emissions, because the longer samples cannot resolve subdaily plumes. Suggestions are made for ways for future distributed pollutant mapping or intervention studies to incorporate high time resolution tools to better understand the potential impacts of industrial plumes.

Implications: Emissions from industrial or other stationary sources can dominate air toxics exposures in communities both near the source and in downwind areas in the form of multihour plume events. Common measurement strategies that use highly aggregated samples, such as weekly or biweekly averages, are insensitive to such plume events and can lead to significant under apportionment of exposures from these sources.