Measurement of Opacity and Particulate Emissions with an On-Stack Transmissometer

An on-stack transmissometer system which is designed to provide a precision measurement of the opacity of visible emissions is described. The sources of error in opacity measurements with regard to recent EPA emission monitoring requirements and planned specifications are discussed. Sources of error are voltage changes, temperature changes, light source and detector aging and effects of ambient light. Other major operational errors are caused by alignment drift and soiling drift. The methods employed to minimize these errors achieve an accuracy of ±3% of span and a maintenance free operational period of 3 months. The relationships between optical density, opacity and transmittance are described. The instrument measurement can be correlated with dust loading provided the particle size distribution is constant. Examples are given of correlations obtained between optical density and particulate concentration in the gas on various types of emission sources and the observed error margins are summarized.     

Narrow stack emissions: Errors in flow rate measurement due to disturbances and swirl

ABSTRACT

European legislation continues to drive down emission limit values, making the emission measurement of narrow stacks of increasing importance. However, the applicable standards (EN ISO 16911–1 and EN 15259) are poorly validated for narrow stacks, and the effect of flow disturbances on the described methods are largely unknown. In this article, measurement errors are investigated in narrow stacks with flow disturbances and swirl, both experimentally and through computational fluid dynamics (CFD) simulations. The results indicate that measurement errors due to misalignment of the flow with typical measuring probes (pitot tubes) are small compared to errors resulting from the positioning of these probes in the measurement plane. Errors up to 15% are reported using the standardized methods, while the measurement error is both smaller and more predictable when using additional measurement points.

Implications: Current international standards provide methods to measure emissions from industrial stacks. With increasingly small emission limit values, the accuracy of these measurements is becoming considerably more important. The data from this study can be used to inform revisions of these standards, in particular with respect to flow disturbances in narrow stacks, and can help law- and policy-makers to obtain insight into the uncertainties of emission measurements in these specific situations.     

Hazardous air pollutant emissions from gas-fired combustion sources: emissions and the effects of design and fuel type

Air emissions from gas-fired combustion devices such as boilers, process heaters, gas turbines and stationary reciprocating engines contain hazardous air pollutants (HAPs) subjected to consideration under the federal clean air act (CAA). This work presents a recently completed major research project to develop an understanding of HAP emissions from gas-fired boilers and process heaters and new HAP emission factors based on field emission tests of gas-fired external combustion devices used in the petroleum industry. The effect of combustion system design and operating parameters on HAP emissions determined by both field and research tests are discussed. Data from field tests of gas-fired petroleum industry boilers and heaters generally show very low emission levels of organic HAPs. A comparison of the emission data for boilers and process heaters, including units with and without various forms of NOx emission controls, showed no significant difference in organic HAP emission characteristics due to process or burner design. This conclusion is also supported by the results of research tests with different burner designs. Based on field tests of units fired with natural gas and various petroleum industry process gases and research tests in which gas composition was intentionally varied, organic HAP emissions were not determined to be significantly affected by the gas composition. Research data indicate that elevated organic HAP emission levels are found only under extreme operating conditions (starved air or high excess air combustion) associated with poor combustion.     

Carbon dioxide emission tallies for 210 U.S. coal-fired power plants: A comparison of two accounting methods

Annual CO₂ emission tallies for 210 coal-fired power plants during 2009 were more accurately calculated from fuel consumption records reported by the U.S. Energy Information Administration (EIA) than measurements from Continuous Emissions Monitoring Systems (CEMS) reported by the U.S. Environmental Protection Agency. Results from these accounting methods for individual plants vary by ± 10.8%. Although the differences systematically vary with the method used to certify flue-gas flow instruments in CEMS, additional sources of CEMS measurement error remain to be identified. Limitations of the EIA fuel consumption data are also discussed. Consideration of weighing, sample collection, laboratory analysis, emission factor, and stock adjustment errors showed that the minimum error for CO₂ emissions calculated from the fuel consumption data ranged from ± 1.3% to ± 7.2% with a plant average of ± 1.6%. This error might be reduced by 50% if the carbon content of coal delivered to U.S. power plants were reported.

Implications:

Potentially, this study might inform efforts to regulate CO₂ emissions (such as CO₂ performance standards or taxes) and more immediately, the U.S. Greenhouse Gas Reporting Rule where large coal-fired power plants currently use CEMS to measure CO₂ emissions. Moreover, if, as suggested here, the flue-gas flow measurement limits the accuracy of CO₂ emission tallies from CEMS, then the accuracy of other emission tallies from CEMS (such as SO₂, NO_x, and Hg) would be similarly affected. Consequently, improved flue gas flow measurements are needed to increase the reliability of emission measurements from CEMS.     

Comparison of measurement methods for the characterization of the black carbon emissions from a T63 turboshaft engine burning conventional and Fischer-Tropsch fuels

Emission measurements of black carbon (BC) mass were conducted on a T63 turboshaft engine, operated at idle and cruise power with conventional and alternative fuels, using an Artium LII-300 laser-induced incandescence analyzer (LII) and AVL model 483 micro soot sensor (MSS) photoacoustic instrument using the manufacturer’s calibration for both instruments. These measurements were compared with elemental carbon (EC) determined by manual and semicontinuous thermal-optical transmission analyses according to National Institute for Occupational Safety and Health (NIOSH) method 5040 as the reference method. The results indicate that both the LII and MSS instruments show good linear correlation with EC for the two fuels and two engine power conditions evaluated. The LII measurements were observed to be biased high (27–49%) and the MSS measurements were biased low (24–35%) relative to EC. The agreement between the instruments and the reference method was substantially improved by applying a calibration of the instruments against a common BC aerosol source. Test data also suggest that the two instruments show some sensitivity to particle size (or properties related to size), specifically for particles with a geometric mean diameter (GMD) <30 nm. This sensitivity is problematic, since new engines or certain combustion conditions in current engines will produce smaller particles compared with the T63 model tested in this study. Further assessments of instrument performance for particles within this size range are therefore warranted.

Implications: Accurate black carbon emission measurements are needed to certify new and in-production commercial aircraft engines. Both the Artium LII-300 and AVL 483 micro soot sensor are currently approved by the International Civil Aviation Organization for this purpose. This study compares the two instruments against elemental carbon (EC) using NIOSH method 5040 as the reference using a T63 turboshaft engine. The results indicate that both instruments correlate reasonably well with EC, and the correlation substantially improved when applying a calibration against a common aerosol source. Sensitivity to particle size may be an issue for both instruments.     

Quality Assured Measurements of Animal Building Emissions: Gas Concentrations

Abstract

Comprehensive field studies were initiated in 2002 to measure emissions of ammonia (NH₃), hydrogen sulfide (H₂S), carbon dioxide (CO₂), methane (CH₄), nonmethane hydrocarbons (NMHC), particulate matter <10 µm in diameter, and total suspended particulate from swine and poultry production buildings in the United States.

This paper focuses on the quasicontinuous gas concentration measurement at multiple locations among paired barns in seven states. Documented principles, used in air pollution monitoring at industrial sources, were applied in developing quality assurance (QA) project plans for these studies. Air was sampled from multiple locations with each gas analyzed with one high quality commercial gas analyzer that was located in an environmentally controlled on-farm instrument shelter. A nominal 4 L/min gas sampling system was designed and constructed with Teflon wetted surfaces, bypass pumping, and sample line flow and pressure sensors. Three-way solenoids were used to automatically switch between multiple gas sampling lines with ≥10 min sampling intervals. Inside and outside gas sampling probes were between 10 and 115 m away from the analyzers. Analyzers used chemiluminescence, fluorescence, photoacoustic infrared, and photoionization detectors for NH₃, H₂S, CO₂, CH₄, and NMHC, respectively. Data were collected using personal computer-based data acquisition hardware and software. This paper discusses the methodology of gas concentration measurements and the unique challenges that livestock barns pose for achieving desired accuracy and precision, data representativeness, comparability and completeness, and instrument calibration and maintenance.     

Total diesel exhaust particulate length measurements using a modified household smoke alarm ionization chamber

To evaluate the effectiveness of various means to combat the negative health effects of ultrafine particles emitted by internal combustion engines, a reliable, low-cost instrument for dynamic measurements of the exhaust emissions of ultrafine particulate matter (PM) is needed. In this study, an ordinary ionization-type building smoke detector was modified to serve as a measuring ionization chamber and utilized for dynamic measurements of PM emissions from diesel engines. When used with diluted exhaust, the readings show an excellent correlation with total particulate length. The instrument worked well with raw and diluted exhaust and with varying emission levels and is well suitable for on-board use.     

Quality assured measurements of animal building emissions: gas concentrations

Comprehensive field studies were initiated in 2002 to measure emissions of ammonia (NH3), hydrogen sulfide (H2S), carbon dioxide (CO2), methane (CH4), nonmethane hydrocarbons (NMHC), particulate matter <10 microm in diameter, and total suspended particulate from swine and poultry production buildings in the United States. This paper focuses on the quasicontinuous gas concentration measurement at multiple locations among paired barns in seven states. Documented principles, used in air pollution monitoring at industrial sources, were applied in developing quality assurance (QA) project plans for these studies. Air was sampled from multiple locations with each gas analyzed with one high quality commercial gas analyzer that was located in an environmentally controlled on-farm instrument shelter. A nominal 4 L/min gas sampling system was designed and constructed with Teflon wetted surfaces, bypass pumping, and sample line flow and pressure sensors. Three-way solenoids were used to automatically switch between multiple gas sampling lines with > or =10 min sampling intervals. Inside and outside gas sampling probes were between 10 and 115 m away from the analyzers. Analyzers used chemiluminescence, fluorescence, photoacoustic infrared, and photoionization detectors for NH3, H2S, CO2, CH4, and NMHC, respectively. Data were collected using personal computer-based data acquisition hardware and software. This paper discusses the methodology of gas concentration measurements and the unique challenges that livestock barns pose for achieving desired accuracy and precision, data representativeness, comparability and completeness, and instrument calibration and maintenance.     

Note on In-Traffic Measurement of Airborne Tire-Wear Participate Debris

Measuring greenhouse gas (GHG) source emissions provides data for validation of GHG inventories, which provide the foundation for climate change mitigation. Two Toyota RAV4 electric vehicles were outfitted with high-precision instrumentation to determine spatial and temporal resolution of GHGs (e.g., nitrous oxide, methane [CH₄], and carbon dioxide [CO₂]), and other gaseous species and particulate metrics found near emission sources. Mobile measurement platform (MMP) analytical performance was determined over relevant measurement time scales. Pollutant residence times through the sampling configuration were measured, ranging from 3 to 11 sec, enabling proper time alignment for spatial measurement of each respective analyte. Linear response range for GHG analytes was assessed across expected mixing ratio ranges, showing minimal regression and standard error differences between 5, 10, 30, and 60 sec sampling intervals and negligible differences between the two MMPs. GHG instrument drift shows deviation of less than 0.8% over a 24-hr measurement period. These MMPs were utilized in tracer-dilution experiments at a California landfill and natural gas compressor station (NGCS) to quantify CH₄ emissions. Replicate landfill measurements during October 2009 yielded annual CH₄ emissions estimates of 0.10 ± 0.01, 0.11 ± 0.01, and 0.12 ± 0.02 million tonnes of CO₂ equivalent (MTCO₂E). These values compare favorably to California GHG Emissions Inventory figures for 2007, 2008, and 2009 of 0.123, 0.125, and 0.126 MTCO₂E/yr, respectively, for this facility. Measurements to quantify NGCS boosting facility-wide emissions, during June 2010 yielded an equivalent of 5400 ± 100 TCO₂E/yr under steady-state operation. However, measurements during condensate transfer without operational vapor recovery yield an instantaneous emission rate of 2–4 times greater, but was estimated to only add 12 TCO₂E/yr overall. This work displays the utility for mobile GHG measurements to validate existing measurement and modeling approaches, so emission inventory values can be confirmed and associated uncertainties reduced.

Implications:?Measuring greenhouse gas (GHG) source emissions provides data and validation for GHG inventories, the foundation for climate change mitigation. Mobile measurement platforms with robust analytical instrumentation completed tracer-dilution experiments in California at a landfill and natural gas compressor station (NGCS) to quantify CH₄ emissions. Data collected for landfill CH₄ agree with the current California emissions inventory, while NGCS data show the possible variability from this type of facility. This work displays the utility of mobile GHG measurements to validate existing measurement and modeling approaches, such that emission inventory values can be confirmed, associated uncertainties reduced, and mitigation efforts quantified.     

A Review of Instrumentation,Test, and Sampling Procedures

An assessment has been made of the state of emission measurement technology to determine whether measurement techniques are sufficiently well advanced to support the development of emission control methods and the implementation of emission standards for aircraft engines. The conclusion drawn from this assessment is that current measurement technology will meet most of the requirements of an emission control program. Certain measurement techniques are inadequate at present but development of improved techniques appears to be proceeding at a satisfactory rate.     

Techniques and Procedures for the Measurement of Aircraft Gas Turbine Engine Emissions

In 1968 the Society of Automotive Engineers formed the Committee on Aircraft Exhaust Emissions Measurement (E-31) whose charge was the development of acceptable standards of measurement for the characterization of aircraft engine exhaust. This committee’s efforts have resulted in the issuance of two Aerospace Recommended Practices, ARP 1179 “Aircraft Gas Turbine Exhaust Smoke Measurement” and ARP 1256 “Procedure for the Continuous Sampling and Measurement of Gaseous Emissions from Aircraft Turbine Engines.” These Recommended Practices have in large part been adopted by the Environmental Protection Agency and promulgated in Federal Register Volume 38, Number 136, Tuesday, July 17, 1973.

For the past three years Pratt & Whitney Aircraft has been measuring emissions from aircraft gas turbine engines using on-line instrumentation systems designed both in accordance with these Aerospace Recommended Practices and in response to the operational needs of a large experimental engineering test facility. In addition to a discussion of these systems this paper describes the experience derived from continuous testing programs in support of this test facility with consideration being given to the specific problems of sampling, sample handling, system accuracy, and data recording and reduction. Comment is made as to the practical limitations of the recommended methods and procedures as applied to emission control technology programs and suggestions are presented for improving the measurement technology.     

The potential of a partial-flow constant dilution ratio sampling system as a candidate for vehicle exhaust aerosol measurements

This paper presents the measurement of airborne particle properties with use of a dedicated sampling protocol and a measurement setup directly installed in the exhaust line of vehicles and engines. The sampling system dilutes a small part of the exhaust directly at the tailpipe without the need of exhaust gas transfer lines that may lead to sampling artifacts. Dilution takes place in two steps with a primary dilution ratio universally set at a value of 12.5:1 for all vehicles and engines tested, and subsequent dilution steps reducing particle concentration within the measuring range of the instruments used. Dilution air temperature and residence time were set at 32 degrees C and 2.5 sec respectively, to allow repeatable measurement of nucleation-mode particles. The paper summarizes the specifications of the system, evaluates its performance in comparison to real-world dilution (chasing experiments), and presents the repeatability and reproducibility of measurements performed in different laboratories. In general, after taking precautions for the setup and condition of instruments, both measurement quality indices reached levels similar to the measurement of particulate matter (PM) mass. Application of the system, using the same protocol, to measure many light-duty vehicles and engines is finally demonstrated, providing useful conclusions for the emission performance of different sized engines. The study concludes that the use of partial-flow sampling systems may offer advantages for the measurement of particle emissions from low-emission engines compared with constant volume sampling facilities, including lower cost of purchase and operation, versatility, lack of artifacts, and possibilities for standardization in different environments.     

Interinstrument comparison of remote-sensing devices and a new method for calculating on-road nitrogen oxides emissions and validation of vehicle-specific power

Emissions of nitrogen oxides (NOx) by vehicles in real driving environments are only partially understood. This has been brought to the attention of the world with recent revelations of the cheating of the type of approval tests exposed in the dieselgate scandal. Remote-sensing devices offer investigators an opportunity to directly measure in situ real driving emissions of tens of thousands of vehicles. Remote-sensing NO₂ measurements are not as widely available as would be desirable. The aim of this study is to improve the ability of investigators to estimate the NO₂ emissions and to improve the confidence of the total NOx results calculated from standard remote-sensing device (RSD) measurements. The accuracy of the RSD speed and acceleration module was also validated using state-of-the-art onboard global positioning system (GPS) tracking. Two RSDs used in roadside vehicle emissions surveys were tested side by side under off-carriageway conditions away from transient pollution sources to ascertain the consistency of their measurements. The speed correlation was consistent across the range of measurements at 95% confidence and the acceleration correlation was consistent at 95% confidence intervals for all but the most extreme acceleration cases. VSP was consistent at 95% confidence across all measurements except for those at VSP ≥ 15 kW t^?1, which show a small underestimate. The controlled distribution gas nitric oxide measurements follow a normal distribution with 2σ equal to 18.9% of the mean, compared to 15% observed during factory calibration indicative of additional error introduced into the system. Systematic errors of +84 ppm were observed but within the tolerance of the control gas. Interinstrument correlation was performed, with the relationship between the FEAT and the RSD4600 being linear with a gradient of 0.93 and an R² of 0.85, indicating good correlation. A new method to calculate NOx emissions using fractional NO₂ combined with NO measurements made by the RSD4600 was constructed, validated, and shown to be more accurate than previous methods.

Implications: Synchronized remote-sensing measurements of NO were taken using two different remote-sensing devices in an off-road study. It was found that the measurements taken by both instruments were well correlated. Fractional NO₂ measurements from a prior study, measurable on only one device, were used to create new NO_x emission factors for the device that could not be measured by the second device. These estimates were validated against direct measurement of total NO_x emission factors and shown to be an improvement on previous methodologies. Validation of vehicle-specific power was performed with good correlation observed.     

Determination of S02 Emission from Oil Combustion Sources by Fuel Analyses and S02/C02 Relations

The current S0₂ emission test methods, based on wet chemistry, are time consuming and costly. Final results depend on at least 10 separate measurements of quantities which may introduce significant errors due to human factors especially under unfavorable field environment. Performance evaluation of several promising S0₂ instrumental methods indicated that results were reliable and reproducible, but instrumental problems still exist. The problems were found to be calibration, mechanical and electronic components, and sample conditioning system. An alternative method proposed requires only two separate analyses: ultimate fuel analysis and stack gas analysis for C0₂. This method can be further simplified with reasonable accuracy by a single measurement of sulfur in the fuel if results are corrected to 12% C0₂. The method can be a useful addition to the current methods, especially if cost and manpower preclude actual stack tests. A test program was conducted at four industrial and utility power plants. Results are presented with discussion to show the application and validity of the simplified method.     

Dilution-based emissions sampling from stationary sources: Part 2--Gas-fired combustors compared with other fuel-fired systems

With the recent focus on fine particle matter (PM2.5), new, self-consistent data are needed to characterize emissions from combustion sources. Such data are necessary for health assessment and air quality modeling. To address this need, emissions data for gas-fired combustors are presented here, using dilution sampling as the reference. The dilution method allows for collection of emitted particles under conditions simulating cooling and dilution during entry from the stack into the air. The sampling and analysis of the collected particles in the presence of precursor gases, SO2 nitrogen oxide, volatile organic compound, and NH3 is discussed; the results include data from eight gas fired units, including a dual-fuel institutional boiler and a diesel engine powered electricity generator. These data are compared with results in the literature for heavy-duty diesel vehicles and stationary sources using coal or wood as fuels. The results show that the gas-fired combustors have very low PM2.5 mass emission rates in the range of approximately 10(-4) lb/million Btu (MMBTU) compared with the diesel backup generator with particle filter, with approximately 5 x 10(-3) lb/MMBTU. Even higher mass emission rates are found in coal-fired systems, with rates of approximately 0.07 lb/MMBTU for a bag-filter-controlled pilot unit burning eastern bituminous coal. The characterization of PM2.5 chemical composition from the gas-fired units indicates that much of the measured primary particle mass in PM2.5 samples is organic or elemental carbon and, to a much less extent, sulfate. Metal emissions are quite low compared with the diesel engines and the coal- or wood-fueled combustors. The metals found in the gas-fired combustor particles are low in concentration, similar in concentration to ambient particles. The interpretation of the particulate carbon emissions is complicated by the fact that an approximately equal amount of particulate carbon (mainly organic carbon) is found on the particle collector and a backup filter. It is likely that measurement artifacts, mostly adsorption of volatile organic compounds on quartz filters, are positively biasing "true" particulate carbon emission results.     

Open-path tunable diode laser absorption spectroscopy for acquisition of fugitive emission flux data

Air pollutant emission from unconfined sources is an increasingly important environmental issue. The U.S. Environmental Protection Agency (EPA) has developed a ground-based optical remote-sensing method that enables direct measurement of fugitive emission flux from large area sources. Open-path Fourier transform infrared spectroscopy (OP-FTIR) has been the primary technique for acquisition of pollutant concentration data used in this emission measurement method. For a number of environmentally important compounds, such as ammonia and methane, open-path tunable diode laser absorption spectroscopy (OP-TDLAS) is shown to be a viable alternative to Fourier transform spectroscopy for pollutant concentration measurements. Near-IR diode laser spectroscopy systems offer significant operational and cost advantages over Fourier transform instruments enabling more efficient implementation of the measurement strategy. This article reviews the EPA's fugitive emission measurement method and describes its multipath tunable diode laser instrument. Validation testing of the system is discussed. OP-TDLAS versus OP-FTIR correlation testing results for ammonia (R2 = 0.980) and methane (R2 = 0.991) are reported. Two example applications of tunable diode laser-based fugitive emission measurements are presented.     

Validation of road vehicle and traffic emission models – A review and meta-analysis

Road transport is often the main source of air pollution in urban areas, and there is an increasing need to estimate its contribution precisely so that pollution-reduction measures (e.g. emission standards, scrapage programs, traffic management, ITS) are designed and implemented appropriately. This paper presents a meta-analysis of 50 studies dealing with the validation of various types of traffic emission model, including ‘average speed’, ‘traffic situation’, ‘traffic variable’, ‘cycle variable’, and ‘modal’ models. The validation studies employ measurements in tunnels, ambient concentration measurements, remote sensing, laboratory tests, and mass-balance techniques. One major finding of the analysis is that several models are only partially validated or not validated at all. The mean prediction errors are generally within a factor of 1.3 of the observed values for CO₂, within a factor of 2 for HC and NO_x, and within a factor of 3 for CO and PM, although differences as high as a factor of 5 have been reported. A positive mean prediction error for NO_x (i.e. overestimation) was established for all model types and practically all validation techniques. In the case of HC, model predictions have been moving from underestimation to overestimation since the 1980s. The large prediction error for PM may be associated with different PM definitions between models and observations (e.g. size, measurement principle, exhaust/non-exhaust contribution).Statistical analyses show that the mean prediction error is generally not significantly different (p < 0.05) when the data are categorised according to model type or validation technique. Thus, there is no conclusive evidence that demonstrates that more complex models systematically perform better in terms of prediction error than less complex models. In fact, less complex models appear to perform better for PM. Moreover, the choice of validation technique does not systematically affect the result, with the exception of a CO underprediction when the validation is based on ambient concentration measurements and inverse modelling. The analysis identified two vital elements currently lacking in traffic emissions modelling: 1) guidance on the allowable error margins for different applications/scales, and 2) estimates of prediction errors. It is recommended that current and future emission models incorporate the capability to quantify prediction errors, and that clear guidelines are developed internationally with respect to expected accuracy.     

A Comparison Study of Various Types of Ozone and Oxidant Detectors Which Are Used for Atmospheric Air Sampling

Four continuous automatic analyzers for measurement of atmospheric levels of ozone were used in a calibration and field study. These were (1) a colorimetric instrument based upon detection of iodine released from neutral potassium iodide reagent, {2) a coulometric instrument utilizing the polarization current as a measurement of iodine released by ozone in a cell contacted by potassium iodide reagent, (8) a galvanic cell measuring bromine release by ozone, and (4) an ultraviolet photometer. Some ozone determinations by the manual rubber cracking procedure were included. After calibration with ozone the average relative response to atmospheric ozone levels for each instrument was determined using the colorimetric oxidant analyzer as an arbitrary standard. These responses ranged from 77 percent for the galvanic cell 90 percent for the photometer. The instrument of choice for any given application would seem to be governed by requirements for precision specificity, portability, reliability, and ease of operation.     

Spatial and temporal variability of the overall error of National Atmospheric Deposition Program measurements determined by the USGS collocated-sampler program, water years 1989-2001

Data from the U.S. Geological Survey (USGS) collocated-sampler program for the National Atmospheric Deposition Program/National Trends Network (NADP/NTN) are used to estimate the overall error of NADP/NTN measurements. Absolute errors are estimated by comparison of paired measurements from collocated instruments. Spatial and temporal differences in absolute error were identified and are consistent with longitudinal distributions of NADP/NTN measurements and spatial differences in precipitation characteristics. The magnitude of error for calcium, magnesium, ammonium, nitrate, and sulfate concentrations, specific conductance, and sample volume is of minor environmental significance to data users. Data collected after a 1994 sample-handling protocol change are prone to less absolute error than data collected prior to 1994. Absolute errors are smaller during non-winter months than during winter months for selected constituents at sites where frozen precipitation is common. Minimum resolvable differences are estimated for different regions of the USA to aid spatial and temporal watershed analyses.     

Numerical model for static chamber measurement of multi-component landfill gas emissions and its application

The quantitative assessment of landfill gas emissions is essential to assess the performance of the landfill cover and gas collection system. The relative error of the measured surface emission of landfill gas may be induced by the static flux chamber technique. This study aims to quantify effects of the size of the chamber, the insertion depth, pressure differential on the relative errors by using an integrated approach of in situ tests, and numerical modeling. A field experiment study of landfill gas emission is conducted by using a static chamber at one landfill site in Xi’an, Northwest China. Additionally, a two-dimensional axisymmetric numerical model for multi-component gas transport in the soil and the static chamber is developed based on the dusty-gas model (DGM). The proposed model is validated by the field data obtained in this study and a set of experimental data in the literature. The results show that DGM model has a better capacity to predict gas transport under a wider range of permeability compared to Blanc’s method. This is due to the fact that DGM model can explain the interaction among gases (e.g., CH₄, CO₂, O₂, and N₂) and the Knudsen diffusion process while these mechanisms are not included in Blanc’s model. Increasing the size and the insertion depth of static chambers can reduce the relative error for the flux of CH₄ and CO₂. For example, increasing the height of chambers from 0.55 to 1.1 m can decrease relative errors of CH₄ and CO₂ flux by 17% and 18%, respectively. Moreover, we find that gas emission fluxes for the case with positive pressure differential (?P_in-out) are greater than that of the case without considering pressure fluctuations. The Monte Carlo method was adopted to carry out the statistical analysis for quantifying the range of relative errors. The agreement of the measured field data and predicted results demonstrated that the proposed model has the capacity to quantify the emission of landfill gas from the landfill cover systems.