Introduction
Stringent automotive exhaust emissions regulations are essential due to the significant health risks associated with air pollution, as mandated by the National Ambient Air Quality Standards (NAAQS) for a cleaner atmosphere Adverse weather conditions, particularly smog formed by the reaction of nitrogen oxides (NOx) and oxygen (O2) in sunlight, further complicate air quality in major cities like Beijing, Shanghai, Los Angeles, and New Delhi In response to these challenges, recent strict emission policies have prompted engine manufacturers to innovate engine designs and after-treatment systems, leading to a notable reduction in automobile emissions.
The primary focus of emissions research is to develop accurate methods for measuring near-zero emissions Over time, newer vehicles have faced stricter certification requirements compared to older models to ensure compliance with emissions limits before being approved for use These policies aim to reduce vehicle emissions, a significant source of pollution linked to adverse weather and health issues Vehicle certification must adhere to the United States Environmental Protection Agency (USEPA) regulations, except in California, where the California Air Resources Board (CARB) enforces more stringent standards Currently, the NOx emission standard for automobiles in the U.S is set at 0.2 g/bhp-hr, based on the 2010 standard, with projected updates forthcoming.
In response to the 2015 NOx emission standards set at 0.02g/bhp-hr, significant technological advancements and strategies have been implemented by manufacturers, researchers, and government agencies to mitigate NOx emissions from automobile exhaust While emission measurement procedures have evolved over time, the methods and devices used to measure these emissions have not significantly improved to meet the stringent near-zero emissions limit This is largely due to the inherent challenges associated with measuring NOx at such low levels Research indicates that measurement difficulties become pronounced when NOx concentrations fall below 100ppm, and the precision, accuracy, and repeatability of measurements are further compromised as concentrations approach near-zero levels of 30ppm or less.
Accurate measurement of NOx emissions is crucial as efforts aim to reduce vehicle emissions to near zero This need for precision has led to the reliance on robust measurement techniques, particularly as future automobile emission certifications depend on accurately quantifying these low NOx levels The Constant Volume Sampling System (CVS) is currently the standard for measuring automobile emissions, effectively diluting exhaust concentrations to prevent the loss of certain emission species However, its effectiveness at near-zero NOx concentrations remains uncertain, necessitating an investigation into the factors and conditions that may contribute to errors in NOx quantification and characterization.
This research aims to enhance the accurate measurement of automobile emissions at low levels by utilizing standardized procedures Traditional measurement methods are approaching their detection limits, raising concerns about the accuracy, precision, and repeatability of emissions characterization The study specifically focuses on investigating the factors that influence the quantification and characterization of NOx emissions near zero levels in a Constant Volume Sampler (CVS) setup.
This study aims to investigate the impact of several key factors on the accuracy of NOx measurement, including background variability, which encompasses dilution air filtration and proportional bag sampling Additionally, it will assess the performance of low concentration NOx analyzers, the effects of virtual NO/NO2 injection, and the influence of tunnel heating on measurement accuracy.
Literature Review
This study examines the effectiveness of existing measuring systems and suggests necessary modifications to the Constant Volume Sampling (CVS) setup to enhance precision, accuracy, and repeatability in measuring emissions at near-zero levels A review of previous research in this field is essential for providing valuable insights and guidance.
The stringent emissions regulations set by the USEPA have significantly lowered the allowable limits for automobile exhaust emissions and spurred advancements in after-treatment technologies These standards have evolved since the mid-20th century, initially aimed at reducing health risks from polluted air and improving visibility affected by these pollutants With automobile exhaust identified as a major pollution source, the first emission control technology was developed in 1961, followed by the implementation of the first automobile emission regulations in the United States in 1966.
The U.S government at state and federal levels established various regulatory agencies to control the pollution from automobile exhaust and to be able to meet up with the NAAQS By
Established in 1967, the California Air Resources Board (CARB) and the United States Environmental Protection Agency (USEPA), founded in 1970, are key regulatory bodies ensuring compliance with air emission standards from automobile exhaust While CARB and USEPA focus on regulating air pollutants, the U.S Department of Transportation (D.O.T) oversees fuel economy standards for manufacturers under the Corporate Average Fuel Economy (CAFE) regulations, aiming to minimize emissions while maximizing fuel efficiency Similar international regulatory agencies include the European Union Emission Standards, the Air Pollution Dispute Resolution Act, and Environment Canada, all adhering to comparable emission control standards.
Automobile Emission Standards and Regulations
Emission standards are promulgated enforcements which are set to control and limit the amount of emissions from different emission sources which can include vehicles, power plants,
Automobile emission standards are implemented to reduce pollutants from vehicle exhaust, promoting cleaner air and a healthier environment These emissions are classified into regulated and unregulated categories based on EPA and CARB standards Regulated emissions include nitrogen oxides (NOX), particulate matter (PM), total hydrocarbons (THC), carbon monoxide (CO), and greenhouse gases (GHG) such as carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) A significant challenge in current emission reduction technologies is balancing the need to lower emissions while managing increased fuel consumption.
Overview of NOx Emission Regulations and Standards
Over the years, NOx emission standards and regulations have evolved, significantly contributing to a cleaner and healthier environment in the U.S The EPA's standards have resulted in a notable reduction in annual NOx emissions from on-road mobile sources This progress is a key strategy for states to meet the National Ambient Air Quality Standards (NAAQS).
Figure 1 - Trend of U.S On-road Mobile NOx Emission Sources [7]
The NOx standards in the U.S have significantly decreased over the years, particularly with the stringent 2010 regulation setting a certification limit of 0.2 g/bhp-hr This pivotal change has driven the development and implementation of advanced after-treatment technologies, such as the Selective Catalytic Converter (SCR), along with other innovative strategies aimed at enhancing combustion efficiency.
U.S National Nitrogen Oxides Emissions: Mobile Source-On-
Engine design is evolving to meet stringent automobile NOx emission standards, which are approaching near-zero levels To achieve compliance, robust technologies are necessary, along with accurate methods to measure and characterize emissions In 2013, the California Air Resources Board (CARB) introduced severe optional low-NOx standards, which serve as a precursor to future NOx emission regulations by the U.S Environmental Protection Agency (USEPA), as the USEPA typically adopts CARB's standards after implementation.
Table 1 - USEPA & CARB NOx Emission Standard for Heavy Duty Diesel Engines [9]
Model Year NOx (g/bhp-hr)
The USEPA has significantly tightened NOx emission standards for heavy-duty vehicles, leading to a notable reduction in automobile exhaust NOx concentrations over the years With the upcoming mandatory CARB low NOx certification standard of 0.02g/bhp-hr set for 2024, the industry is moving closer to achieving near-zero emissions Recent advancements in technology have demonstrated that heavy-duty natural gas engines can meet these low NOx standards In the near future, it is anticipated that NOx emissions from automobile exhaust will drop below 10ppm, particularly under most operating conditions, with the exception of cold starts and low-load scenarios Ongoing research aims to address emission spikes during these specific conditions, striving for a robust solution to further minimize NOx emissions.
The shift towards a "near zero NOx emission era" necessitates a reliable process for accurately measuring emissions across all vehicle operating conditions This development is essential for certification, quantification, and characterization purposes, ensuring compliance with stringent NOx limits.
Figure 2 - NOx Emission Comparison for Trucks used for Port Operations [11]
The NOx emission standards outlined in Table 1 demonstrate that modern vehicles are approaching near-zero exhaust emissions, with concentrations below 50 ppm To meet the current standard of 0.2 g/bhp-hr and the optional target of 0.02 g/bhp-hr for automobile NOx emissions in the U.S., significant advancements in technology and compliance are essential.
Emission Standards for Heavy Duty Vehicles
Over the years, NOx emission standards for heavy-duty diesel engines have progressively tightened, achieving a remarkable reduction of over 99.8% since 1985, as shown in Table 1 Due to their combustion characteristics and operating conditions, heavy-duty engines are particularly prone to higher NOx emissions.
Current SCR technology alone is insufficient to meet existing NOx emission standards To comply with future low NOx standards, closed couple SCR technology is proposed as an effective Exhaust After-treatment System (EATS) for heavy-duty vehicles Additionally, enhancing the current SCR after-treatment system by optimizing the operating temperature of SCR catalysts can broaden the optimum temperature range, leading to improved performance.
Technologies for NOx Emission Reduction and Control
Recent advancements in emission control technologies have been crucial for meeting increasingly stringent emission standards Key developments include the introduction of the Positive Crankcase Ventilation (PCV) system in 1961 and the elimination of leaded fuel, which enabled the use of catalytic converters for automobile emission control Additional technologies that have emerged include the Lean NOx Trap (LNT), Exhaust Gas Recirculation (EGR), Diesel Oxidation Catalyst (DOC), Three-Way Catalyst (TWC), Selective Catalytic Reduction (SCR), and Diesel Particulate Filter (DPF).
An effective emission control strategy aims to minimize NOx emissions from automobiles, as high combustion temperatures contribute to increased NOx production By reintroducing a portion of the exhaust back into the engine's combustion chamber as intake air, this method effectively lowers combustion temperatures, leading to a significant reduction in NOx emissions.
A device designed for emission control effectively oxidizes automobile exhaust, ensuring that the emissions are non-harmful It targets carbon monoxide (CO) and hydrocarbons (HC), converting them into less harmful substances, while also oxidizing nitric oxide (NO) to nitrogen dioxide (NO2) This process increases the concentration of NO2 in the exhaust, which can be utilized by other downstream emission control devices.
Three Way Catalyst (TWC) Converter
An emission control device suited for rich burn engines as the converter requires stoichiometric or slightly rich combustion to operates effectively It concurrently reduces the NOX, CO and THC
Experimental Set-up and Procedures
This article provides a comprehensive overview of the experimental setup utilized in this study, detailing the configuration of the test cell, the measurement procedures, and the devices employed Additionally, it includes a concise outline of the test cell design.
The testing and data collection for this study were conducted at the Vehicle Emission and Testing Laboratory (VETL), part of the Center for Alternative Fuels, Engines, and Emission (CAFEE) at West Virginia University (WVU) The research adhered to the standards outlined in the Code of Federal Regulations (CFR), Title 40, Part 1065, as specified by the USEPA.
Overview of the General Test Set-Up
The study utilized an engine test cell where an engine was mounted and controlled to achieve the desired output for emission sampling This engine was paired with an after-treatment system designed to reduce NOx emissions to near-zero levels Emissions from the engine were channeled to a Constant Volume Sampler (CVS) system for sampling in a well-monitored and conditioned environment, adhering to established emission regulations Certain sections of the general setup were selectively utilized for the study, with further emphasis placed on specific components in subsequent sections of the report.
The study utilized a 2017 D13 engine mounted in the Engine Test cell, equipped with an after-treatment system designed to achieve low NOx emissions and comply with current standards The Emission After-Treatment System (EATS) included Selective Catalytic Reduction (SCR), Diesel Particulate Filter (DPF), and Diesel Oxidation Catalyst (DOC), working in conjunction with an Exhaust Gas Recirculation (EGR) strategy for effective emission reduction.
Fuel Ultra Low Sulfur Diesel
Fuel System Common rail fuel injection
Rated Horse-Power (HP) 375 – 500 HP
Emission After-Treatment System EGR+DOC+DPF+SCR
NOx Emission Certification 0.2g/bhp-hr
The engine, detailed in Table 5, was integrated with the DAQ system to monitor its operating conditions and enable control through a computer software interface.
Figure 11 -Engine Test Cell: Dyno [1], EATS [2],2015 DD15 engine [3], Connection to CVS
In this study, a full flow dilution sampling system was employed to analyze engine emissions, where the entire exhaust from the engine test cell is diluted with ambient air The resulting homogeneous mixture is sampled at the sampling port after mixing in the CVS tunnel and is then directed to analyzers that measure the concentrations of various gas constituents An overview of the emission sampling system utilized is depicted in Figure 12, in accordance with EPA 40 CFR standards.
Figure 12 - Schematic of the Emission Sampling Set-up adapted from [48]
This study primarily focused on measuring low levels of NOx, utilizing gas analyzers specifically designed for high-accuracy detection near zero levels To ensure precise results, multiple analyzers were employed for comparison and measurement alignment.
This section focuses on the modifications implemented in the conventional CVS setup and sampling system at the VETL laboratory to achieve near-zero accurate NOx measurements These enhancements, guided by recommendations from prior studies, include the integration of low NOx analyzers for precise measurements and a dilution air filtration system to minimize variability and ensure consistent intake air concentration.
Trace level NOx analyzers, such as the Brand-Gauss and Eco-Physics models, are designed for low-range NOx measurement These analyzers are utilized in conjunction with the Horiba MEXA One - CVS and MKS FTIR 2030 systems to accurately assess NOx emissions from engine exhaust within a CVS setup Specifically, the Brand-Gauss 7705 NOx Analyzer provides comprehensive total NOx measurements.
The ECO-Physics CLD 64 NOx analyzer utilizes the CLD measurement technique in conjunction with a high-temperature NO2 converter, featuring a measurement range of 0-50ppm and a maximum zero drift of 0.1ppm It operates with a noise level of 0.02ppm and boasts rapid response times, with T95 under 15 seconds and T50 under 7 seconds Additionally, the analyzer has an extended measurement capacity of up to 100ppm and a minimum detection limit of 2ppb, achieving a T50 response time of less than 1 second.
Figure 13- Trace level NOx analyzers: Brandgaus 7705[1], Ecophysics CLD64 [2], MKS
The ULPA V-cell filter from Air Filters, Inc utilizes activated charcoal technology, demonstrating significant effectiveness in reducing harmful gases including NOx, THC, Volatile Organic Compounds (V.O.C), and SOx in dilution air, as evidenced by previous studies.
Figure 14 – Schematic of the Dilution Air Filter [61]
The filter model boasts a Minimum Efficiency Reporting Value (MERV) of EN1822-U15, enabling it to effectively reduce high nitrogen oxide (NOx) levels in ambient air to approximately 0.1 ppm Additionally, it decreases non-methane hydrocarbon (NMHC) concentrations in the dilution air, ensuring a consistent concentration throughout the measurement process This reduction in hydrocarbon levels is crucial, as hydrocarbon interference can significantly affect the accuracy of CLD NOx analyzers when measuring low NOx levels.
The CVS measurement system was improved by varying background and dilution air sampling at all times, which accounts for concentration fluctuations throughout the measurement period This approach significantly reduces or eliminates errors associated with background correction, as previous studies have indicated variations in background NOx levels.
34 concentration with time The dilute bag was filled proportionally to the exhaust flow while the background bag was filled proportional to the dilution air flow
The assumption of a constant ambient concentration has been disproven, leading to necessary adjustments with mass flow controllers that maintain a proportional ratio of background air mass to dilution air mass, accommodating variability in ambient air concentration Emission samples for the Ecophysics and Brandgauss NOx analyzers were dried using Nafion tubes, while the Mexa analyzer processed emissions as dry samples after water removal via a chiller, and the FTIR measured samples in their wet state Corrections from dry to wet measurements were applied prior to analyzing emissions from the various analyzers employed.
Figure 15 - Proportional Bag Sampling System: Mass flow controller [1], Vacuum pumps [2],
Robust control of filling [3], Bagging System [4]
This study focused on various areas, as outlined in the sections below, to investigate their impact on near-zero NOx measurements through a series of experiments.
35 level in a CVS set-up As mentioned earlier this section further emphasize the specific section of the general experimental set-up used for conducting each of these studies
Table 6- Summary of Experiments Carried Out
Dilution air filtration study 3.4.1.1 Proportional Bag Sampling 3.4.1.2 Low Concentration NOx analyzer study 3.4.2 Virtual NO/NO2 injection Checks 3.4.3
Results and Analysis
This section of the report gives an overview of the results gotten for each study and the analysis carried out in achieving the objectives of this study
This study aimed to analyze daily fluctuations in ambient NOx concentrations Utilizing Brandgaus and Ecophysics low NOx analyzers, the researchers collected data that was segmented into 10-minute intervals to observe variations in NOx levels over 600 seconds each day.
Figure 21- Variation of Background NOx Concentration across Day 1
The background NOx levels measured by two low analyzers exhibited variation within a 10-minute interval, likely due to differences in their specifications and capabilities Additionally, random errors related to sampling connections and measurements may have contributed to this variation The Brandgauss analyzer recorded NOx measurements ranging from 0 to 0.05 ppm, while the eco-physics analyzer showed smaller fluctuations within a 0.01 ppm range Notably, daily variations in NOx concentration were observed for both analyzers on day 2.
44 figure 20 for both trace level analyzers.This connotes variability in the amount of NOx in the dilution air used for the CVS emission measurement system
Figure 22- Variation of Background NOx Concentration across Day 2(Test 2)
On day 2, higher NOx concentrations were observed for each analyzer compared to day 1, with the Brandgaus analyzer measuring approximately 0.06ppm and the Ecophysics analyzer measuring 0.014ppm, reflecting increases of 20% and 40% respectively The average NOx concentrations for the Ecophysics analyzer were 0.00088ppm on day 1 and 0.0077ppm on day 2, while the Brandgaus analyzer recorded averages of 0.0069ppm for day 1 and 0.017ppm for day 2 This data highlights the variability in measurements due to differences in analyzer specifications.
Daily fluctuations in NOx concentration over 10-minute intervals can significantly hinder the accurate quantification of NOx at near-zero levels This challenge arises because the diminishing difference between background NOx levels and diluted exhaust NOx makes it increasingly difficult for analyzers to measure accurately Additionally, variations may lead to the dilution air's NOx concentration exceeding that of the diluted exhaust, further complicating the quantification process using the CVS system at such low concentrations.
A study was conducted to analyze the daily variation of total hydrocarbon (THC) constituents in background air alongside NOx variability Using the MEXA analyzer, THC measurements revealed that on the first day, levels fluctuated between 2.3 ppm and 2.9 ppm In contrast, the second day exhibited a higher THC variation, ranging from 2.4 ppm to approximately 3.2 ppm, indicating a significant increase of 133.33% in the range of THC across both days.
Figure 23-Background THC Concentration: Day 1 and Day 2
The average THC concentrations on day 1 and day 2 were 2.53 ppm and 2.2 ppm, respectively, indicating a 13% decrease over the two days Notably, while THC levels decreased on day 1, there was an increase observed on day 2, as shown in Figure 23 This fluctuation in THC concentrations is likely to impact the accurate measurement of NOx at low levels, as THC contamination can lead to quenching, thereby compromising the precision of the CLD measurement technique used for NOx analysis.
Following an investigation into the variability of background NOx and THC levels, evidence revealed fluctuations in the concentrations of both species To address this variability during the sampling period, a dilution air filter was incorporated into the dilution air system Consequently, sampling was conducted both before (PRE) and after (POST) the implementation of the dilution air filtration system to evaluate its effectiveness.
The modification of the CVS system aims to minimize variability in ambient NOx and THC concentrations during the sampling period, ensuring stable species concentrations throughout Testing was conducted under various conditions, as detailed in Table 7, using Brand-gauss and Eco-physics low NOx analyzers for NOx measurements, while THC concentrations were measured with the MEXA analyzer.
Table 7 -Dilution Air Filter Average NOx and THC Reduction with Different Testing Conditions
Figure 24- Dilution Air Filter % Reduction in ambient NOx : Test 1 [1], Test 2 [2],Test 3 [3],
Conditions Test 1 Test 2 Test 3 Test 4
% N Ox Diff ere n ce (Po st w rt p re )
Respective Pre and Post Sampling at different testing conditions
Table 8- Summary of Average NOx Reduction Across Testing Conditions
Analyzers Conditions Test 1 Test 2 Test 3 Test 4
The incorporation of a dilution air filter into the CVS measurement setup resulted in significant NOx reduction, with a maximum of 250% recorded by Eco-physics and 100% by Brand-gauss analyzers Additionally, a percent increase in measured NOx was observed between pre and post-sampling, as illustrated in Figure 24 This increase is attributed to the consecutive nature of the sampling rather than concurrent measurements The background variability study indicated that NOx levels fluctuate within short time frames, potentially causing the recorded increase during post-sampling The average percent difference in measured NOx was 47% for Eco-physics and 14.3% for Brand-gauss analyzers.
Figure 25- Effect of Increase in Temperature on % NOx Reduction
The effect of increase in temperature on NOx reduction as measured by both Ecophysics and Brandgaus analyzers does not showcase a definite linear relationship as shown in Figure 25
Also, the effect of increase in humidity does typically showcase a specific trend on reducing NOx concentration in the ambient air as shown in Figure 26
Figure 26- Effect of Increase in Relative Humidity on % NOx reduction
Figure 27 - Dilution Air Filter % Reduction in ambient THC: Test 1 [1],Test 2 [2],Test
Table 9- Summary of Average THC Reduction Across Testing Conditions
Analyzer Conditions Test 1 Test 2 Test 3 Test 4
The study observed a maximum reduction of 10% in THC levels, as measured by the MEXA analyzer, indicating that the DAR was more effective in lowering background THC during test 1 compared to test 2 Notably, the most significant reduction occurred in Test 4, achieving a 10% decrease in ambient THC concentration The fluctuations in THC reduction may be attributed to variations in ambient air Overall, the average percentage difference in THC levels before and after sampling was 5% This reduction in ambient THC concentrations is crucial for enhancing the accuracy of NOx measurements, as it minimizes background THC variability.
The dilution air filter demonstrates a greater reduction in % THC as temperatures rise, while higher relative humidity results in decreased % THC reduction This indicates that optimal conditions for the dilution air filter's effectiveness in reducing THC occur at elevated temperatures and lower humidity levels.
Figure 28 - Effect of increasing Temperature and Relative Humidity on %THC Reduction
Proportional bag sampling was implemented to account for measurement variations, ensuring that the average bag measurement accurately reflects actual emissions Emission measurements were conducted through a bagging system using four different modes, as detailed in Table 8, to assess the impact of proportional sampling on NOx emissions in grams via the CVS setup A comparison of the NOx measurements between the CVS system and the bagging system revealed an increase in accuracy when transitioning from constant to proportional sampling modes for both dilute and background bag sampling.
Table 10 - Modes of Bag Sampling
The ECO-physics analyzers exhibited an average error difference of approximately 30% for modes 1 and 2, escalating to around 50% for mode 3, with a peak error of about 80% observed in mode 4 when both bags were filled proportionally Similarly, the Brandgaus and FTIR analyzers recorded an approximate 30% error difference in NOx measurements for the first two modes, with a 50% average difference in mode 3 Notably, mode 4 revealed the largest error difference for both Brandgaus and FTIR analyzers, reaching around 90%, aligning closely with the findings from the ECO-physics analyzer.
The results indicate a significant variation between proportional bag sampling and constant bag sampling methods Previous studies have shown that proportional bag sampling provides greater accuracy by considering fluctuations in exhaust flow rates and concentration changes during testing As illustrated in Figure 29, discrepancies in NOx measurements between constant bag sampling and proportional bag sampling were noted when compared to CVS measurements.
Sampling Bag Mode 1 Mode 2 Mode 3 Mode 4
Background Constant Proportional Constant Proportional
Dilute Constant Constant Proportional Proportional
Figure 29- Percent Error Difference in NOx Measured to CVS Measurement
An analysis of variance reveals that CVS measurements are independent of sampling modes, with a P-value of 0.2441, indicating no significant differences among the four modes at the 0.05 level Conversely, PBS measurements from FTIR show significant differences between the means of the sampling modes at the same significance level, as illustrated in Figure 30.
T test comparison of the sampling modes carried out as shown in Figure 30
Conclusions and Recommendations
The study investigated the factors influencing the accurate quantification and characterization of NOx emissions at near-zero levels, aligning with the research objectives.
➢ There exists variation in ambient NOx and THC concentration within 10 minutes interval
The dilution air filtration system demonstrated superior effectiveness in reducing THC levels compared to NOx, achieving an average NOx reduction of 47% and 14.3% as measured by the Ecophysics and Brandgaus analyzers, respectively In contrast, the THC reduction measured by the MEXA analyzer was only 5%.
➢ THC reduction efficiency increased with increased temperature and decreased with increased relative humidity
➢ An overall average analyzer drift of -1.8% and -1.6% was recorded for Ecophysics and Brandgaus analyzers respectively
➢ The Brandgauss and Ecophysics NOx analyzers both passed the linearization verification test according to the EPA “CFR, Title 40, Part 1065.307 standards
➢ A stable air temperature of 22℃ and a stable reduction in relative humidity to 60% was observed at the sampling plane as a result of tunnel heating
The study observed that NOx concentration in ambient air, used for exhaust dilution in the CVS tunnel, varied significantly within 10-minute intervals These fluctuations in NOx levels were primarily influenced by human activities, which contributed to an increase in environmental NOx concentration.
➢ The dilution air filter used according to this study was more relatively effective in reducing THC than NOx concentration
This study identifies key factors that may contribute to inaccuracies in measuring NOx at near-zero levels Recommendations include addressing these factors to improve measurement accuracy.
• Dilution air filtration study, pre and post sampling should be done concurrently to be able to account for possible variations in NOx concentration within 600seconds
Further research is essential to identify and comprehend the causes of errors and variations observed in this study, particularly concerning factors such as analyzer drift, dilution air filter loading, rapid adjustments of the proportional bag sampling mass flow controller in relation to varying flow rates, and the effects of pressure pulsation on CVS measurement during sampling.
• A need to quantify each error associated to each study and seek means to reduce this error and their cause
• Overall error propagation calculation is required to identify sub-components of the CVS system contributing significant error
Appendix
Figure A1 – Day 2 Test 1 Background NOx Variation(Brandgauss)
Figure A2 – Day 2 Test 1 Background NOx Variation(EcoPhysics)
72 Figure A3 – Day 3 Background NOx Variation(Brandgaus)
Figure A4 – Day 3 Background NOx Variation(EcoPhysics)
Table A1 - Brandgaus 7705 NOx Analyzer Specifications
Chemiluminescence using all solid-state detection
Span Calibration drift ± 1% of reading
Linearity Error ≤ 2% of full scale
Interferences < 2% of full scale Response Time T95 < 15secs
Table A2 - Ecophysics CLD 64 NOx Analyzer Specifications
Noise at Zero point (1𝜎) 1ppb
Humidity Tolerance 5-95% tolerance relative humidity (non- condensing ambient air and sample gas)
Sample flow rate 300 ml/min
Standards NO/NOx analyzer with internal molybdenum converter
74 Figure A5 – JMP Output of ANOVA and Student-T test comparison of CVS measurements at four Different Bag Sampling modes
75 Figure A6- JMP Output ANOVA and Student-T test comparison of FTIR measurements at four Different Bag Sampling modes
76 Figure A7 - JMP Output ANOVA and Student-T test comparison of Brandgaus measurements at four Different Bag Sampling modes
77 Figure A8- JMP Output ANOVA and Student-T test comparison of Ecophysics measurements at four Different Bag Sampling modes.