Evaluation and Comparison of Portable Emissions Measurement

There is considerable interest in portable emissions measurement systems (PEMS) for .... The loading of the BUG was provided by a resistive load bank...
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Environ. Sci. Technol. 2007, 41, 6199-6204

Evaluation and Comparison of Portable Emissions Measurement Systems and Federal Reference Methods for Emissions from a Back-Up Generator and a Diesel Truck Operated on a Chassis Dynamometer THOMAS D. DURBIN,* KENT JOHNSON, DAVID R. COCKER III, AND J. WAYNE MILLER College of Engineering-Center for Environmental Research and Technology, University of California, Riverside, California 92521 HECTOR MALDONADO California Air Resources Board, Research Division, 1001 I Street, P.O. Box 2815, Sacramento, California 95815 ATUL SHAH AND CARL ENSFIELD Sensors, Inc., 6812 South State Road, Saline, Michigan 48176 CHRIS WEAVER Engine, Fuels, and Emissions Engineering (EEFE) Inc., Rancho Cordova, California 95827 MIKE AKARD AND NEAL HARVEY Horiba Instruments Inc., 17671 Armstrong Ave., Irvine, California 92614 JAMES SYMON AND THOMAS LANNI New York State Department of Environmental Conservation, 625 Broadway, Albany, New York 12233 WILLIAM D. BACHALO AND GREG PAYNE Artium Technologies, Inc., 150 West Iowa Avenue, Suite 202, Sunnyvale, California 94086 GREG SMALLWOOD National Research Council of Canada, Building M-9, 1200 Montreal Road, Ottawa, ON K1A 0R6 MANFRED LINKE AVL GmBH, Hans-List-Platz 1, Graz, Austria 8045

There is considerable interest in portable emissions measurement systems (PEMS) for emission inventory and regulatory applications. For this study, four commercial PEMS were compared with a Federal Reference Method (FRM) for measuring emissions from a back-up generator (BUG) over steady-state loads and a diesel truck on transient and steady-state chassis dynamometer tests. The agreement between the PEMS and the FRM varied depending on the pollutant and the particular PEMS tested * Corresponding author phone: (951) 781-5791; fax: (951) 7815790; e-mail: [email protected]. 10.1021/es0622251 CCC: $37.00 Published on Web 07/26/2007

 2007 American Chemical Society

for both the BUG and chassis dynamometer testing. The best performing PEMS for both the BUG and chassis testing was within ∼12% for NOx of the FRM. For the BUG testing, several PEMS showed agreement with the FRM within ∼5% for CO2. For the chassis dynamometer testing, the best PEMS showed agreement typically within ∼5% for CO2. PM measurements for the BUG testing were low compared to the FRM, with the best measurements ∼20% lower. For the chassis testing, two PM PEMS showed a good correlation but a high bias, while the correlation was worse for the other two PEMS. For each emissions component, some PEMS under different test conditions showed considerably larger deviations than those for the best performing PEMS.

Introduction On-road heavy-duty diesel (HDD) engines/vehicles are significant contributors to the emissions inventories for oxides of nitrogen (NOx) and particulate matter (PM) because of their high emission rates and the longevity of the vehicles. In recent years, the U.S. Environmental Protection Agency (EPA) and the California Air Resources Board (CARB) have promulgated regulations to further control diesel emissions. The most recent regulation has targeted in-use emissions in a defined portion of the engine map known as the Not-ToExceed (NTE) control area and the protocols required to make those measurements (1-3). The new requirement to measure in-use emissions means that portable emissions measurement systems (PEMS) will be needed in addition to fixed laboratory measurements. PEMS are also important tools for assessment of in-use HDD emissions for emissions inventory purposes. In the past few years, the use of PEMS has expanded considerably for in-use measurements and in the development of regulations. The EPA has put considerable emphasis on the development of PEMS for use in regulatory applications, as well as for the development of emissions data for its Mobile Vehicle Emission Modeling System (MOVES) (26). The engine manufacturers, as part of the consent decree program, conducted several programs to evaluate and develop PEMS through West Virginia University (7-9). Other studies have also evaluated the performance of PEMS, including studies with comparisons against laboratory grade measurements (10-17). Most of these studies were focused on only a few PEMS, were performed with earlier generations of the technology that were not designed for application to the NTE regulation, or were performed by the instrument manufacturers themselves. For this study, PEMS representing commercially available technologies in the late 2004 to early 2005 time frame were compared against laboratory measurements. The project is intended to represent a snapshot of the current state-of-art and a reference point from which PEMS can be evaluated as a tool for either emissions inventory development or in-use compliance. This PEMS evaluation determined the basic measurement capabilities (e.g., accuracy, precision, etc.) and compared those results with measurements made with laboratory grade emissions analyzers that are specified in the federal reference methods (FRM) and as used in the University of California at Riverside’s (UCR)’s mobile emissions laboratory (MEL). The goal of the project is to evaluate the PEMS suitability for inventory/model building work and for NTE compliance. VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Description of Measurement Capabilities of Individual PEMS THC CO CO2 NOx NO2 PM flow sample a

PEMS1 NDIRa NDIRa NDIRa electrochemical cella not measured light scattering calculated raw exhaust

PEMS2 FID NDIRa NDIRa ND-UVa ND-UVa not measured differential pressure raw exhaust

PEMS3 not measured NDIRa NDIRa chemiluminescence not measured filter not measured dilute exhaust

PEMS4 NDIR NDIR NDIR zirconia sensor not measured not measured differential pressure raw exhaust

Measurements were made on a “dry” basis (unmarked emissions are measured on a “wet” basis).

The PEMS systems selected for inclusion in the project were evaluated over two main test programs with increasing levels of complexity in measurement and potential variability in operation. The PEMS were initially evaluated using emissions from a diesel engine driving a back-up generator (BUG) over a series of steady-state load points. These steadystate conditions represent a lower level of measurement complexity than PEMS would experience during measurements under in-use conditions with mobile sources, which typically will operate under transient conditions. Second, the PEMS were evaluated over chassis dynamometer test cycles. The chassis dynamometer tests included steady-state cycles, transient cycles, and cycles designed to provide test conditions that create transitions into and out of NTE events. The chassis dynamometer transient tests represent an additional level of complexity for the PEMS measurements in that they require accurate measurement and correlation of the exhaust concentrations and the exhaust flow rate. For both tests, the PEMS were evaluated directly against measurements made by UCR’s MEL. Although this study includes measurements in NTE zone and information relevant to NTE gaseous measurements, it is not directly related to the larger Measurement Allowance program (18). The results of this study are discussed in this manuscript and are described in greater detail in ref 19.

Experimental Design and Procedures Measurement Method, CE-CERT’s MEL with FRMs. University of California at Riverside’s (UCR)’s mobile emissions laboratory (MEL) was used for the measurements utilizing the Federal Reference Method, subsequently referred to in this manuscript as FRM. The MEL consists of a full dilution tunnel with instruments meeting regulatory measurement requirements. The gas-phase analytical instruments measure NOx using chemiluminescence, methane (CH4), and total hydrocarbons (THC) with flame ionization detectors (FID), carbon monoxide (CO) and carbon dioxide (CO2) using nondispersive infrared (NDIR). PM emissions were measured using a gravimetric method. A full description of UCR’s lab is available in the peer-reviewed literature (20, 21). Measurement Method, PEMS Units Tested. The PEMS tested for the BUG and chassis dynamometer testing were the Clean Air Technology Incorporated (CATI) Montana System (13), the Sensors Inc., Semtech D system (8), the Engine, Fuels, and Emissions Engineering, Inc. Ride-Along Vehicle Emission Measurement System (RAVEM) (11, 12), and the Horiba OBS-1300 system (9, 10). The CATI, Semtech D, RAVEM, and Horiba units are generically identified as PEMS1, 2, 3, and 4, respectively, throughout this paper since the focus of the work was to provide a broad characterization of PEMS, as opposed to an evaluation of specific technologies. Each PEMS manufacturer approached their instrument design in a unique manner, so the measurement capabilities from each PEMS differed as summarized in Table 1. Note in the last row of Table 1 that three of the instruments are designed to work on raw exhaust and one of them is designed for dilute exhaust. The manufacturer representatives were 6200

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on-site during the testing and performed the installation, operation, and quality assurance/quality control (QA/QC) for their PEMS and subsequent data analysis, except for PEMS1 during the chassis testing. PEMS4 was also not available for the chassis dynamometer testing. The emissions measurements and calibration and blended gas checks were all done as blind comparisons in that the manufacturers provided their data before the FRM results were provided. It should be noted that in some cases, measurements for one or more emissions component may not be available for a particular PEMS for a variety of reasons. In addition to the four main PEMS tested, several other instruments designed specifically for the measurement of PM were also incorporated into the chassis dynamometer testing portion of the program. This portion of the study was strictly an exploratory survey since the PM PEMS represented a range of instruments using methods that are not direct measures of PM mass as measured using gravimetric sampling. This included the Artium laser induced incandesence (PEMS5), the AVL photoacoustic analyzer (PEMS7), and a TSI DustTrak light scattering PM monitor (PEMS8). These instruments each determine PM emissions using different measurement principles. PEMS5 and PEMS7, for example, measure soot (or black carbon) and not total PM. Elemental and organic carbon filter measurements were also made by the FRM. Further detail of the operation of these instruments is provided in the Supporting Information, along with details of the experimental methodology and setup. These PM measurements were all compared to the filterbased mass obtained from the FRM. Audit Bottle and Blended Audit Gas Checks. The PEMS and FRM measured the concentration of gaseous constituents from the same audit bottles as a consistency check in conjunction with both test programs. During the BUG testing, measurements were also made at four concentration levels from a primary audit bottle diluted using a gas divider with (1.5% accuracy mass flow controllers. The results of the audit and blended audit gas checks are provided in the Supporting Information. Test Matrix, Measurements of Emissions from an Operating BUG. The engine used for this testing was a model year 2000 CAT 3406C engine that powered a BUG rated at 350 kW. The exhaust measurements were made on the BUG at four different load points at the rated speed, 1800 revolutions per minute (RPM). The actual test points were as follows: mode 1, 100% load; mode 2, 65% load; mode 3, 25% load; and mode 4, 5% load. During the testing, the engine was operated following the protocols specified in 40 CFR Part 89, except that the load points differed from the levels specified in the CFR. The loading of the BUG was provided by a resistive load bank. A total of seven tests were run for each of the test points. Each test run was setup such that all of the PEMS units were sampling simultaneously from the exhaust stream at the same time. PEMS1-3 were tested in November 2004 while the data for PEMS4 were collected in February 2005. The measurements made with the FRM in February are slightly different

FIGURE 1. Mass emissions (gm/hr) for PEMS relative to FRM (a) NOx and (b) CO2. emission rates in gm/hp-hr provided for FRM. from those of November because of slightly different load values at some test points. The fuel used for both test periods was a commercially available CARB ultralow sulfur diesel (ULSD or sulfur 0.99) for all PEMS, with associated bias and Caterpillar C-15 ACERT engine with 200 h or about 5000 other differences found in the slope and intercept values. miles on it since being rebuilt. The engine was equipped The agreement for CO2 was relatively good for PEMS1 with dual exhausts and a pair of oxidation catalysts and was and 2. PEMS4 showed good agreement at the lowest load certified to the 2.5 g/bhp-hr NOx + NMHC and 0.1 g/bhp-hr point, but ∼10% difference at the higher flow rates. The PM levels. For the emissions inventory test cycles (UDDS agreement for PEMS3 was good at the highest flow rates, but and 50 mph cruise mode), the test weight was set at the difference was about 50% at the lowest load. PEMS1, 2, approximately 53 000 lbs. For the three-mode steady-state and 3 all showed good agreement at the highest load point. test cycle and the four-speed, seven-mode steady-state test It is interesting to note that the FRM measured higher CO2 cycle, the dynamometer settings were determined iteratively emissions than all PEMS. The correlation results between based on achieving the desired engine speeds, loads, and the PEMS and FRM (not shown) showed good R2 values vehicle speeds. (>0.99) for all PEMS, with associated bias and other differResults ences found in the slope and intercept values. Three manufacturerssPEMS1, PEMS2, and PEMS4s BUG Measurements, Gaseous Emission Rates. BUG emisreported hydrocarbons (HCs) and resultant values were sion rates for each gas were calculated in grams per hour VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Comparison of NOx emission rates and 95% confidence limits. (a) g/bhp-hr. (b) Percentage difference relative to NTE standard.

TABLE 2. Percentage Differences for Integrated Cycle Results (in Percent) NOx (g/cycle)

UDDS 50 mph cruise NTE 1290 NTE 1550 NTE 1770 NTE stepped

CO2 (g/cycle)

9

THC (g/cycle)

PEMS1

PEMS2

PEMS3

PEMS1

PEMS2

PEMS3

PEMS1

PEMS2

PEMS3

PEMS1

PEMS2

-13 -9 5 13 25 -8

12 7 9 11 5 8

21 16 16 15 14 18

-35 -19 -16 -14 -4 -14

1.7 2.1 5.3 2.2 2.1 2.7

9 14 13 13 12 13

-42 -16 -10 -16 -8 -25

20 21 51 36 38 43

-29 -25 -38 -29 -25 -64

-103 -78 -88 -46 -83 -4

57 -3 -36 -33 2 -18

significantly different relative to the FRM, with a range from 5 to 390% deviation. It should be noted that the overall HC values are relatively low. In comparisons with the anticipated THC NTE standard, the HC deviations were approximately 15% for the best performing PEMS, and in the range of 40160% for the other PEMS at the 25% and greater load points, depending on the load point and PEMS. Since PEMS1, 2, and 4 had accurate flow measurements, the source of the difference must be in the measured HC concentration. CO emissions were relatively low compared to the applicable emissions standard. On a percentage basis, CO emissions for the PEMS differed from +15 to -83% from the FRM, with one instrument showing differences within 5% on an absolute basis. In comparison with the anticipated CO standard, the deviations for the CO emissions for the PEMS were 3% or less for all PEMS and all load points 25% or greater. More details on THC and CO can be found in ref 19. Chassis Dynamometer Integrated Cycle Gaseous Emissions. The percentage differences between the PEMS and FRM for the integrated chassis dynamometer cycle gaseous emissions are provided in Table 2 for all gaseous emissions. The NTE steady-state cycles are denoted by the speed at which they were conducted (1290, 1550, and 1770 rpm). Exhaust flow comparisons for the chassis dynamometer testing are provided in the Supporting Information. Except for PEMS1, the NOx readings for the PEMS were higher than those obtained by the FRM. PEMS2 showed the best agreement for NOx, with readings ranged from 5 to 12% higher than the FRM over the different cycles. Other PEMS ranged from 13% lower to 25% higher than the FRM for NOx. The correlation results between the PEMS and FRM for NOx showed good R2 values (>0.93) for all PEMS. CO2 emissions also showed a trend with 2 of 3 PEMS having higher emissions than the FRM. The PEMS2 CO2 emissions were within 5% of those measured by the FRM for most of the test runs. These differences are comparable to those found between the exhaust flow rates for the FRM and PEMS2. The differences in CO2 for the other PEMS ranged from -35 to +14%. The correlation results between the PEMS and FRM for CO2 showed good R2 values (>0.93 or higher). 6202

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THC and CO emissions generally showed larger differences on a percentage basis than NOx or CO2. THC and CO emissions were well below the NTE limits, however, for nearly all cycles. Hence, the percentage differences relative to the NTE standards were generally less than 5% for THC and less than 2% for CO for the best performing PEMS. Data Analysis with a Focus on the NTE Zone. Comparisons between emissions measurements for the PEMS and FRM under conditions in the NTE zone were an important emphasis of this work due to the upcoming implementation of the NTE regulations. The evaluation of the NTE event data was performed only for PEMS2. This instrument was considered to the most likely to be used for this regulatory application, and the PEMS2 manufacturer provided data on an NTE event basis. NTE events were determined based on information obtained from the engine control module (ECM) data and the J1939 signal. The data analysis here focuses primarily on NOx and CO2 and the steady-state NTE cycle. Comparisons of NTE emissions for the other test cycles are presented elsewhere (19) and generally show trends comparable to those found for the steady-state NTE cycle. THC and CO results are presented elsewhere (19). The average NTE results in g/bhp-hr and in percentage difference units for NOx over the steady-state cycles are provided in Figure 3a and b. The percentage differences for the NTE events were calculated relative to anticipated NTE thresholds, since these are the differences most relevant for regulatory implementation and since the final NTE threshold will include a margin of error for the in-use PEMS measurement accuracy (21). For NOx, this was done by taking the absolute difference in the mass emission rates for the FRM and a particular PEMS, dividing by an approximate NTE threshold (2.0 g/bhp-hr for NOx), and presenting the value as a percent. The NOx emission rates for PEMS2 were approximately 3-17% higher than those for the FRM in g/bhp-hr units relative to the NOx NTE threshold. The correlation between the FRM and PEMS2 over all NTE events showed an R2 ) 0.92 with a positive bias from the FRM.

FIGURE 4. (a) Comparison of mass emission rates for CO2 in g/bhp-hr. (b) Percentage difference between PEMS and FRM for CO2 in g/bhp-hr.

FIGURE 5. PEMS emissions relative to FRM (a) on a percentage basis for the BUG testing and b) for absolute magnitude for the chassis testing. The average CO2 NTE results in g/bhp-hr and the percentage difference for the PEMS in comparison with the FRM are provided in Figure 4a and b, respectively. Results from PEMS2 were generally on the order of 5% higher than the FRM. The correlation between the emissions measurements for the FRM and PEMS2 was R2 ) 0.99 with a slight positive bias. PM Emissions Comparisons. Figure 5 shows the comparison between the PM emission rates for the FRM and the PEMS. The BUG testing results are presented on a relative percentage difference basis in Figure 5a, while the chassis dynamometer results are presented as a comparison of absolute values in Figure 5b. For the BUG testing, two manufacturers, PEMS1 and PEMS3, measured PM. For PEMS1 results were significantly lower than the FRM at all ranges. For PEMS3 values were within ∼20% of the FRM at higher loads, with larger deviations at the lower loads. PM/CO2 ratios for PEMS3 for the 5 and 25% load points were 22-25% lower than those measured by the FRM. It should be noted that the PEMS3 manufacturer indicated that at a point subsequent to the BUG and chassis dynamometer testing, they discovered a software error where every once in a while, at random intervals, the RS-232 signal would be misread and give a much larger flow value. The effect was that the mass flow through the filters was overestimated by a random amount, averaging about 10%. Thus, the estimated PM mass emissions for PEMS3 are likely UNDER estimated by an average of about 10%. This could have contributed to errors for PEMS3 for both the BUG and chassis testing PM results below. For the chassis dynamometer testing, PEMS3 PM measurements ranged from 18% lower to 49% higher than the FRM. The correlation coefficient from the linear regression was rather weak, however, with R2 ) 0.68. PEMS7 results were consistently below the FRM (13-22% lower), but showed

a high correlation coefficient of 0.95. PEMS8 also had a high correlation coefficient of 0.9, but was consistently higher than the FRM (24-43% higher). The good correlation for PEMS7 and 8 indicate that the agreement for these instruments might be improved if calibrated against the gravimetric filter weights. PEMS5 was also consistently higher than the FRM (6-57%), but this instrument had a lower correlation coefficient of 0.53 compared to the FRM. Discussion of Overall Results. The goal of this program was to provide a snapshot of the capabilities of PEMS for regulatory and emissions inventory applications. The results show that PEMS are continuing to evolve and show the potential for expanding measurement capabilities beyond fixed laboratories. The measurement techniques and operating practices for the PEMS differed between manufacturers and will likely evolve toward a more uniform protocol as the product moves toward standardization. Although the results from this study are meant to represent the best equipment and expert operators, a number of issues were still encountered in the field measurements. The agreement of the PEMS with the FRM ranged depending on the pollutant and the particular PEMS tested. The closest agreement was generally found for NOx and CO2. The best performing PEMS for both the BUG and chassis testing was with ∼12% for NOx of the FRM on a relative basis. For the BUG testing, several PEMS showed agreement with the FRM within ∼5% for CO2 over most of the operating conditions. For the chassis dynamometer testing, the best PEMS showed agreement within ∼5% for CO2 over most of the test conditions. It should be noted that for each emissions component some PEMS under different testing conditions showed considerably larger deviations than observed for the PEMS that provided the best performance. The correlation results between the PEMS and FRM for NOx and CO2 showed good relatively R2 values (>0.93 or higher), indicating VOL. 41, NO. 17, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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relatively strong linear relationships between the PEMS and FRM for those emission components, with associated bias and other differences found in the slope and intercept values. THC and CO emission levels were relatively low for both the BUG and chassis dynamometer testing, and the PEMS showed larger deviations from the FRM for these emission components. For THC, in comparison with to the NTE standard, percentage differences for the best performing PEMS with the FRM were within 15% for the BUG testing. Other PEMS did show higher deviations for THC, however, even relative to the NTE standard. The deviations for CO were relatively low compared with the NTE standard, with the best performing PEMS showing deviations of 2% or less relative to the NTE standard for the chassis dynamometer testing and all PEMS below 3% for the BUG testing. For the BUG PM measurements, both instruments were biased low compared to the FRM, with the best measurements approximately 20% lower than FRM. For the chassis dynamometer PM results, two instruments showed either a lower or a higher bias relative to the FRM, but showed good correlations with the FRM (R2 > 0.9). The correlations for the other two instruments were worse (R2 > 0.53 and 0.68), with one instrument showing a high bias and the other no showing a definitive bias. It is anticipated that PM PEMS instruments will continue to evolve and that there will be an increased emphasis on understanding the differences between various methods of measuring PM and more traditional filter-based methods.

Acknowledgments We thank the following organizations and individuals for their contributions to this project. We acknowledge the manufacturers of the Portable Emissions Measurements Systems (PEMS)s Clean Air Technologies Inc.; Engine Fuels, Emissions, Engineering (EEFE) Inc.; Horiba Inc., Sensors Inc., Artium Technologies, Inc., and AVLs for providing staff, instruments, and technical expertise. We acknowledge the Emission Measurement and Testing Committee (EMTC), Mr. Rey Agama of Caterpillar, and Dr. Nigel Clark of West Virginia University for technical assistance. Caterpillar Inc. also provided a heavy-duty diesel truck for use in this project as an in-kind contribution. We acknowledge the CARB staff at the heavy-duty vehicle chassis dynamometer facility in Los Angeles and Mr. Don Pacocha, University of California, Riverside for their contributions during the experimental measurements. We acknowledge the funding from the California Air Resources Board (CARB), the U.S. Environmental Protective Agency (US EPA), and the help from the New York Department of Environmental Conservation.

Supporting Information Available Additional tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review September 18, 2006. Revised manuscript received May 23, 2007. Accepted June 21, 2007. ES0622251