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Characteristics of On-road Diesel Vehicles: Black Carbon Emissions in Chinese Cities Based on Portable Emissions Measurement Xuan Zheng, Ye Wu, Jingkun Jiang, Shaojun Zhang, Huan Liu, Shaojie Song, Zhenhua Li, Xiaoxiao Fan, Lixin Fu, and Jiming Hao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b04129 • Publication Date (Web): 13 Oct 2015 Downloaded from http://pubs.acs.org on October 13, 2015
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Characteristics of On-road Diesel Vehicles: Black Carbon Emissions in Chinese Cities Based on Portable Emissions Measurement Xuan Zheng1, Ye Wu1,
2,*
, Jingkun Jiang1, 2, Shaojun Zhang3, Huan Liu1, 2, Shaojie Song4,
Zhenhua Li1, Xiaoxiao Fan1, Lixin Fu1, 2, Jiming Hao1, 2 1. School of Environment, State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, P. R. China. 2. State Environmental Protection Key Laboratory of Sources and Control of Air Pollution Complex, Beijing 100084, P. R. China 3. Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan 48109, U.S.A. 4. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, U.S.A. *Corresponding author. Phone: +86-10-62796947; Fax: +86-10-62773597; E-mail:
[email protected] 1
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ABSTRACT
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Black carbon (BC) emissions from heavy-duty diesel vehicles (HDDVs) are rarely
3
continuously measured using portable emission measurement systems (PEMSs). In this
4
study, we utilize a PEMS to obtain real-world BC emission profiles for twenty-five
5
HDDVs in China. The average fuel-based BC emissions of HDDVs certified according to
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Euro II, III, IV, and V standards are 2224 ± 251 mg kg-1, 612 ± 740 mg kg-1, 453 ± 584
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mg kg-1, and 152 ± 3 mg kg-1, respectively. Notably, HDDVs adopting mechanical pump
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engines had significantly higher BC emissions than those equipped with electronic
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injection engines. Applying the useful features of PEMSs, instantaneous BC emissions
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can be related to driving conditions using an operating mode binning methodology, and
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the average emission rates for Euro II to Euro IV diesel trucks can be constructed. From
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a macroscopic perspective, we observe that average speed is a significant factor affecting
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BC emissions and is well correlated with distance-based emissions (R2=0.71). Therefore,
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the average fuel-based and distance-based BC emissions on congested roads are 40% and
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125% higher than those on freeways. These results should be taken into consideration in
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future emission inventory studies.
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1. INTRODUCTION
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Because of China’s rapid motorization, fine particulate matter (PM2.5) and gaseous
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precursors (e.g., nitrogen oxides and hydrocarbons) emitted from gasoline and diesel
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vehicles have significantly contributed to urban air pollution
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light-duty gasoline vehicles (LDGVs), primary PM2.5 emissions from heavy-duty diesel
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vehicles (HDDVs) can be 1-2 orders of magnitude higher [5-7]. Among various species of
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diesel particulate matter (DPM), black carbon (BC) is a key component contributing
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36-90% of the mass emitted by HDDVs without advanced after-treatment devices.
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However, the fraction of BC in DPM can be significantly reduced by diesel particle filters
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(DPFs), which have been commercially adopted by new diesel vehicles in the U.S. and
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Europe [8-11]. In terms of total climate forcing (e.g., +1.1 W m−2, as reported by Bond et al.
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[12]
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through cloud, snow and sea-ice processes), BC is identified as the second most
[1-4]
. Compared with
), including direct radioactive forcing and semi-direct and indirect effects (e.g., effects
2
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significant anthropogenic emission, after carbon dioxide (CO2) [12-14]. In addition, BC has
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been recognized as an essential indicator of diesel exhaust in carcinogenicity studies,
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supporting a positive association between exposure to diesel engine exhaust and cancer
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risk [15, 16]. These well-documented properties and impacts of BC strongly motivated us to
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measure real-world BC emissions from HDDVs.
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BC emissions from HDDVs have been evaluated worldwide primarily through
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laboratory dynamometer testing and remote sensing measurements using tunnel, plume
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chasing or tent-like exhaust measurement systems
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challenges remain regarding the accurate measurement of BC emissions from HDDVs.
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For example, in dynamometer tests, potential uncertainties may result from the weak
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representativeness of test conditions (e.g., test cycles). Additionally, in remote sensing
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measurements, despite their ability to capture large-sized samples, the test duration is
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usually relatively short (ranging from a few seconds for tunnel studies or a few minutes
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for road chasing to hours or more for portable emission measurement systems (PEMSs)).
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Therefore, such approaches may not be able to adequately reflect the complexity of
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traffic conditions and provide statistical evidence for high emitters
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noting that in an effort to overcome the limitations of dynamometer tests and specifically
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address the significant off-cycle exceedances, on-board PEMSs have been employed in
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establishing regulatory test protocols in several developed countries (e.g., the U.S. and
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Europe)
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pollutants and fuel consumption and were only recently extended to particle mass or
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particle number [25-29]. To date, PEMS measurements of second-by-second BC emissions
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for HDDVs worldwide have been rare. China has always closely followed advanced
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international experiences in vehicle emission control
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PEMS protocols are currently being formulated. In addition to existing measurement
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challenges, severe air pollution problems and lax supervision of the diesel vehicle market
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have created a strong need for more domestic emissions measurements, and advanced
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PEMS instruments can be used to investigate in-use BC emissions from HDDVs and the
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real-world impacts of operating conditions.
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[23, 24]
[17-19]
. Nevertheless, numerous
[20-22]
. It is worth
. However, previous PEMS tests were typically focused on gaseous
[4]
, and national HDDV-specific
In this study, we develop a new PEMS platform by integrating on-board instruments 3
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to allow the continuous monitoring of BC emissions for HDDVs. Twenty-five HDDVs
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that are declared to comply with Euro II to Euro V emission standards were tested under
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various traffic conditions in China. Although the number of vehicle tested is small
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compared with remote sensing studies, the PEMS approach enables (1) the evaluation of
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real-world emissions from HDDVs over entire trip lengths, (2) the elucidation of
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instantaneous BC emission rates by operating mode, and (3) the quantification of impacts
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due to transient driving conditions (e.g., vehicle speed change). The advantages of the
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PEMS allow us to report useful data and provide insightful suggestions to researchers and
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policy makers.
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2. METHODOLOGY
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2.1. The PEMS platform
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Our PEMS sampling system consisted of an exhaust flow meter, a global position
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system (GPS) receiver, a gas analyzer, an air compressor, an air filtration and drying unit, a
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dilution system and a BC detector (see Figure 1). The exhaust flow meter (Model EcoStar,
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Sensors Inc., U.S.) was used to record real-time exhaust volume data. The GPS receiver
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was employed to obtain instantaneous vehicle speed and location. The air filtration and
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drying unit was used to ensure that the dilution air was dry and particle free. The dilution
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system (Model FPS4000, Dekati Ltd, Finland) consisted of an ejector dilution device able
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to heat the exhaust gas to 350 °C in the mixing box to avoid water and volatile organic
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compound condensation. The exhaust gas passed through a hose from the exhaust pipe to
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the platform; to maintain the gas temperature and effectively reduce particle loss during
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transmission, the hose was constructed of high temperature plastics. A portion of the
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exhaust gas was sampled directly by the gas analyzer, and the remaining exhaust was
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routed through the dilutor to the BC detector.
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Second-by-second CO2 and CO emission rates were measured by a gas analyzer
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monitor (Model EcoStar, Sensors Inc., U.S.) from the full-flow sampling exhaust before
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the dilution process (see Figure 1) using a non-dispersive infrared (NDIR) analyzer. BC
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mass concentrations were measured using an Aethalometer (Model AE-51, Magee
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Scientific, U.S.). The flow rate of the BC measurement was 100 ml min-1, with time 4
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resolutions of 1 min and 1 s. Second-by-second BC concentration data were collected
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from 19 diesel trucks in Beijing after a software update for the Aethalometer in late 2014;
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for previously tested HDDVs, we only collected minute-by-minute BC profiles (see
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Table S1).
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2.2. Tested vehicles and routes
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On-road BC emissions tests based on the PEMS platform were conducted in two
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Chinese cities, Beijing and Macao, from 2013 to 2015. We recruited 25 HDDVs,
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including 23 heavy-duty diesel trucks (HDDTs, gross vehicle weight over 12 t) and 2
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diesel buses; detailed vehicle information is listed in Table S1. The test vehicles were
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declared by their manufacturers to comply with Euro II to Euro V (i.e., equivalent to
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China II to China V) emission standards. Currently, HDDVs complying with Euro III and
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Euro IV standards account for a major portion of the total diesel fleet because the Euro V
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standard has been only adopted in public fleets (e.g., public transit buses) for a limited
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number of cities (e.g., Beijing)
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(i.e., Vehicle ID #1, see Table S1), all the remaining 24 vehicles were manufactured in
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China. The numbers of sampled vehicles for the Euro III and Euro IV categories were
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eleven and nine, an adequate sampling for PEMS studies.
[30]
. In this study, except for one 1998 model year truck
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Prior to road tests, we carefully checked the vehicle labels, engine types and
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post-treatment devices and compared the data with the corresponding type-approval
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information provided by China’s authority in charge of vehicle emission control.
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However, it should be noted that many local environmental protection agencies do not
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strictly enforce type-approval conformity during the selling process. For example, we
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found that one diesel truck (Vehicle ID #23) manufactured to meet the Euro IV standard
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employed a mechanical pump engine, which is a typical engine type for pre-Euro III
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emission standards and should not be adopted to meet the stringent Euro IV emission
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limit (e.g., for PM, 0.02 g kWh-1 under the steady test cycle). Furthermore, none of the
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tested vehicles, including two Euro V HDDVs, were equipped with a DPF; Beijing will
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not require the application of DPFs until 2016, when ultra-low sulfur diesel is expected
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to be delivered to the municipal area and neighboring regions (e.g., Hebei Province and
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Tianjin Municipal City) [31, 32]. 5
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The sample routes in Beijing and Macao can be divided into two road
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categories—congested roads and freeways (see Figure S1)—representing various driving
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conditions. The total tested distance and average speed by road category for each vehicle
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are presented in Table S2. The tested vehicles were cycled two to four times along the
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sampling routes in Beijing and Macao to ensure a typical continuous test duration of
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nearly 2 hours for each sample. The diesel fuels used in our tests were obtained directly
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from retail fuel stations, and the sulfur contents were lower than 15 ppm in both cities.
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2.3. Quality assurance and control
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It should be noted that there is no reference method to measure BC emissions for
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HDDVs. Indeed, the use of the PEMS method is rather novel for this applications. Thus,
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a series of quality control and assurance processes were carefully considered for the key
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experimental and data analysis procedures.
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Using GPS-derived vehicle speeds, which could be uncertain because of the
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substantial signal noise in building-dense areas, we compared second-by-second speed
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profiles for two Euro IV trucks determined by the GPS receiver and the on-board
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diagnostic (OBD) system (required since the Euro IV stage in China). The comparison
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results demonstrate that the GPS receiver was able to provide sufficiently accurate speed
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data (relative discrepancy within ±2%, see Figure S2 as an example).
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For the exhaust dilution and sampling system, the dilution rates of our measurement
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range from “100:1” to “120:1” to match the Aethalometer’s range of 0 to 1 mg m-3 and
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avoid instantaneous over-loading. Furthermore, we chose an appropriate caliber hose to
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ensure that the exhaust gases of all samples were in a completely turbulent state
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(Re~24000, see Supporting Information). Thus, the BC concentration in the sampling
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flow was representative of the entire exhaust flow.
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Because the Aethalometer neglects any influence of particle light scattering, previous
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studies have already reported that BC concentrations might be erroneously estimated
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when using this instrument
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Aethalometer were adjusted using equations (1) and (2) [33].
[33]
. Therefore, in this study, the raw data detected by the
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BC o (1) (0.88Tr + 0.12)
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BC =
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− ATN Tr = exp 100
(2)
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where BCo and BC are the originally measured and corrected BC concentrations,
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respectively, [ng m-3], Tr is the filter transmission, and ATN is the absorbance
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parameter,
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Aethalometer-derived BC concentrations have been demonstrated to exhibit good
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correlations with elementary carbon concentration values measured using thermal–
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optical analysis (TOA) methods
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vehicle emissions measurement studies [20, 21]. We also determined the agreement for BC
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emissions estimated using 1-min and 1-s profiles from two paralleled and pre-calibrated
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Aethalometers for 19 vehicle samples (see Figure S3 as one example) and quantified the
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discrepancies in BC emissions between two time resolutions: 7±7%. For each vehicle
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sample, we carefully modified the time alignment among the various instruments (i.e.,
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the gas analyzer EcoStar vs. the Aethalometer AE-51) to minimize potential mismatch
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between instantaneous data sources.
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2.4. Emission calculations and operating mode binning methodology
which
is
directly
obtained
[33, 34]
by
the
Aethalometer.
Corrected,
, and this correction has been commonly applied in
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Based on continuous second-by-second emission profiles for BC and gaseous
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species combined with simultaneous driving condition data, we were able to estimate the
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distance and fuel consumption specifically related to BC emissions (i.e., units in mg km-1
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and mg kg-1), as equations (3) and (4) illustrate: n
∑ BC ⋅V ⋅ DR i
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EFdis =
i
n
∑S
i
×106
i =1
(3)
i
i =1
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EFfuel
n BCi ⋅Vi ⋅ DRi ∑ i =1 = 103 ⋅ wc ⋅ n (12 / 44 ⋅ CO +12 / 28 ⋅ CO ） 2i i ∑ i =1
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(4)
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where EFdis is the distance-based BC emissions [mg km-1], BCi is the corrected BC
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concentration for episode i (minute or second) [ng m-3], Vi represents the exhaust gas
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volume for episode i [m-3], DRi represents the instantaneous dilution ratio for episode i,
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Si is the distance traveled during episode i [km], EFfuel is the fuel-based BC emissions
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[mg kg-1], wc represents the mass fraction of carbon in the fuel (0.87 for diesel), CO2i is
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the CO2 mass of emissions for minute i [g], and COi is the CO mass of emissions [g]. We
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did not include hydrocarbons in equation (4) because hydrocarbon emissions were not
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derived for all samples and because the carbon mass contribution from hydrocarbon
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emissions should be minor (
