On-road emission measurements of reactive nitrogen compounds from heavy-duty diesel trucks in China

Journal Pre-proof On-road emission measurements of reactive nitrogen compounds from heavy-duty diesel trucks in China Liqiang He, Shaojun Zhang, Jingnan Hu, Zhenhua Li, Xuan Zheng, Yihuan Cao, Guangyi Xu, Min Yan, Ye Wu PII:

S0269-7491(19)35636-2

DOI:

https://doi.org/10.1016/j.envpol.2020.114280

Reference:

ENPO 114280

To appear in:

Environmental Pollution

Received Date: 29 September 2019 Revised Date:

25 February 2020

Accepted Date: 25 February 2020

Please cite this article as: He, L., Zhang, S., Hu, J., Li, Z., Zheng, X., Cao, Y., Xu, G., Yan, M., Wu, Y., On-road emission measurements of reactive nitrogen compounds from heavy-duty diesel trucks in China, Environmental Pollution (2020), doi: https://doi.org/10.1016/j.envpol.2020.114280. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

Graphical abstract

Weather Probe

GPS

Analyzers Communication lines (including OBD data logger) Sample lines Exhaust Flow Meter

1

On-road emission measurements of reactive nitrogen compounds from

2

heavy-duty diesel trucks in China

3

Liqiang He a, Shaojun Zhang

4

Guangyi Xu e, Min Yan e, Ye Wu a, b

a, b

*, Jingnan Hu c, Zhenhua Li a, Xuan Zheng d, Yihuan Cao a,

5 6

a

7

Pollution Control, Tsinghua University, Beijing 100084, China

8

b

9

Complex, Beijing 100084, China

School of Environment, State Key Joint Laboratory of Environment Simulation and

State Environmental Protection Key Laboratory of Sources and Control of Air Pollution

10

c

Chinese Research Academy of Environmental Sciences, Beijing 100012, China

11

d

College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen

12

518060, China

13

e

Shenzhen Research Academy of Environmental Sciences, Shenzhen 518001, China

14 15

* Corresponding author. [email protected]

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Abstract: Emissions of major reactive nitrogen compounds, including nitric oxide (NO),

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nitrogen dioxide (NO2) and ammonia (NH3), from heavy-duty diesel vehicles (HDDVs) place

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substantial pressure on air quality for many large cities in China. To control nitrogen oxide

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(NOX) emissions from HDDVs, selective catalytic reduction (SCR) systems have been widely

20

used since the China IV standards. To investigate the impacts of aftertreatment technologies

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and driving conditions on real-world emissions of reactive nitrogen compounds, a portable

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emissions measurement system was employed to test eighteen heavy-duty diesel trucks in

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China. The results showed that the China IV and China V HDDVs with appropriate SCR

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functionality could reduce NOX emissions by 36% and 53%, respectively, compared to the

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China III results, although their real-world emissions were still higher than the corresponding

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emission limits for regulatory engine tests. For these HDDVs, five samples were tested with

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NH3 emissions, ranging from 1.67 ppm to 51.49 ppm. The NH3 emission rates tended to

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significantly increase under high-speed driving conditions. The results indicate that the

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current SCR technology may have certain risks in exceeding the future China VI NH3 limit.

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However, five China IV/V HDDVs were found to have SCR temperature sensors that were

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intentionally tampered with, resulting in comparable or even higher NOX emissions and zero

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NH3 emissions. Increased NO2 emissions due to the adoption of diesel oxidation catalysts and

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diesel particulate filters were also found from our experiments. This study highlights the

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importance of enhancing in-use compliance requirements and eliminating aftertreatment

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tampering for China IV and China V HDDVs.

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Main finding:

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NOX and NH3 emission profiles of heavy-duty diesel trucks indicate significant effects from

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aftertreatments (e.g., selective catalytic reduction) and driving conditions, which should be

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carefully considered to develop real-world emission factors.

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Keywords: NOX, NH3, selective catalytic reduction (SCR), diesel, vehicle emissions, portable

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emissions measurement systems (PEMS)

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1. Introduction

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On-road vehicles have been identified as one of the most important contributors to air

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pollution in metropolitan areas across the world (Anenberg et al., 2017; Yang et al., 2019),

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resulting in significantly adverse health impacts (Shindell et al., 2011; Wolfe et al., 2019).

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Vehicular nitrogen oxide (NOX) emissions directly trigger serious nitrogen dioxide (NO2)

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pollution problems in traffic-dense areas, where ambient NO2 concentrations could

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substantially exceed air quality standards (Cheng et al., 2018; UNEP, 2019). Furthermore,

49

through complex atmospheric processes, vehicle emissions were the largest local contributors

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to ambient fine particulate matter (PM2.5, particulate matter with aerodynamic diameters

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smaller than 2.5 µm) concentrations in populous cities (Wu et al., 2017). Major reactive

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nitrogen compounds including nitric oxide (NO), NO2 and ammonia (NH3) are important

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precursors involved in secondary particulate matters and ozone (O3) (Bishop and Stedman,

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2015; Elser et al., 2018). In the Beijing-Tianjin-Hebei region, one of the most polluted areas

55

in China, ambient nitrate has become the most important secondary inorganic component,

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with an average mass fraction of 19% of the total PM2.5 mass (NAPC, 2019), which was

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observed up to 40% during several pollution episodes (Beijing Daily, 2018). The proportion

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of ammonium salts was responsible for 11% of the total PM2.5 mass during the heating season

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in 2017-2018 (NAPC, 2019). In 2018, the Chinese government announced a “Three-Year

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Action Plan for Winning the Blue Sky Defense War” (referred to as the action plan) to ensure

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greater achievements in air pollution control (State Council, 2018). In this action plan,

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controlling diesel truck emissions was one of the most imminent tasks (MEE, 2018a) due to

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their high contributions of NOX and primary PM2.5 emissions among all sectors MEE (2018b).

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To reduce air pollutant emissions, China has implemented increasingly stringent emission

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regulations on heavy-duty diesel vehicles (HDDVs). Selective catalytic reduction (SCR)

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systems that reduce NOX emissions through redox reactions between NOX and NH3 have been

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applied by HDDVs since the China IV stage. Urea aqueous solution is used as a reducing

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agent of SCR, which is injected into the exhaust stream at the SCR inlet when exhaust

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temperatures are sufficiently high for reaction activity (typically above 200℃) (Suarez-Bertoa

70

et al., 2016; Guan et al., 2014). The adoption of urea-SCR systems might lead to NH3 leakage 3 / 25

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because partial NH3 compounds that are not oxidized could be emitted from the tailpipe (Liu

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et al., 2016). Thus, NH3 leakage is considered an important cause of NH3 emissions from

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modern diesel vehicles (Suarez-Bertoa et al., 2017). Of note, previous studies reported that

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SCR-equipped HDDVs may not be successfully reducing real-world NOX emissions as

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expected, possibly due to low exhaust temperatures under urban driving conditions or

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tampering of SCR systems (Zhang et al., 2014a; Yang, 2018). Therefore, a series of stringent

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requirements have been added in the latest China VI standards. For example, enhanced

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on-board diagnostic (OBD) functions (e.g., anti-tampering) (Yang, 2018) and exhaust thermal

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management systems should be utilized to maintain good performance of aftertreatment

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systems (e.g., SCR). In addition, real-world portable emission measurement system (PEMS)

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testing and remote on-board sensing protocols have been applied to monitor the in-use

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emission compliance. Similar to the Euro VI standards, the China VI emission standards also

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propose a new NH3 concentration limit over the regulatory cycle in the laboratory (i.e.,

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weighted average below 10 ppm) (EU 2011; MEE and SAMR, 2018).

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Previous studies have not reported concurrent measurements of on-road NOX and NH3

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emissions for HDDVs in China. As real-world driving conditions could significantly affect

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aftertreatment performance and thus pose complex effects on NOX and NH3 emissions, it is

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necessary to conduct real-word emission measurements for reactive nitrogen compounds. In

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this study, a PEMS was employed to measure eighteen HDDVs that varied by emission

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standards and aftertreatment technologies. Instantaneous gaseous emissions of NO, NO2, NH3

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and other major species (e.g., carbon monoxide (CO), total hydrocarbon (THC), and carbon

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dioxide (CO2)) were tested and further analyzed in association with the vehicle operating

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mode. Furthermore, the impacts from more stringent emission standards, driving conditions

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and aftertreatments on NOX and NH3 emissions were explored. This study aimed to provide

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policymakers with useful data and to highlight the urgency of enhancing in-use compliance

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requirements and eliminating aftertreatment tampering for HDDVs to control real-world

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emissions of reactive nitrogen compounds.

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2. Methodology

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2.1 Experimental section 4 / 25

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Vehicle information. On road emission tests were conducted in Beijing and Shenzhen

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from 2017 to 2019. The tested fleet included 11 rigid trucks and 7 semitrailer tractors

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recruited from local freight companies. These vehicles were declared to comply with China

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III to China V emission standards. The vehicle specifications are summarized in Table 1.

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Table S1 lists the detailed information about the engine and aftertreatment systems (e.g.,

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manufacturer, type and model).

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All of these vehicles were tested as received. Before each test, we carefully checked the

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actual status of the aftertreatment systems. Two China III vehicles (#4 and #5) were retrofitted

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by installing diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) systems. To

109

evaluate the impact from DOC and DPF on reactive nitrogen compounds, engine and tailpipe

110

emissions from the two vehicles were separately measured using PEMS combined with an

111

NH3 analyzer. We noticed that the urea injection systems of five vehicles (#7, #8, #11, #15

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and #17) had been tampered with by keeping the SCR inlet temperature sensors

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approximately 3~5 cm away from the original set position in the engine exhaust (see Figure

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1), because the vehicle owners intended to save on the cost of diesel exhaust fluid (DEF). We

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selected one China V vehicle (#17) among the tampered vehicles to compare real-world

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emissions between tampered conditions vs. normal conditions (i.e., by keeping the

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temperature sensor working appropriately).

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Fuel and test routes. According to the PEMS measurement regulations in China (MEP,

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2017; MEE and SAMR, 2018), we used commercial diesel fuels obtained from certified gas

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stations, which complied with China VI standards with a sulfur content below 10 ppm. The

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test routes consisted of urban, rural and motorway operations representing the typical driving

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routes of heavy-duty trucks (see Figure S1). The average vehicle speeds were 15 to 30 km/h

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for urban driving, 45 to 70 km/h for rural driving, and approximately 70 km/h for motorway

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driving. The average test conditions are illustrated in Table 2 and the detailed trip information

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of each test in Table S2. All drivers were professional and were instructed to maintain their

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usual driving behaviors during the test trips.

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Emission measurement systems. A PEMS (Model SEMTECH-ECOSTAR, Sensors Inc.,

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USA) integrated with an NH3 analyzer (Model NMS, IAG Inc., Austria) was used to measure 5 / 25

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real-time vehicle emissions. The SEMTECH-ECOSTAR employed a nondispersive

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ultraviolet (NDUV) module to separately measure NO and NO2 concentrations, a

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nondispersive infrared (NDIR) analyzer was used for measurements of CO and CO2, and a

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heated flame ionization detector (HFID) was used for THC measurements. The NH3 analyzer

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used a heated extractive diode laser ammonia detection technology to measure instantaneous

134

NH3 concentrations. Exhaust flow rates were monitored through a 4-inch heated sampling

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tube assembly. Based on exhaust flow rates and concentrations of air pollutants, instantaneous

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emission rates (g/s) were calculated. Instantaneous vehicle speed and location information

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were collected using a global positioning system (GPS) receiver, and second-by-second

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engine operating data, including engine torque, engine speed, fuel injection rate and vehicle

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speed from the engine control unit (ECU), were derived by using an OBD data logger. The

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technical details of pollutant analyzers are illustrated in Table S3.

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2.2. Data Processing

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An operating mode binning method was applied to mitigate the impact of various traffic

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conditions between each individual on-road test by normalizing the emission results under a

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benchmark operating pattern (Wu et al., 2012). As Table S4 indicates, 22 bins representing

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instantaneous operating modes were classified by vehicle specific power (VSP) and vehicle

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speed (Zhang et al., 2014a; Zheng et al., 2015). In this study, we calculated instantaneous VSP

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with Eq.1 (USEPA, 2009).

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VSP = av +

149

where VSP is estimated vehicle specific power, kW/t; a is instantaneous vehicle

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acceleration, m/s2; v is instantaneous vehicle speed, m/s;

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coefficient, kW
s/m;

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aerodynamic drag coefficient, kW
s3/m3; and m is vehicle weight, t. In this study, the values

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of

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and 0.000331, respectively (Wu et al., 2012).

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A B C v + v2 + v3 (1) m m m

B

A

is the rolling resistance

is the rotational resistance coefficient, kW
s2/m2; C is the

A B C , and applied for HDDVs with GVWs greater than 12 tons were 0.0857, 0, m m m

Based on the instantaneous profiles, we estimated the average emission rates of gaseous 6 / 25

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pollutants and derived the engine power for each operating mode bin with Eq.2 and Eq.3.

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ERi , j =

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1 Pi = Ti

1 Ti ∑ ERi, j,t (2) Ti t =1 Ti

∑P t =1

i ,t

(3)

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where ER i , j is the average emission rate of pollutant j for a tested vehicle in operating

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mode bin i, g/s; Ti is the number of seconds for a tested vehicle in operating mode bin i;

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ERi , j ,t is the real-time emission rate of pollutant j for a tested vehicle in operating mode

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bin i at second t, g/s; P i and Pi ,t are the average and instantaneous derived engine power

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for a tested vehicle in operating mode bin i, kW.

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To mitigate effects of various traffic conditions and compare to the emission limits unit in

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g/kWh, a normalized brake-specific emission factor to a baseline traffic pattern was

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developed for each vehicle. Eq. 4 illustrates the calculation based on average emission rates,

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engine operating data and the time allocation of operating mode bins within a driving cycle.

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 ER i , j  nBSEF j = 3600 × ∑  × npi  (4) Pi i  

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where nBSEF j is the brake-specific emission factor of pollutant j , g/kWh; np i is time

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allocation of operating mode bin i to the benchmark driving pattern (see Figure S2). In this

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study, we refer to a modified version of the World Harmonized Vehicle Cycle (i.e., the

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C-WTVC illustrated in Figure S3) for the chassis dynamometer test as the benchmark driving

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pattern used for fuel consumption certification purposes of heavy-duty commercial vehicles in

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China (AQSIQ and SA, 2011).

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Engine operating data were not obtainable from seven vehicles (i.e., #3, #4, #5, #6, #11,

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#12 and #13). For comparison with the emission limits (g/kWh), we applied the

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brake-specific fuel consumption (BSFC) values (kg-fuel/kWh) for HDDVs provided by the

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MOBILE6 model (US EPA, 2002) to convert the fuel-based emission factors (g/kg-fuel) into

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brake-specific emission factors (g/kWh). Furthermore, the mean relative error of

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brake-specific emission factors between directly calculating using engine operating data and 7 / 25

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converting the fuel-based emission factors was 1.50% (ranging from -28.8% to 62.2%, see

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Figure S4) and the normalized mean square error was 0.09 (N=11) in this study, suggesting

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the usefulness of the BSFC values for HDDV fleets.

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For these vehicles without available engine operating data, we first calculated the average

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fuel consumption rates of each operating mode bin with Eq. 5 (Zhang et al., 2014b), and then

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the normalized fuel-based emission factors were estimated with Eq.6 which was used to

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convert into brake-specific emission factors (g/kWh) with Eq. 7 (Wu et al., 2012).

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FRi =

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 ER i , j  nFEF j = 1000 × ∑  × np i  (6) i  FR i 

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nBSEFj =

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where FR i is the average fuel consumption rate in operating mode bin i , g/s; ER i , C O ,

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ER i , C O and ERi,THC are average emission rates of CO2, CO and THC, respectively, in

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operating mode bin i, g/s; WC is the ratio of carbon mass to total fuel mass, 0.866 for diesel;

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and nFEF j is the fuel-based emission factor of pollutant j , g/kg-fuel.

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3. Results and Discussion

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3.1 Overview of real-world emissions of NOX and NH3

0.273 × ERi ,CO 2 + 0.429 × ERi ,CO + 0.866 × ERi ,THC (5) WC

0.454 × nFEFj × BSFC (7) 0.746 2

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Figure 2 presents the normalized brake-specific emission factors of NOX (the sum of NO

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and NO2) and NH3 based on a benchmark operating pattern (C-WTVC) for all tested vehicles.

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For NOX emissions, we observed a significant difference between normal and tampered

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vehicles for the China IV and China V samples. To better evaluate emissions of in-use

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HDDVs in China, the tampered vehicles were classified into a separate category in the

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following sections. Average NOX emission factors were 12.79±3.52, 8.25±1.99, 6.05±2.42

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and 14.15±2.68 g/kWh, respectively, for China III (N=5), China IV (N=3), China V (N=5)

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and tampered China IV/V vehicles (N=5), higher than their corresponding in-lab engine

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emission limits (i.e., 5.0, 3.5 and 2.0 g/kWh for China III, IV and V vehicles, respectively) 8 / 25

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due to the road-to-lab discrepancy and tampering issues (Wu et al., 2012; Yang, 2018).

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Compared to China III vehicles, the average emission factors of the normal China IV and

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China V vehicles decreased by 35.52% and 52.70%, respectively. For these normal China IV

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and China V HDDVs, their average reductions in NOX emissions compared to China III are

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comparable to the trends in Europe (i.e., Euro IV/V vs. Euro III) (Ligterink et al., 2009;

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Ntziachristos and Samaras, 2018). However, the results of the tampered China IV/V HDDVs

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showed an opposite trend and were increased by approximately 10.62% than the tested China

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III HDDVs.

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As were measured NO and NO2 emissions separately, Figure 2 indicated a decreasing trend

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in primary NO2 emission fractions (i.e., NO2/NOX) from China III to China IV/V. The average

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NO2 emission factors were 0.90±0.07 g/kWh for China III vehicles (N=5), 0.17±0.05 g/kWh

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for China IV vehicles (N=3), 0.29±0.13 g/kWh for China V vehicles (N=5) and 0.32±0.09

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g/kWh for tampered China IV/V vehicles (N=5), indicating that the average fractions of

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primary NO2 in total NOX emissions were 7.21%±2.04%, 2.16%±0.96%, 5.33%±2.97% and

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2.24%±0.29%, respectively. The NO2 emission factors of the tested China IV and China V

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HDDVs including the tampered vehicles are lower than China III samples, which suggests

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that the decreased NO2 emissions for China IV and China V HDDVs might be attributed to

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the optimization of the combustion conditions in the diesel engines (Carslaw and Rhys-Tyler,

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2013).

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NH3 emissions varied significantly among all tested HDDVs. Of the five vehicles (#6, #9,

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#10, #13 and #18) of the total eighteen tested vehicles, we detected NH3 emissions (detection

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limit of 0.8 ppm, see Table S3) and the emission factors distributed widely from 8.16 to

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443.67 mg/kWh. Average NH3 emission factors were 196.50±215.45 mg/kWh for China IV

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HDDVs (N=3), far higher than that for China V vehicles (13.29±7.26 mg/kWh, N=2). As

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shown in Table S5, the average NH3 emission concentrations of two in-use HDDVs (51.49

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ppm for #6; 12.66 ppm for #10) failed to comply with the upcoming China VI emission limits

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under engine tests, indicating that the current SCR technologies are facing the risk of

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exceeding the NH3 limit. The ammonia slip catalyst (ASC) systems have been suggested for

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adoption for China VI diesel vehicles. We did not detect NH3 emissions from China III, the 9 / 25

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tampered China IV/V vehicles, and some normal China V vehicles (#12, #14 and #16).

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In-depth analyses regarding the instantaneous NH3 emissions are provided in the following

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section.

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3.2 Impact of driving conditions on NOX and NH3 emissions

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Figure 3 shows the instantaneous NOX, NO2 and NH3 emission rates for the tested HDDVs

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associated with emission standards and operating modes. For low-speed zones, little

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distinction in the NOX emission rates was found among the four vehicle categories (China III,

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China IV, China V and tampered China IV/V vehicles). This finding implies that the SCR

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systems might have less satisfactory performances under low-speed modes (Bins 11 to 18),

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which tend to result in low exhaust temperatures (Wu et al., 2012). By contrast, under the

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medium-speed modes (Bins 21 to 28) and high-speed modes (Bins 35 to 38), we observed a

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substantial decrease in NOX emission rates for normal China IV/V HDDVs compared to the

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China III and tampered China IV/V vehicles. For example, the average fuel-based emission

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factors for China III vehicles were 53.22 g NOX per kg fuel under medium-speed modes and

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38.09 g NOX per kg fuel under high-speed modes, separately. The decreases in the average

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ratios of NOX emissions to fuel consumption were 31.76% (medium-speed modes) and

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42.49% (high-speed modes) for China IV, and 57.78% (medium-speed modes) and 60.38%

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(high-speed modes) for China V. However, the NOX emissions of the tampered China IV/V

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vehicles were comparable to or even higher than the China III vehicles in this study (see

254

Figure S5).

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Unlike the NOX emissions, the China IV and China V vehicles emitted lower NO2 than

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China III vehicles even under low-speed driving conditions, regardless of whether the SCR

257

systems had been tampered. These results confirmed that the low NO2 emissions are more

258

related to the optimization of the combustion conditions in the engines rather than the SCR

259

systems because less urea was injected under low-speed modes (Carslaw and Rhys-Tyler,

260

2013). In Figure S5, the NO2 emission to fuel consumption shows significant inverse trends

261

with VSP and vehicle speed due to the higher tendency of NO2 formation in the tailpipe at

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lower temperatures which are associated with lower driving speeds and engine loads (Coda

263

Zabetta and Kilpinen, 2001). For example, the average emissions for China III vehicles were 10 / 25

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4.90±1.53, 3.51±0.96 and 2.50±1.00 g NO2 per kg fuel under low-speed, medium-speed and

265

high-speed mode, respectively.

266

Several factors are known to affect NH3 emissions of HDDVs, such as the application of

267

SCR systems (Suarez-Bertoa et al., 2016) and ASC systems (Mendoza-Villafuerte et al.,

268

2017), the quality of DEF and the calibration strategy of urea injections (i.e., injection

269

quantity of DEF under different conditions) (Jeon et al., 2016). In this study, NH3 was emitted

270

mainly under medium-speed and high-speed modes for five SCR-equipped HDDVs (see

271

Figure 3). Figure 4 and Figure S6 presents the real-world NOX and NH3 emission profiles for

272

four typical China IV and China V HDDVs. These results revealed a profound discrepancy in

273

NH3 emissions between in-use HDDVs in China. Peak NH3 concentrations could be more

274

than 65 ppm for the five vehicles (#6, #9, #10, #13 and #18), especially over 300 ppm for the

275

#6 and #10 vehicles, suggesting that the introduction of SCR systems to diesel fleets might

276

risk higher NH3 emissions. To increase the SCR efficiency, higher NH3 emissions usually

277

occurs under certain real-driving conditions (e.g., #6 and #9, see Figure 4(a) and Figure

278

S6(a)). However, #16 vehicle is an example that a good SCR performance not only reduced

279

NOX emissions but also prevented NH3 slip (see Figure 4 (b)). In addition, there are a number

280

of high NOX emitters among the China IV and China V vehicles in China because their SCR

281

systems were tampered with to use less or no DEF (EEO, 2017), although NH3 emissions

282

from these vehicles were not detected (e.g., #15 vehicle, see Figure S6(b)). Thus,

283

anti-tampering management of in-use China IV and China V HDDVs need to be further

284

strengthened to control NOX emissions.

285

3.3 Impact of aftertreatments on NOX and NH3 emissions

286

Figure 5 compares the NOX emission factors under various driving conditions for the two

287

China III HDDVs (#4 and #5) with and without DOC and DPF. Likewise, no NH3 emissions

288

were detected for the #4 and #5 vehicles equipped with DOC and DPF in this study. The

289

average primary NO2 emissions increased substantially, with the average primary NO2 ratios

290

increasing from 9.63% to 20.00%. The increases were more significant under medium- and

291

high-speed conditions. The average primary NO2 ratios for #4 and #5 without DOC and DPF

292

were 8.04%±0.44% under medium-speed conditions and 10.28%±0.52% under high-speed 11 / 25

293

conditions. However, the ratios are as high as 22.56%±9.64% and 29.84%±14.04% with DOC

294

and DPF, respectively, confirming that the use of oxidation catalysts significantly increase

295

primary NO2 emissions (Velders et al., 2011; Vojtíšek-Lom et al., 2018). The overall primary

296

NO2 fraction for HDDVs in China is expected to be lower than that in Europe (Kousoulidou

297

et al., 2008; Velders et al., 2011). This is because not all major cities in China have retrofitted

298

some of the China III HDDVs with DOC and DPF. The wide-scale adoption of DPF will not

299

be deployed until the full implementation of China VI standards (2020 for Heavy-duty diesel

300

vehicles).

301

For the #17 vehicle, comparative experiments between tampered and normal SCR

302

temperature sensors revealed that the temperature data has a significant impact on the

303

performance of SCR systems and thus the emissions of reactive nitrogen compounds. What is

304

striking in Figure 6 is the discrepancy of NH3 emissions under high-speed driving conditions.

305

As previously stated, no NH3 emissions were detected when the SCR inlet temperature sensor

306

was set away from the original position. However, once the temperature sensor was reset

307

normally, the peak NH3 concentration was as high as 85 ppm (see Figure 6(b)). Table S6

308

reports that compared to the tampered condition, the estimated NOX emission factors of the

309

normal condition were reduced by approximately 25.09% under medium-speed driving

310

conditions and 62.21% under high-speed driving conditions. The average NH3 emissions of

311

10.01 mg/kWh for high-speed driving conditions also confirmed the injection of urea in the

312

SCR system. These results suggest that tampering with the SCR systems could affect both

313

NOX and NH3 emissions.

314

4 Conclusions

315

Real-world gaseous emissions of reactive nitrogen compounds were measured for eighteen

316

HDDVs using PEMS and an NH3 analyzer. We applied an operating mode binning method to

317

estimate on-road emission factors under a benchmark operating cycle (i.e., C-WTVC), with a

318

special focus on NO, NO2 and NH3 emissions. The average emission factors were 12.79±3.52,

319

8.25±1.99, 6.05±2.42 and 14.15±2.68 g/kWh, respectively, for China III (N=5), China IV

320

(N=3), China V (N=5) and tampered China IV/V HDDVs (N=5), higher than the emission

321

limits of regulatory engine tests, especially for the tampered China IV/V vehicles. 12 / 25

322

The fractions of primary NO2 of the total NOX emissions were lower than 10% for all

323

HDDVs without oxidation catalysts such as DOC and DPF. In addition, the optimization of

324

the combustion conditions in the diesel engines could be a major driver to reduce primary

325

NO2 emissions for the China IV and China V HDDVs. For example, tampered China IV/V

326

trucks reduced NO2 emissions by 61.7% compared to China III trucks. For NH3 emissions,

327

only five HDDVs were detected, among which the average NH3 concentrations of two

328

HDDVs were higher than the upcoming China VI limit (10 ppm).

329

Furthermore, we explored the impacts of driving conditions and aftertreatment systems on

330

real-world NOX and NH3 emissions. The emission results associated with operating modes

331

revealed that NOX emission factors were higher under low-speed modes. Compared to China

332

III vehicles, the NOX emissions under medium-speed and high-speed modes decreased by

333

19.5% and 51.7% for China IV vehicles, and 60.0% and 72.1% for China V vehicles,

334

respectively. Unlike medium-speed and high-speed modes, there was little distinction in the

335

NOX emissions under lower-speed modes. The adoption of DOC and DPF could increase the

336

average fraction of primary NO2 in NOX emissions from 9.6% to 20%, and the comparative

337

experiments between tampered and normal SCR conditions revealed that SCR tampering

338

could significantly affect NOX and NH3 emissions. For example, when the SCR temperature

339

sensor was set appropriately, high NH3 concentrations were identified under high-speed

340

modes and NOX emissions decreased.

341

For HDDV fleets, this study suggests an urgent need for China’s policy-makers to enhance

342

in-use compliance testing programs and relevant regulations (e.g., PEMS, remote OBD and

343

remote sensing standards) to guarantee a real-world NOX emission reduction benefit in China.

344

Acknowledgements

345

This study was sponsored by the National Key Research and Development Program of China

346

(2017YFC0211100 and 2017YFC0212100); and the National Natural Science Foundation of

347

China (NSFC) (No. 91544222). The contents of this paper are solely the responsibility of the

348

authors and do not necessarily represent the official views of the sponsors.

349

13 / 25

350

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351

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Figures

492 493

Figure 1. The most common example of defeat SCR systems for in-use HDDVs in China. (a)

494

Tampered SCR systems by keeping the SCR inlet temperature sensors approximately 3~5 cm

495

away from the original set position in engine exhaust, (b) Defeat device with a cost of about

496

$2, and (c) Normal condition of the exhaust temperature sensors (i.e., the manufacturer

497

default setting).

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498

499 500

Figure 2. The normalized NOX and NH3 emission factors for the eighteen tested HDDVs. The

501

emission factors of the #4 and #5 vehicles represent the results measured without the

502

aftertreatment devices.

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503 504

Figure 3. Average NOX, NO2 and NH3 emission rates according to operating mode

20 / 25

505

506 507

Figure 4. Examples of instantaneous NOX and NH3 emission profiles and vehicle speed: (a)

508

Lower NOX and higher NH3 emissions under high speed, (b) Lower NOX and NH3 emissions

509

under medium and high speed.

21 / 25

510 511

Figure 5. NOX emission factors under various driving conditions for the two China III

512

HDDVs (#4 and #5) with and without DOC and DPF.

22 / 25

513 514

Figure 6. Instantaneous NOX and NH3 emission profiles and vehicle speed for #17 HDDVs,

515

(a) tampering with the SCR systems by keeping the SCR inlet temperature sensor away from

516

the original set position in the engine exhaust and (b) resetting the SCR inlet temperature

517

sensor (i.e., the manufacturer default setting).

23 / 25

518

Tables

519

Table 1. Summary of vehicle specifications

Vehicle

Vehicle

Production

Odometer

GVW a

Load b

Emission Aftertreatments

No.

type

year

(103 km)

(kg)

#1

Truck

2009

294.4

25000

6.8%

Without

China III

#2

Truck

2009

295.7

25000

6.8%

Without

China III

#3c

Truck

2009

296.8

12490

16.4%

Without

China III

#4c, d

Tractor

2013

530.0

17995

13.4%

DOC+DPF

China III

#5c, d

Tractor

2013

640.7

17995

13.4%

DOC+DPF

China III

#6c

Truck

2015

94.7

15685

12.2%

SCR

China IV

#7e

Tractor

2015

282.2

13950

18.4%

SCR

China IV

#8e

Tractor

2015

290.7

16900

13.7%

SCR

China IV

#9

Truck

2014

339.2

20400

9.9%

SCR

China IV

#10

Truck

2014

542.8

20400

9.9%

SCR

China IV

#11c, e

Truck

2014

684.3

20400

9.9%

SCR

China IV

#12c

Tractor

2018

8.0

25000

9.3%

SCR

China V

#13c

Truck

2018

11.3

31000

5.5%

SCR

China V

#14

Truck

2018

84.4

20490

10.0%

SCR

China V

#15e

Tractor

2017

102.0

25000

9.3%

SCR

China V

#16

Truck

2018

113.3

20490

10.0%

SCR

China V

#17e

Tractor

2017

136.8

25000

9.3%

SCR

China V

#18

Truck

2016

256.6

20490

10.0%

SCR

China V

standard

520

Note: a Gross vehicle weight; b The percentage of actual load mass to the maximum load capability;

521

c

522

tests; d Retrofitted vehicles by installing DOC and DPF; e Tampered SCR systems by keeping the

523

SCR inlet temperature sensors 3~5 cm away from the original set position in the engine exhaust

524

(see Figure 1).

Engine operating data of these vehicles were not collected via an OBD data logger during the

24 / 25

525

Table 2. Summary of average test conditions per vehicle. City Vehicle Speed (km/h) Distance (km) Maximum relative positive acceleration (m/s2) Ambient temperature (℃) Relative humidity (%) Altitude (m)

526

Beijing #1, #2, #3, #6, #9, #10, #11, #13, #14, #16, #18 46 (38-54) a 104 (88-108)

Note: a Mean (Minimum-Maximum).

25 / 25

Shenzhen #4, #5, #7, #8, #12, #15, #17 47 (42-52) 90 (83-98)

2.6 (1.8-3.5)

2.1 (1.4-3.1)

22 (15-33) 29 (13-70) 53 (50-54)

23 (19-29) 45 (25-64) 30 (27-32)

Highlights Tampered trucks emitted twice as much NOX as normal trucks although no NH3 detected. Current selective catalytic reduction may emit NH3 above the China VI limit. Oxidation catalysts increased NO2 by 79%, but little effect on NOX and NH3.