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Environmental Processes

Fresh and Oxidized Emissions from In-Use Transit Buses Running on Diesel, Biodiesel and CNG Ågot Kirsten Watne, Magda Psichoudaki, Evert Ljungstrom, Michael Le Breton, Mattias Hallquist, Martin Jerksjö, Henrik Fallgren, Sara Jutterström, and Åsa Marita Hallquist Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01394 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 15, 2018

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Environmental Science & Technology

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Fresh and Oxidized Emissions from In-Use Transit Buses Running on

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Diesel, Biodiesel and CNG

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Ågot K. Watne1, Magda Psichoudaki1, Evert Ljungström1, Michael Le Breton1, Mattias

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Hallquist1, Martin Jerksjö2, Henrik Fallgren2, Sara Jutterström2, Åsa M. Hallquist2,*

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Gothenburg, SE- 412 96 Gothenburg, Sweden

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Department of Chemistry and Molecular Biology, Atmospheric Science, University of

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IVL Swedish Environmental Research Institute, Box 530 21, SE-400 14 Gothenburg, Sweden Correspondence to: Å. M. Hallquist ([email protected])

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Abstract

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The potential effect of changing to a non-fossil fuel vehicle fleet was investigated by

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measuring primary emissions (by extractive sampling of bus plumes) and secondary mass

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formation, using a Gothenburg Potential Aerosol Mass (Go:PAM) reactor, from 29 in-use

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transit buses. Regarding fresh emissions, diesel (DSL) buses without a diesel particulate filter

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(DPF) emitted the highest median mass of particles, whereas compressed natural gas (CNG)

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buses emitted the lowest (MdEFPM 514 and 11 mg kgfuel , respectively). Rapeseed methyl ester

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(RME) buses showed smaller

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hybrid-electric RME (RMEHEV) buses exhibited the highest particle numbers (MdEFPN 12 × 1014

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# kgfuel ). RMEHEV buses displayed a significant nucleation mode (Dp< 20 nm). EFPN of CNG

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buses spanned the highest to lowest values measured. Low

25

observed for a DPF-equipped DSL bus. Secondary particle formation resulting from exhaust

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ageing was generally important for all the buses (79% showed an average EFPM:AGED/EFPM:FRESH

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ratio >10) and fuel types tested, suggesting an important non-fuel dependent source. The

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results suggest that the potential for forming secondary mass should be considered in future

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fuel shifts, since the environmental impact is different when only considering the primary

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emissions.

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Md

EFPM and particle sizes than DSL buses. DSL (no DPF) and

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Md

EFPN and

Md

EFPM were


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Introduction

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On-road vehicles are a major source of emission of air pollutants in urban areas, and the

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choice of fuel and technology for transportation can have a significant impact on the air

34

quality.1,2 To meet the challenges of increased energy demand and greenhouse gas

35

emissions in connection with transportation, the European Union has set national

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mandatory targets of 10% substitution of conventional fuel (petrol and diesel) by biofuels in

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the road transport sector by 2020. Such shifts in fuel use will eventually change not only the

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numbers and masses of primary emitted particles but also the properties of secondary

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chemical-induced particles formed from atmospheric oxidation of the exhaust. However,

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there is still a lack of information on the primary emissions, especially for ultrafine particles3,

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4

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generation of alternative fuels (e.g., CNG (compressed natural gas) and RME (rapeseed

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methyl ester)).5 Furthermore, public transportation has an increasing responsibility for the

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transition to more sustainable cities.6 Here, the present bus fleet can be a representative of

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modern heavy duty vehicles (HDVs) in the urban environment and offer possibilities for in

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depth analysis of new technologies and fuel types in real traffic.

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In addition to the uncertainties regarding primary emissions when introducing new

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technologies, there is a general lack of understanding of secondary aerosol formation and its

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relationship to regulations.7, 8 A classic example of the secondary formation of particles is the

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oxidation of SO2 to H2SO4 in the atmosphere, which indirectly can be regulated by limiting

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the fuel sulfur content. However, emissions may also contain a large fraction of organic

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compounds that are not explicitly regulated, which in the atmosphere contribute to

, from real traffic and the secondary formation of particles associated with the newest



9

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secondary organic aerosol (SOA) formation.7,

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indirectly influence the SOA potential of the emissions.10, 11

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Oxidation flow reactors (OFRs) offer a new tool for assessing, by inter-comparison,

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secondary chemical-induced aerosol formation from emissions.10, 12, 13 For studies of on-road

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vehicles one may desire to enhance the number of vehicles studied to cover the large

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variability between individuals in a fleet. One limiting factor of using OFRs for that purpose

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could be the time resolution, which can make it hard to resolve emission factors on an

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individual vehicle basis.14

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In the present work, emissions from individual buses representative of an actual rather

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modern traffic fleet were investigated. Measurements were conducted during acceleration

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from standstill, resembling driving in stop and go traffic15 as well as leaving a bus stop.

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Emissions from individual in-use buses running on diesel (DSL), RME (100%) and CNG (i.e.,

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methane, natural and biogas mixture) were thoroughly investigated regarding both primary

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emissions and potential for forming secondary mass from the co-emitted gaseous fraction

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using state-of-the-art instrumentation and a new type of OFR, i.e., the Go:PAM. This paper

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describes the novel set-up of the experiments and focuses on the primary emissions and

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secondary chemical-induced particle formation. Detailed chemical characterization of the

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particle phase is presented separately.16 Overall, this study provides essential information

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for transport sector development regarding effects on air quality and climate, providing a

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scientific platform for policy making.

Obviously, any regulation on NMHC might

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Methods

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The previously developed method for characterizing individual emission sources17-19 with

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respect to primary particle and gaseous emissions was used together with an OFR (Go:PAM)

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and a High Resolution Time-of-Flight Chemical Ionization Mass Spectrometer (HR-ToF-CIMS)

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to enable analysis of the potential for secondary particle formation and chemical

80

characterization.

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In total, 348 plumes from 29 buses were studied: 5 DSL, 11 RME (of which 5 were hybrid-

82

electric vehicles (RMEHEV)) and 13 CNG (technical characteristics are shown in Table S1).

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Sampling of the emissions was conducted according to Hallquist et al.17, i.e., extractive

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sampling of passing bus plumes during full throttle acceleration from standstill. In this

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method, the concentration of a specific constituent was measured relative to the CO2

86

concentration. Prior to the measurements, a warm-up route was driven to prevent cold

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engines. A minimum of three accelerations were made for each individual bus and setting

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(i.e., at least three Fresh and three Aged plumes), but often, more repetitions were

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performed.

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Particles were physically characterized using a high time resolution particle instrument, EEPS

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(Engine Exhaust Particle Sizer, Model 3090 TSI Inc., time resolution 10 Hz) and the

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concentration of CO2 was measured with a non-dispersive infrared gas analyzer (LI-840A,

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time resolution 1 Hz). When calculating particle mass, spherical particles of unit density were

94

assumed. One may note that fresh soot particles may be agglomerates of spherules with

95

densities far from unity that after emission are transformed into more spherical shape.20 No

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correction for this was made. Gaseous compounds CO, NO and THC were measured using a

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remote sensing device (AccuScan RSD 3000, Environmental System Products Inc.). Briefly, 5 ACS Paragon Plus Environment


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this instrument generates and monitors a co-linear beam of IR- and UV-light passed through

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the plume and concentrations are determined relative to the concentration of CO2 (for

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further details, see Hallquist et al.17). For chemical characterization of the gas and condensed

101

phase, a HR-ToF-CIMS equipped with a FIGAERO-inlet was used.21 Details about this

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instrument, the set–up, operating conditions and MS-evaluation procedures used in these

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experiments can be found in Le Breton et al.16

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A schematic of the measurement principle and instruments included in the set-up is given in

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Figure 1.

106 107

Figure 1. Measurement principle and example of emission signals measured for three

108

successive bus passages for fresh emissions during acceleration from standstill: RSD (Remote

109

Sensing Device), EEPS (Engine Exhaust Particle Sizer), HR-ToF-CIMS (High Resolution Time-of-

110

Flight Chemical Ionization Mass Spectrometer), Go:PAM (Gothenburg Potential Aerosol Mass

111

reactor).

112

Emission factors (EFs) of different constituents per kg fuel burnt were determined by relating

113

the concentration change of a specific compound in the diluted exhaust plume to the change

114

in CO2 concentration compared to background concentrations. In the calculations, complete

115

combustion and a carbon content of 86.1, 77.3, and 69.2% for DSL, RME and CNG,

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respectively, were assumed.22 6 ACS Paragon Plus Environment

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To study the potential for forming secondary mass, an OFR was used, i.e., Gothenburg

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Potential Aerosol Mass Reactor (Go:PAM), whereby the extracted emission sample was

119

exposed to high concentrations of OH radicals formed via the photolysis of ozone and

120

subsequent reaction with water vapor, as described in detail in the SI. The general principle

121

of all OFRs is to strongly enhance oxidation initiated by OH reactions and monitor changes in

122

products and aerosol formation between various systems. In our application, the aim was to

123

compare how emissions from different in-use buses contribute to secondary products and

124

aerosol particles when oxidized by OH radicals as an indicator of their atmospheric

125

secondary pollution formation potential. All data was background corrected (lamps on no

126

plume). As recently pointed out by Zhao et al.23 the effective SOA yield could be influenced

127

by background organics if the background particles do not provide enough condensable

128

sinks. However, here we use oxidation of ambient air as the background condition, prior and

129

after a plume, producing a significant condensational sink (typically a surface area > 1.5×1010

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nm2/cm3). It should be noted that the chemistry in an OFR is enhanced and that some

131

reaction pathways (e.g., RO2+RO2) might be overestimated in relation to the real

132

atmosphere. For a comprehensive discussion on the atmospheric relevance of OFRs, see e.g.,

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Peng et al.24 and references therein.

134

In short, the Go:PAM is an OFR designed to be operated in a dynamic mode where potential

135

secondary products, including aerosol particles, can be monitored with high time resolution,

136

i.e., less than 30 s. The application of characterising bus plumes, as in the present study, is

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therefore well suited as a first test of the performance of the Go:PAM. The Go:PAM consists

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of a 100 cm long, 9.6 cm i.d. flow reactor made of quartz glass (Raesh GmbH RQ 200). About

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84 cm of the reactor may be illuminated by either one or two Philips TUV 30 W fluorescent

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tubes, each radiating about 10 W at 254 nm. To reduce the inhomogeneity of the photon 7 ACS Paragon Plus Environment


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field inside the reactor, it is enclosed in a compartment of aluminum mirrors. The extracted

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emission sample is introduced centrally at the top of the reactor, while the reactants, i.e.,

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ozone in particle-free, humidified air, are distributed over the remaining reactor top cross

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section. The gases undergo convective mixing due to differences in velocity before

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approaching a laminar flow profile (Re 7 compared to the highest emitting Euro V bus in

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Hallquist et al. 17 which indicates less complete combustion conditions, hence favoring soot

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formation.36


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In the CNG class, all buses were EEV (Euro V) and two of the buses (ID17&18) stood out with

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respect to the mass and number of particles emitted, emitting more than 16 (EFPM) and 25

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(EFPN) times as much as the average emission of the other CNG buses. Also, higher EFHC and

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EFCO were observed from these individuals (Table 1). Possible reasons for this may be

230

differing functionality of the catalyst, lubrication oil consumption or maintenance.

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Table 1. Average particle and gaseous EFs of individual buses for fresh emissions and

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average EFPM for aged emissions. Given errors are at the statistical 95% confidence interval. Bus ID Fuel 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

234 235 236 237 238 239 240 241 242 243

DSLd DSL DSL DSL DSL e CNG e CNG e CNG CNGe CNGe CNGe CNGe CNGe CNGe e CNG e CNG e CNG e CNG RMEf RMEf RMEf RME RME g RME RMEHEV RMEHEV RMEHEV RMEHEV RMEHEV

EFPM:FRESH (mg

-1 kgfuel )

8.9±0.2 145±70 470±80 560±200 1200±600 0.034 2.0±2.6 3.5±3.0 4.7±2.6 7.2±6.5 12±7.6 10±19 11±12 15±8.6 17±16 30±19 170±110 320±330 18±2 33±19 40±12 64±14 85 170±15 23±7 26±14 34±24 47±7 64±24

EFPN:FRESH

CMDa

EFCOb

EFTHCc

EFNO

EFPM:AGED

-1 kgfuel )

-1 (g kgfuel )

-1 (g kgfuel )

(mg kgfuel )

0.02 n.a. 0.3±0.1 0.6±0.5 0.2±0.1 0.3±0.2 0.2±0.1 0.2±0.1 0.2±0.2 4.2±0.8 0.6±0.3 0.3±0.1 0.4±0.5 0.3±0.3 0.3±0.2 2.6±0.5 4.2±1.6 8.4±1.3 0.4±0.2 0.3±0.1 0.1±0.1 1.1±0.4 9.6±10 2.7±1.3 0.9±0.4 0.9±0.2 0.7±0.5 0.5±0.2 0.6±0.3

0.01 n.a. 7.6±0.9 6.4±1.5 6.8±0.4 5.1±9.3 10±9 13±15 0.01 0.5±13 21±12 18±24 0.01 9.2±7.7 8.5±5.3 0.4±0.7 8.9±2.9 9.9±6.9 21±1 18±2 28±2 39±2 39±46 69±19 17±2 9.2±4.6 20±3 15±2 15±1

1400±600 1400±800 7100±5400 2100±900 3800±1600 1300±600 1800±1000 120±60 560±210 450±520 6300±3300 1800±1900 430±260 610±200 570±420 720±460 1500±500 7700±1700 1400±800 860±360 4900±2300 9600±7800 840 2300±500 260±130 1700±1100 110±120 n.a. 3100±1600

-1 # kgfuel )

(nm)

(g

0.12±0.12 3.0±1.7 12±2 13±4 18±7 0.30 0.096±0.105 0.093±0.065 0.26±0.24 1.2±1.1 0.25±0.10 1.10±1.58 0.28±0.24 1.08±1.30 0.28±0.19 7.5±4.8 33±6 29±8 1.7±0.5 2.4±1.1 2.2±0.5 35±7 2.0 3.3±0.7 12±7.9 11±4.6 12±5.1 34±20 9.3±4.2

24 49 61 61 82 10 12 20 20 18 16 21 27 27 19 15 24 37 32 33 35 14 58 55 15 16 16 15 11,49

13±10 n.a.h 220±10 230±20 330±20 36±30 3.7±4.9 1.7±0.5 1.7±1.1 82±47 3.0±2.0 10±13 38±47 2.0±0.7 18±32 33±41 76±17 130±50 6.1±1.6 8.9±1.9 4.4±3.5 4.2±1.1 60±23 19±11 1.6±0.3 2.7±0.3 2.6±0.8 2.8±1.0 2.7±0.6

14

(10

-1

a

Count median diameter, calculated assuming lognormal distribution For calculating averages, minimum recorded EFs were used for passages giving concentrations below the detection limits of 1.12, 0.020 and 0.012 g kg-1 for CO, HC and NO respectively c Measured as propane equivalents for all buses except CNG (measured as methane equivalents) d DSL, abbreviation for diesel e Methane, natural and biogas mixture f Possibility of some diesel in the RME (unknown fraction) g Mixture of diesel and RME (unknown fraction) h n.a., abbreviation for not available b

244

In the RME class, three of the buses (ID19-21) were similar regarding particle number, mass

245

and size emitted. However, for two of the buses (ID23&24), the EFPM was 2-9 times higher and

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for ID22, the EFPN was 14-20 times higher with a large nucleation mode peaking around 10 14 ACS Paragon Plus Environment

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nm. The high emitting RME buses (ID22-24) also had the highest EFHC (Table 1) and two of

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them (ID23&24) had the highest EFCO, indicating a malfunction that may explain the higher

249

particle emissions. Additionally, it was later revealed that bus ID24 was running on a mixture

250

of RME and diesel (unknown fraction), which may be another reason for the higher mass

251

emitted. This was also supported by the number size distribution being similar to the

252

distribution obtained from the DSL buses, CMD 55 nm (Table 1 & Figure 2 & 3).

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Among the RMEHEV class, the emissions were more homogenous regarding both the particle

254

characteristics (mass, number and size) and emitted gases (CO, HC, NO) compared to the

255

other fuel classes and one reason may be because this class only comprised vehicles of the

256

same model year and technology.

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There were generally larger variations in the gaseous emissions of CO, NO and HC for the

258

investigated CNG and RME buses compared to the other fuel classes (Table 1). A higher

259

Md

260

15 vs. 6.8 g kgfuel , respectively), which is in line with literature data.37, 38 One reason for this

261

may be the higher oxygen content in RME fuel.39 NO emissions from the CNG buses varied

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from below the detection limit to 21 g kgfuel . MdEFHC and MdEFCO were the highest for the DSL

263

and CNG buses, but

264

studies.17

EFNO was observed for the RME and RMEHEV buses than for the DSL buses (MdEFNO 33 and -1

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Md

EFCO for the DSL buses was much higher than observed in earlier

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Aged Emissions

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In order to compare the potential for forming secondary particle mass from exhaust

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emissions between buses using different fuels, engine and aftertreatment technologies, the

269

emissions were oxidized using OH radicals as a reactant. Figure 4 shows the bus average

270

EFPM:FRESH vs. the corresponding EFPM:AGED for the individual bus passages, where the average

271

EFPM:AGED for each bus is indicated by a solid line. For all the buses studied, the mass

272

increased when the emissions were oxidized, and for the majority of the bus individuals

273

(79%), the average EFPM:AGED were a factor of 10 higher compared to the average EFPM:FRESH.

274

Md

275

decreased in the order RME (1830), RMEHEV (990) and CNG (720). However, the median ratio

276

of aged to fresh EFPM was the largest for the RME and CNG buses (52 and 42, respectively),

277

followed by the RMEHEV and DSL buses (30 and 7, respectively).

278

As shown in Figure 4a, there was a significant variation in the secondary mass formed

279

between different passages of the same bus. Primarily, this depended on the emitted

280

compounds and their dilution before being sampled into the Go:PAM. This is illustrated in

281

Figure 4b, where the size of the symbols represents the estimated reactivity with CO and HC

282

for each plume. A larger reactivity reduces the amount of OH radicals in the Go:PAM and

283

thus decreases the amount of OH available for particle precursor oxidation.40

284

To further examine the effects of OH reactivity and other known processes, such as the loss

285

of ozone due to NO titration, an estimated minimum OH-exposure was calculated for each

286

plume using a simplified chemical kinetic model (the full details of the model and related

287

assumptions are found in SI). This value represents the oxidation capacity at the plume peak

288

maximum (Figure 1), and hence provides an upper limit of the attenuation of OH-exposure.

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EFPM:AGED was the highest for the DSL buses (without a DPF) (2940 mg kgfuel ) and then


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For all plumes, the minimum OH-exposure (in the plume maximum) varied between 3.0×1010

290

and 2.2×1012 molecules cm-3 s, whereas the range without a plume was 4.2×1011-2.3×1012

291

molecules cm-3 s. For the RME, RMEHEV and DSL buses, most of the variation in EFPM:AGED

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between different passages of the same individual was due to different OH-exposure (Figure

293

4c), i.e., secondary mass formation generally increased with increasing OH-exposure. For the

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CNG buses, the trend was less clear, suggesting there were larger differences in the chemical

295

composition of the emissions between different passages of the same bus, as supported by

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the data on the primary particle and gaseous emissions from the CNG buses (Table 1). For

297

the majority of passages where there was a small difference between the fresh and aged

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particle mass (points lying close to the 1:1 line in Figure 4), the OH-exposure was low.

299

However, for some of the bus individuals, a similarly low OH-exposure resulted in a much

300

higher secondary mass, most likely due to the emission of compounds more prone to form

301

secondary particle mass.

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304

305


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Figure 4. EFPM:AGED vs. average EFPM:FRESH for all the studied bus passages with respect to fuel

307

type (A) and as a function of OH reactivity in the different fuel classes (B) and OH-exposure

308

(range: 5.0×1010- 1.29×1012 (CNG 2.16×1012) molecules cm-3 s) (C). The dashed lines denote

309

the 1:10 and 1:1 lines and the solid lines in (A) represent bus averages.

310 311

Regarding the secondary mass formed (ΔPM), calculated as EFPM:AGED for a plume subtracted

312

by the average EFPM:FRESH for the specific bus individual, the investigated RME buses showed

313

a generally greater potential for forming secondary particulate mass compared to the other

314

fuel classes (Figure 5). For OH-exposure corresponding to a day or more of atmospheric

315

oxidation, assuming an OH-concentration of 1×106 molecules cm-3 12 h per day, the lowest

316

ΔPM was ~260, ~200, ~4 mg kgfuel and an insignificant increase for the RME, DSL, CNG and

317

RMEHEV buses, respectively. However, notably, among the five bus individuals generating the

318

highest secondary mass (at an OH-exposure of ~4-5×1011 molecules cm-3 s), all the fuel

319

classes were represented. Further, the RMEHEV and CNG buses exhibited the largest

320

variability in the potential for forming secondary mass compared to the RME and DSL buses

321

(Figure 5).

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Figure 5. Secondary mass formed (ΔPM), calculated as EFPM:AGED subtracted by the average

324

EFPM:FRESH, vs. modeled OH-exposure (OHexp) for individual bus plumes. Dashed lines indicate 1

325

and 3 day OH-exposure assuming an OH concentration of 1×106 molecules cm-3 12 h per day.

326

ΔPM values less than 0.1 are shown at 0.1 for illustrative purposes.

327

From the chemical speciation of the aged condensed phase using a CIMS in combination

328

with a FIGAERO-inlet showed that about 11-20% of the aged mass, as measured by the EEPS,

329

for all fuel types was organics,16 which is in line with Tkacik et al.41 Further, a large fraction

330

(80-96%) of the identified mass, using the CIMS-FIGAERO, consisted of compounds detected

331

for all the fuel types, indicating an important non-fuel related source for the secondary

332

particle formation, e.g., lubrication oil and/or fuel additives.16 This may explain the

333

observation that all the fuel classes were represented among the five highest ΔPM buses.

334

The mass not identified by the CIMS was most likely soot, metals and other organic

335

compounds not accounted for by the CIMS. In addition, some of the unexplained mass may

336

have been inorganic ions/salts, e.g., ammonium nitrate. Tkacik et al.41 showed that more 20 ACS Paragon Plus Environment

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than 50% of the aged particle mass was NO3- and about 25% was NH4+. However, other

338

studies have suggested low formation of nitrate.e.g.

339

dominated by gasoline vehicles equipped with three way catalysts (TWCs), which have been

340

suggested to be a possible source of NH3,44 and hence also the high levels of NH4+ and NO3-

341

observed. In contrast, in the other studies,42,

342

investigated, which may explain the lower concentrations of inorganic NO3- measured.

343

However, in our study, all the DSL and RME buses were equipped with an SCR using urea as a

344

reducing agent, which may have been a possible source of NH3.44 In addition, the CNG buses

345

were equipped with TWCs.

346

Atmospheric Implications

347

By 2020, the European Union has decided on national mandatory targets of 10% substitution

348

of petrol and diesel fuels in the road transport sector by biofuels in order to meet the

349

challenges of increased transportation and associated greenhouse gas emissions. In some EU

350

countries, e.g., Sweden, efforts are being made to achieve a fossil fuel free vehicle fleet by

351

2030. In the present study, the renewable fuels RME and methane (if using biogas) were

352

investigated with respect to particle characteristics (PM, PN and size) and gaseous emissions

353

(HC, NO and CO) during real-world dilution. DSL buses without a DPF emitted the highest

354

EFPM, whereas CNG buses emitted the lowest. Compared to RME buses, DSL buses (Euro V,

355

no DPF) emitted almost four times higher particle mass, suggesting a transition from DSL to

356

RME may be beneficial. The highest MdEFPN was observed for the DSL buses, followed closely

357

by the RMEHEV buses with a significant nucleation mode (Dp< 20 nm). This indicates that it is

358

important to carefully evaluate the introduction of hybrids and the impact of their smaller

359

engine sizes. It should be noted that the fuel consumption is reduced for the hybrid-electric

43

42, 43

In Tkacik et al.41 the fleet was

diesel engines without TWCs were



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vehicles, thus reducing the effective emissions. For the CNG buses, there was a large

361

variation in EFPN, including one of the highest and lowest EFPN. Also, NOx emissions varied

362

greatly between different CNG buses and between different plumes for the same bus

363

(median variance of 88 compared to 19, 11 and 2.2 for RME, RMEHEV and DSL respectively).

364

The reasons for this are not fully clear, but the chemical characterization indicated that it

365

could be linked to variation in emissions from lubrication oil and/or fuel additives or related

366

to the catalytic converter. The number of particles from the RME buses was almost four

367

times lower compared to the DSL buses (Euro V, no DPF), again favoring the alternative fuel,

368

while the particle sizes were smaller (MdCMD 34 and 61 nm, respectively). It should be noted

369

that for the DSL bus equipped with a functioning DPF, significantly lower number and mass

370

emissions were observed compared to the Euro V no-DPF RME buses.

371

Once emitted into the atmosphere, the gaseous and particle fractions are subjected to

372

atmospheric processing, i.e., ageing. This process is not included in today’s legislation but

373

may be of importance for climate and health reasons. Our results showed that secondary

374

particle formation following exhaust ageing was generally important for all the buses and

375

fuel types studied. More than 79% of the buses had an aged mass that was 10 times higher

376

than the fresh PM emissions. For a few buses with a lower ratio, it was concluded that the

377

OH-exposure in Go:PAM was small due to co-emitted HC, CO or NO. All fuel types were

378

represented among the buses that generated the highest secondary mass (at similar OH-

379

exposure), indicating an important non-fuel dependent source. Significant secondary mass

380

formation was also observed from a DPF-equipped DSL bus, in contrast to the findings of

381

Gordon et al.,10 who observed no SOA formation from a DPF-equipped diesel HDV engine.

382

Further, the chemical speciation from our measurements showed e.g., C6 oxidation products

383

and organosulfates.16 This discrepancy in results may be because the HDV engine in Gordon 22 ACS Paragon Plus Environment

Page 23 of 26


384

et al.10 was equipped with a diesel oxidation catalyst, which suppresses the emissions of

385

organic species available for further oxidation. Generally, the significant secondary

386

contribution from all types of buses irrespective of fuel or aftertreatment technology for this

387

rather modern bus fleet (mainly Euro-V-SCR or EEV and one retrofitted with a DPF)

388

demonstrates that it is important to consider the potential for forming secondary mass in

389

future legislation and when evaluating the environmental impact of different sources, e.g.,

390

fuel types. Until one has appropriately addressed and abated the issue of secondary aerosol

391

formation there is no combustion related technology or fuel available on the market that

392

could be considered to represent a clean transport option.

393

394

Acknowledgement

395

This work was financed by Vinnova, Sweden’s Innovation Agency (2013-03058) and Formas

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(214-2013-1430). Support from Västtrafik, who is responsible for public transport in all of

397

Västra Götaland, Sweden, is gratefully acknowledged. The drivers and personnel at the

398

measurement sites are also gratefully acknowledged for their assistance and hospitality.

399

400

Supporting Information

401

Tables of technical data of the buses studied and reactions, rate coefficients and

402

concentrations used for the model calculations are given in the Supporting Information,

403

together with more detailed information about the Go:PAM and chemical kinetic model

404

used.

405 23 ACS Paragon Plus Environment


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References:

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