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

Jun 12, 2018 - The potential effect of changing to a non-fossil fuel vehicle fleet was investigated by measuring primary emissions (by extractive samp...
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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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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

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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

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

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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

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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

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characterization.

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

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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

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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

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

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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

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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.

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

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

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

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

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reactor).

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

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

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

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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

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

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

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

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

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

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

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

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

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

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

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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

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

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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.

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

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

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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

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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

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(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

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

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

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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

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

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

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

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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

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

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

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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

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

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Md

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

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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

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and CNG buses, but

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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

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

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

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

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

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

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Md

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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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

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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

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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

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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.

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

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

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secondary particle mass.

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304

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

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

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

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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

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

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

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

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

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

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

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

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

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

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(Figure 5).

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

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

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

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

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

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

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

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

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

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

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

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

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

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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

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

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

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

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

342

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

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

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

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were equipped with TWCs.

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Atmospheric Implications

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

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

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

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

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

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

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

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

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

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

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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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360

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

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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

396

(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.

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References:

407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455

1. 2.

3. 4.

5.

6.

7.

8.

9.

10.

11.

12.

13.

Page 24 of 26

Heal, M. R.; Kumar, P.; Harrison, R. M., Particles, air quality, policy and health. Chem. Soc. Rev. 2012, 41, (19), 6606-6630. Cohen, A. J.; Brauer, M.; Burnett, R.; Anderson, H. R.; Frostad, J.; Estep, K.; Balakrishnan, K.; Brunekreef, B.; Dandona, L.; Dandona, R.; Feigin, V.; Freedman, G.; Hubbell, B.; Jobling, A.; Kan, H.; Knibbs, L.; Liu, Y.; Martin, R.; Morawska, L.; Pope, C. A.; Shin, H.; Straif, K.; Shaddick, G.; Thomas, M.; van Dingenen, R.; van Donkelaar, A.; Vos, T.; Murray, C. J. L.; Forouzanfar, M. H., Estimates and 25-year trends of the global burden of disease attributable to ambient air pollution: an analysis of data from the Global Burden of Diseases Study 2015. The Lancet 2017, 389, (10082), 1907-1918. Kumar, P.; Morawska, L.; Birmili, W.; Paasonen, P.; Hu, M.; Kulmala, M.; Harrison, R. M.; Norford, L.; Britter, R., Ultrafine particles in cities. Environment International 2014, 66, 1-10. Kumar, P.; Wiedensohler, A.; Birmili, W.; Quincey, P.; Hallquist, M., Chapter 15 - Ultrafine Particles Pollution and Measurements. In Comprehensive Analytical Chemistry, Miguel de la, G.; Sergio, A., Eds. Elsevier: 2016; Vol. Volume 73, pp 369-390. Gentner, D. R.; Jathar, S. H.; Gordon, T. D.; Bahreini, R.; Day, D. A.; El Haddad, I.; Hayes, P. L.; Pieber, S. M.; Platt, S. M.; de Gouw, J.; Goldstein, A. H.; Harley, R. A.; Jimenez, J. L.; Prévôt, A. S. H.; Robinson, A. L., Review of Urban Secondary Organic Aerosol Formation from Gasoline and Diesel Motor Vehicle Emissions. Environ. Sci. Technol. 2017, 51, (3), 1074-1093. Basagaña, X.; Triguero-Mas, M.; Agis, D.; Pérez, N.; Reche, C.; Alastuey, A.; Querol, X., Effect of public transport strikes on air pollution levels in Barcelona (Spain). Science of The Total Environment 2018, 610-611, 1076-1082. Robinson, A. L.; Donahue, N. M.; Shrivastava, M. K.; Weitkamp, E. A.; Sage, A. M.; Grieshop, A. P.; Lane, T. E.; Pierce, J. R.; Pandis, S. N., Rethinking organic aerosols: Semivolatile emissions and photochemical aging. Science 2007, 315, (5816), 1259-1262. Hallquist, M.; Munthe, J.; Hu, M.; Wang, T.; Chan, C. K.; Gao, J.; Boman, J.; Guo, S.; Hallquist, Å. M.; Mellqvist, J.; Moldanova, J.; Pathak, R. K.; Pettersson, J. B. C.; Pleijel, H.; Simpson, D.; Thynell, M., Photochemical smog in China: scientific challenges and implications for air-quality policies. Natl Sci Rev 2016, 3, (4), 401-403. Hallquist, M.; Wenger, J. C.; Baltensperger, U.; Rudich, Y.; Simpson, D.; Claeys, M.; Dommen, J.; Donahue, N. M.; George, C.; Goldstein, A. H.; Hamilton, J. F.; Herrmann, H.; Hoffmann, T.; Iinuma, Y.; Jang, M.; Jenkin, M. E.; Jimenez, J. L.; Kiendler-Scharr, A.; Maenhaut, W.; McFiggans, G.; Mentel, T. F.; Monod, A.; Prevot, A. S. H.; Seinfeld, J. H.; Surratt, J. D.; Szmigielski, R.; Wildt, J., The formation, properties and impact of secondary organic aerosol: current and emerging issues. Atmos. Chem. Phys. 2009, 9, (14), 5155-5236. Gordon, T. D.; Presto, A. A.; May, A. A.; Nguyen, N. T.; Lipsky, E. M.; Donahue, N. M.; Gutierrez, A.; Zhang, M.; Maddox, C.; Rieger, P.; Chattopadhyay, S.; Maldonado, H.; Maricq, M. M.; Robinson, A. L., Secondary organic aerosol formation exceeds primary particulate matter emissions for light-duty gasoline vehicles. Atmos. Chem. Phys. 2014, 14, (9), 4661-4678. Zhao, Y.; Saleh, R.; Saliba, G.; Presto, A. A.; Gordon, T. D.; Drozd, G. T.; Goldstein, A. H.; Donahue, N. M.; Robinson, A. L., Reducing secondary organic aerosol formation from gasoline vehicle exhaust. Proceedings of the National Academy of Sciences 2017, 114, (27), 6984-6989. Timonen, H.; Karjalainen, P.; Saukko, E.; Saarikoski, S.; Aakko-Saksa, P.; Simonen, P.; Murtonen, T.; Dal Maso, M.; Kuuluvainen, H.; Bloss, M.; Ahlberg, E.; Svenningsson, B.; Pagels, J.; Brune, W. H.; Keskinen, J.; Worsnop, D. R.; Hillamo, R.; Rönkkö, T., Influence of fuel ethanol content on primary emissions and secondary aerosol formation potential for a modern flex-fuel gasoline vehicle. Atmos. Chem. Phys. 2017, 17, (8), 5311-5329. Karjalainen, P.; Timonen, H.; Saukko, E.; Kuuluvainen, H.; Saarikoski, S.; Aakko-Saksa, P.; Murtonen, T.; Bloss, M.; Dal Maso, M.; Simonen, P.; Ahlberg, E.; Svenningsson, B.; Brune, W. H.; Hillamo, R.; Keskinen, J.; Ronkko, T., Time-resolved characterization of primary particle 24 ACS Paragon Plus Environment

Page 25 of 26

456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507

Environmental Science & Technology

14.

15.

16.

17.

18. 19.

20.

21.

22. 23.

24.

25. 26.

27.

28. 29.

30.

emissions and secondary particle formation from a modern gasoline passenger car. Atmos. Chem. Phys. 2016, 16, (13), 8559-8570. Simonen, P.; Saukko, E.; Karjalainen, P.; Timonen, H.; Bloss, M.; Aakko-Saksa, P.; Rönkkö, T.; Keskinen, J.; Dal Maso, M., A new oxidation flow reactor for measuring secondary aerosol formation of rapidly changing emission sources. Atmos. Meas. Tech. 2017, 10, (4), 1519-1537. Goel, A.; Kumar, P., A review of fundamental drivers governing the emissions, dispersion and exposure to vehicle-emitted nanoparticles at signalised traffic intersections. Atmos. Environ. 2014, 97, 316-331. Le Breton, M.; Psichoudaki, M.; Hallquist, M.; Watne, Å. K.; Hallquist, Å. M., Utilization of FIGAERO ToF-CIMS for evaluation of fresh and aged particulate emissions from diesel, CNG and RME fueled buses. Submitted to Aerosol Science and Technology 2018. Hallquist, Å. M.; Jerksjö, M.; Fallgren, H.; Westerlund, J.; Sjödin, Å., Particle and gaseous emissions from individual diesel and CNG buses. Atmos. Chem. Phys. 2013, 13, (10), 53375350. Jonsson, A. M.; Westerlund, J.; Hallquist, M., Size-resolved particle emission factors for individual ships. Geophys. Res. Lett. 2011, 38. L13809, doi: 10.1029/2011GL047672. Hak, C. S.; Hallquist, M.; Ljungstrom, E.; Svane, M.; Pettersson, J. B. C., A new approach to insitu determination of roadside particle emission factors of individual vehicles under conventional driving conditions. Atmos. Environ. 2009, 43, (15), 2481-2488. Liu, D.; Whitehead, J.; Alfarra, M. R.; Reyes-Villegas, E.; Spracklen, Dominick V.; Reddington, Carly L.; Kong, S.; Williams, Paul I.; Ting, Y.-C.; Haslett, S.; Taylor, Jonathan W.; Flynn, Michael J.; Morgan, William T.; McFiggans, G.; Coe, H.; Allan, James D., Black-carbon absorption enhancement in the atmosphere determined by particle mixing state. Nature Geoscience 2017, 10, 184. Lopez-Hilfiker, F. D.; Mohr, C.; Ehn, M.; Rubach, F.; Kleist, E.; Wildt, J.; Mentel, T. F.; Lutz, A.; Hallquist, M.; Worsnop, D.; Thornton, J. A., A novel method for online analysis of gas and particle composition: description and evaluation of a Filter Inlet for Gases and AEROsols (FIGAERO). Atmos. Meas. Tech. 2014, 7, (4), 983-1001. Edwards, R.; Larive, J.-F.; Rickeard, D.; Weindorf, W. Conversion factors and fuel properties; Joint Research Centre ISPRA: ISPRA, 2014. Zhao, Y.; Lambe, A. T.; Saleh, R.; Saliba, G.; Robinson, A. L., Secondary Organic Aerosol Production from Gasoline Vehicle Exhaust: Effects of Engine Technology, Cold Start, and Emission Certification Standard. Environ. Sci. Technol. 2018, 52, (3), 1253-1261. Peng, Z.; Day, D. A.; Ortega, A. M.; Palm, B. B.; Hu, W.; Stark, H.; Li, R.; Tsigaridis, K.; Brune, W. H.; Jimenez, J. L., Non-OH chemistry in oxidation flow reactors for the study of atmospheric chemistry systematically examined by modeling. Atmos. Chem. Phys. 2016, 16, (7), 4283-4305. Kang, E.; Root, M. J.; Toohey, D. W.; Brune, W. H., Introducing the concept of Potential Aerosol Mass (PAM). Atmos. Chem. Phys. 2007, 7, (22), 5727-5744. Montero, L.; Duane, M.; Manfredi, U.; Astorga, C.; Martini, G.; Carriero, M.; Krasenbrink, A.; Larsen, B. R., Hydrocarbon emission fingerprints from contemporary vehicle/engine technologies with conventional and new fuels. Atmos. Environ. 2010, 44, (18), 2167-2175. Karavalakis, G.; Hajbabaei, M.; Jiang, Y.; Yang, J. C.; Johnson, K. C.; Cocker, D. R.; Durbin, T. D., Regulated, greenhouse gas, and particulate emissions from lean-burn and stoichiometric natural gas heavy-duty vehicles on different fuel compositions. Fuel 2016, 175, 146-156. EMEP/EEA air pollutant emission inventory guidebook 2016,; Report No 21/2016; EEA: 2016. Wang, Y.; Liu, H.; Lee, C. F. F., Particulate matter emission characteristics of diesel engines with biodiesel or biodiesel blending: A review. Renewable & Sustainable Energy Reviews 2016, 64, 569-581. Pirjola, L.; Dittrich, A.; Niemi, J. V.; Saarikoski, S.; Timonen, H.; Kuuluvainen, H.; Jarvinen, A.; Kousa, A.; Ronkko, T.; Hillamo, R., Physical and Chemical Characterization of Real-World Particle Number and Mass Emissions from City Buses in Finland. Environ. Sci. Technol. 2016, 50, (1), 294-304. 25 ACS Paragon Plus Environment

Environmental Science & Technology

508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548

31.

32. 33. 34.

35.

36. 37.

38.

39.

40.

41.

42.

43.

44.

Page 26 of 26

Nystrom, R.; Sadiktsis, I.; Ahmed, T. M.; Westerholm, R.; Koegler, J. H.; Blomberg, A.; Sandstrom, T.; Boman, C., Physical and chemical properties of RME biodiesel exhaust particles without engine modifications. Fuel 2016, 186, 261-269. Mayer, A.; Czerwinski, J.; Wyser, M.; Mattrel, P.; Heitzer, A., Impact of RME/Diesel Blends on Particle Formation, Particle Filtration and PAH Emissions. In SAE International: 2005. Srivastava, D. K.; Agarwal, A. K.; Gupta, T., Particulate Characterization of Biodiesel Fuelled Compression Ignition Engine. In The Automotive Research Association of India: 2009. Jayaratne, E. R.; Meyer, N. K.; Ristovski, Z. D.; Morawska, L., Volatile Properties of Particles Emitted by Compressed Natural Gas and Diesel Buses during Steady-State and Transient Driving Modes. Environ. Sci. Technol. 2012, 46, (1), 196-203. Hassaneen, A.; Munack, A.; Ruschel, Y.; Schroeder, O.; Krahl, J., Fuel economy and emission characteristics of Gas-to-Liquid (GTL) and Rapeseed Methyl Ester (RME) as alternative fuels for diesel engines. Fuel 2012, 97, (Supplement C), 125-130. Kittelson, D. B., Engines and nanoparticles: A review. J. Aerosol Sci. 1998, 29, (5-6), 575-588. Tsolakis, A.; Megaritis, A.; Wyszynski, M. L.; Theinnoi, K., Engine performance and emissions of a diesel engine operating on diesel-RME (rapeseed methyl ester) blends with EGR (exhaust gas recirculation). Energy 2007, 32, (11), 2072-2080. Johansson, M.; Yang, J.; Ochoterena, R.; Gjirja, S.; Denbratt, I., NOx and soot emissions trends for RME, SME and PME fuels using engine and spray experiments in combination with simulations. Fuel 2013, 106, (Supplement C), 293-302. Fiebig, M.; Wiartalla, A.; Holderbaum, B.; Kiesow, S., Particulate emissions from diesel engines: correlation between engine technology and emissions. Journal of Occupational Medicine and Toxicology 2014, 9, (1), 6. Emanuelsson, E. U.; Hallquist, M.; Kristensen, K.; Glasius, M.; Bohn, B.; Fuchs, H.; Kammer, B.; Kiendler-Scharr, A.; Nehr, S.; Rubach, F.; Tillmann, R.; Wahner, A.; Wu, H. C.; Mentel, T. F., Formation of anthropogenic secondary organic aerosol (SOA) and its influence on biogenic SOA properties. Atmos. Chem. Phys. 2013, 13, (5), 2837-2855. Tkacik, D. S.; Lambe, A. T.; Jathar, S.; Li, X.; Presto, A. A.; Zhao, Y.; Blake, D.; Meinardi, S.; Jayne, J. T.; Croteau, P. L.; Robinson, A. L., Secondary Organic Aerosol Formation from in-Use Motor Vehicle Emissions Using a Potential Aerosol Mass Reactor. Environ. Sci. Technol. 2014, 48, (19), 11235-11242. Jathar, S. H.; Friedman, B.; Galang, A. A.; Link, M. F.; Brophy, P.; Volckens, J.; Eluri, S.; Farmer, D. K., Linking Load, Fuel, and Emission Controls to Photochemical Production of Secondary Organic Aerosol from a Diesel Engine. Environ. Sci. Technol. 2017, 51, (3), 1377-1386. Weitkamp, E. A.; Sage, A. M.; Pierce, J. R.; Donahue, N. M.; Robinson, A. L., Organic aerosol formation from photochemical oxidation of diesel exhaust in a smog chamber. Environ. Sci. Technol. 2007, 41, (20), 6969-6975. Link, M. F.; Kim, J.; Park, G.; Lee, T.; Park, T.; Babar, Z. B.; Sung, K.; Kim, P.; Kang, S.; Kim, J. S.; Choi, Y.; Son, J.; Lim, H.-J.; Farmer, D. K., Elevated production of NH4NO3 from the photochemical processing of vehicle exhaust: Implications for air quality in the Seoul Metropolitan Region. Atmos. Environ. 2017, 156, 95-101.

549

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