Oxygenated VOCs as significant but varied contributors 1 to VOC emissions from vehicles 2

: 23 Vehicular emission is an important source for volatile organic compounds (VOCs) in 24 urban and downwind regions. In this study, we conducted a chassis dynamometer study 25 to investigate VOC emissions from vehicles using gasoline, diesel, and liquefied 26 petroleum gas (LPG) as fuel. Time-resolved VOC emissions from vehicles are 27 chemically characterized by a proton-transfer-reaction time-of-flight mass 28 spectrometry (PTR-ToF-MS) with high frequency. Our results show that emission 29 factors of VOCs generally decrease with the improvement of emission standard for 30 gasoline vehicles, whereas variations of emission factors for diesel vehicles with 31 emission standards are more diverse. Mass spectra analysis of PTR-ToF-MS suggest 32 that cold start significantly influence VOCs emission of gasoline vehicles, while the 33 influences are less important for diesel vehicles. Large differences of VOC emissions 34 between gasoline and diesel vehicles are observed with emission factors of most VOC 35 species from diesel vehicles were higher than gasoline vehicles, especially for most 36 oxygenated volatile organic compounds (OVOCs) and heavier aromatics. These results 37 indicate quantification of heavier species by PTR-ToF-MS may be important in 38 characterization of vehicular exhausts. Our results suggest that VOC pairs (e.g. C 14 39 aromatics/toluene ratio) could potentially provide good indicators for distinguishing 40 emissions from gasoline and diesel vehicles. The fractions of OVOCs in total VOC 41 emissions are determined by combining measurements of hydrocarbons from canisters 42 and online observations of PTR-ToF-MS. We show that OVOCs contribute 9.4% ± 5.6% 43 of gasoline vehicles of the total VOC emissions, while the fractions are significantly 44 higher for diesel vehicles (52-71%), highlighting the importance to detect these OVOC 45 species in diesel emissions. Our study demonstrated that the large number of OVOC 46 species measured by PTR-ToF-MS are important in characterization of VOC emissions 47 from vehicles. dataset, we provide emission of many VOCs from these three different types of vehicles associated with various emission in China. Our results show that emission factors of VOCs generally decrease with the increased stringency of emission standards for gasoline vehicles, whereas variations of emission factors for diesel vehicles with emission standards are more diverse. Mass spectra analysis of PTR-ToF-MS suggest that cold


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Volatile organic compounds (VOCs) are important trace components in the 51 troposphere, as important precursors of ground-level ozone (Shao et al., 2009) and 52 secondary organic aerosol (SOA) (Seinfeld and Pandis, 2006;Kansal, 2009;Ziemann 53 and Atkinson, 2012). As the result, it is particularly important to identify emission 54 sources of VOCs in the atmosphere. Vehicular emission is an important source of VOCs 55 in cities around the world (Liu et al., 2008;Parrish et al., 2009), contributing 56 approximately 25% to total VOC emissions in China (Ou et al., 2015;Wu et al., 57 2016; Sun et al., 2018). In order to control atmospheric pollution in urban and 58 surrounding regions, it is necessary to understand source profiles and emission 59 characteristics of VOCs from vehicles. 60 Emissions of VOCs from vehicles have been investigated extensively from 61 tunnel studies (Cui et al., 2018;Zhang et al., 2018;Song et al., 2020), on-road mobile 62 measurements (Li et al., 2017), and chassis dynamometer tests (Guo et al., 2011;Wang 63 et al., 2013;Yang et al., 2018). Previous studies demonstrated that fuel types of vehicles 64 strongly impact VOC emissions. Aromatics along with other hydrocarbons are known 65 as compounds with high emissions in exhausts of gasoline vehicles (Wang et al.,66 2013; Ly et al., 2020). Some carbonyl compounds contribute significantly to emissions 67 of diesel vehicles, at fractions much higher than gasoline vehicles (Tsai et al., 2012;Qiao 68 et al., 2012;Yao et al., 2015;Mo et al., 2016). Moreover, there are still a large number 69 of unidentifiable compounds in diesel vehicles (May et al., 2014). Furthermore, VOC 70 emissions from vehicles significantly decreased in China due to stricter emission 71 standards (Liu et al., 2017;Sha et al., 2021). In order to reduce emissions of most 72 primary pollutants, more stringent emission standards and after-treatment devices have China VI emission standard (see details in the Supplement) (Wu et al., 2017). With the 77 continuous development of engine and exhaust after-treatment technologies, emission 78 4 characteristics of VOCs from vehicles may change and need to be frequently updated. 79 Oxygenated volatile organic compounds (OVOCs) were found to be an important 80 class of compounds in vehicle exhausts, accounting for more than 50% of the total VOC 81 emissions for diesel vehicles from both chassis dynamometer tests (Schauer et al.,82 1999; Mo et al., 2016) and on-road mobile measurements (Yao et al., 2015). 83 Traditionally, VOCs are collected in the canister or Tedlar bags, and then analyzed by 84 gas chromatography-mass spectrometer/flame ionization detector (GC-MS/FID), 85 mainly reporting emissions of hydrocarbons (Wang et al., 2017;Qi et al., 2019). 86 Previous work usually collected 2,4-dinitrophenyhydrazine (DNPH) cartridges and 87 analyzed using high-performance liquid chromatography (HPLC) for carbonyls 88 (aldehydes and ketones), which are both time-consuming and prone to contaminations 89 (Mo et al., 2016;Han et al., 2019).  Yuan et al., 2017). More OVOC species could be quantified from the measured 102 mass spectra based on parameterization methods for sensitivity of instrument 103 (Sekimoto et al., 2017;Wu et al., 2020).

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In this study, we applied a PTR-ToF-MS along with a suite of other instruments 105 to measure VOCs emitted from gasoline, diesel, and liquefied petroleum gas (LPG) 106 vehicles. We investigated emission factors from different fuel types and emission 107 standards for representative VOC species exhausted from these vehicles. We used the  Table S2 and Table S3.

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The short transient driving cycle (GB 18285-2018, Figure S1a), as one of the 135 widely used test methods for vehicle emissions in China (Li et al., 2012;Wang et al., 136 2013), was used for measurements of gasoline vehicles and LDDT, each running for 137 6 three to five times. The short transient driving cycle methods were initially adapted 138 based on emission regulations of the Economic Commission for Europe (ECE) cycle 139 (Yao et al., 2003), which is developed and used in European countries (Laurikko, 1995).

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The short transient driving cycle consist of four conditions, namely idling, acceleration, 141 deceleration and uniform speed, as shown in Fig. S1. For the MDDT and HDDT, we 142 customized a step-by-step test method, in which the vehicle accelerates to 20 km·h -1 , 143 40 km·h -1 and 60 km·h -1 in sequence after the engine activates, keeping at 20 km·h -1 144 and 40 km·h -1 for 2 minutes, and 60 km·h -1 for 1 minute, respectively ( Fig. S1) (Li et 145 al., 2021;Liu et al., 2021;Liao et al., 2021). In addition, the cold start was tested for a 146 number of vehicles after a cold soak for more than 12 hours at ambient temperature 147 (20-25 ℃) before engine started. The measurements of cold start are compared to 148 measurements of hot start after a ~10 minutes break for the vehicles after previous 149 measurement. More details about cold start and hot start in this campaign can be found 150 in Li et al. (2021).

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A custom-built sampling and dilution system for vehicles combining online and 152 offline sampling techniques was used in this study. As shown in Fig. S2, a portable 153 emission measurement system (PEMS, SEMTECH-DS, Sensors. USA) was employed 154 to measure emissions of CO, CO2, NOX, and total hydrocarbon (THC) directly from the 155 tailpipe of vehicles. A custom-built dilution system (Li et al., 2021;Liao et al., 2021) 156 was used for dilution of vehicular emissions, achieving dilution ratios of 10-100 for 157 different vehicles. After dilution, CO2 and CO were measured using a Li-840A 158 CO2/H2O Gas Analyzer (Licor, Inc. USA) and a Thermo 48i-TLE analyzer (Thermo 159 Fisher Scientific Inc. USA), respectively. Measurements of CO2 before and after the  H3O + chemistry was used to measure VOCs (Sulzer et al., 2014). The mass spectra of 166 7 PTR-ToF-MS was recorded every 1 s as to capture characteristics of VOC species from 167 vehicle exhausts in real-time. Background measurements of the instrument were 168 performed using sampled air through a custom-built platinum catalytical converter 169 heated to 365 °C for 30 s before vehicle starts in each test. The more detailed setting 170 parameters for the instrument can be found elsewhere (Wu et al., 2020;Wang et al., 171 2020a; He et al., 2022). Data analysis of PTR-ToF-MS was performed using the Tofware 172 software package (version 3.0.3, Tofwerk AG, Switzerland) (Stark et al., 2015). Innsbruck, Austria) was used to calibrate a total of 11 organic acids and nitrogen-180 containing species (Table S4). The limits of detection for calibrated VOC species are 181 below 100 ppt for the 1-s measurement, except for ethanol (423 ppt) and formic acid 182 (166 ppt). Additionally, the humidity dependence for a few VOC species in PTR-ToF-183 MS (Yuan et al., 2017;Koss et al., 2018) were corrected using humidity-dependence 184 curves determined in the laboratory, as previously shown in Wu et al. (2020). To

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Whole air samples were collected using canisters after the dilution system for  (Table S5). We compared emission factors from PTR-  An instrument based on Hantzsch reaction-absorption method was used to 203 measure formaldehyde . Good agreement for formaldehyde between 204 PTR-ToF-MS and the Hantzsch instrument was obtained (Fig. S6a). An iodide-adduct Research, Inc.) (Wang et al., 2020c;Ye et al., 2021) was used to measure organic acids, 207 hydrogen cyanide (HCN), and isocyanic acid (HNCO) from vehicles (Li et al., 2021).    temperature of the after-treatment device (Gentner et al., 2017;George et al., 2015). In 253 10 contrast to the gasoline vehicle, we observe higher concentrations of the two OVOC 254 species than the two aromatics species from the diesel vehicle. These higher OVOC 255 concentrations in diesel vehicle exhausts are in line with the observations of organic 256 acids using the I-ToF-CIMS from the same campaign (Li et al., 2021).  representative VOC species. This is consistent with the results in previous studies with 264 lower emissions for newer emission standards (Wang et al., 2017;Sha et al., 2021). In for acetaldehyde and 0.8 to 10.0 mg·km -1 for acetone) than almost all gasoline vehicles 285 (a maximum of 3.9 mg·km -1 for acetaldehyde and a maximum of 3.2 mg·km -1 for 286 acetone). Higher emission factors from diesel vehicles are also observed for many other 287 common OVOC species, as shown in Fig. 4. As the largest OVOCs emitted from 288 gasoline vehicles (4.6 ± 5.1 mg·km -1 ), methanol is found to be the only common OVOC  in previous studies (Gentner et al., 2012;Erickson et al., 2014).   The scatterplot shown in Fig. 8 can also be expressed in terms of the determined 402 fuel-based emission factors between gasoline and diesel vehicles (Fig. S10a).

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Generally, similar variability is obtained except the determined slope of the data points, 404 with higher slopes determined from the scatterplot based on fuel-based emission factor 405 (0.19 versus 0.15). The emission ratios to CO between gasoline and diesel vehicles 406 (Fig. S10b) show similar results. Furthermore, the difference between the slopes 407 reflects the different average mileage for the same weight of fuel between gasoline 408 (9.7 km·kgfuel -1 ) and diesel vehicles (7.1 km·kgfuel -1 ), as demonstrated for emission 409 factors of CO2 in Table S6.

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Comparing gasoline and diesel vehicles, we can also observe profound  cold/hot start, except somewhat higher fractions for China VI from hot start (Fig. S11).

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The OVOC fractions obtained in this study for gasoline vehicles are generally OVOCs measured by PTR-ToF-MS, the OVOC fractions determined in this study are 500 more comparable with previous studies (Fig. 12), since most previous studies only 501 detected carbonyls among various types of OVOCs. Finally, we determine that OVOCs 502 account for 41% ± 10% of total VOC emissions for LPG vehicles, which is also higher 503 than in one previous study (Wang et al., 2020b) with only carbonyls and a few

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In this work, we conducted a chassis dynamometer study to measure VOC