Measurement campaigns

We combine results from two measurement campaigns where 2S scooter exhaust was injected through a heated inlet into smog chambers3,24,25 to produce SOA via photochemistry. During the first study, an in-use Euro 1 (E1) and a new Euro 2 (E2a) 2S scooter were run in idle or simulated low power. During the second campaign, emissions from a different Euro 2 2S scooter (E2b) were sampled during ECE47 driving cycles26. Supplementary Table 5 provides specifications of these vehicles. European (exhaust) emission standards are shown in Supplementary Table 1. Average OH concentrations were ~

5 × 106 cm−3. OH concentrations were determined from the decay of a nine times deuterated butanol (butanol-D9, 98% Aldrich) tracer as measured using a quadrupole proton transfer reaction mass spectrometer (idling 2S scooters) or proton transfer reaction time-of-flight mass spectrometer (Ionicon Analytik, driving cycle 2S scooters), see also ref. 27.

Estimation of the NO X regime during experiments

Experiments were under ‘high NO X ’ conditions, which we define as the chemical regime where the main reactions of peroxy radicals (RO 2 ) are with NO rather than other peroxy radicals (self-reaction, or reaction with hydroperoxy radicals). An estimate of the ratio of the RO-forming reactions (RO 2 +NO) versus peroxide-forming reactions(RO 2 +RO 2 , RO 2 +HO 2 ) is possible for experiments conducted on idling scooters at the Paul Scherrer Institute chamber, instrumentation for which includes a NO X monitor equipped with a ‘blue light converter’ (ensuring NO X is truly measured as NO 2 +NO). Figure 5b shows the measured concentrations of NO and O 3 , from an experiment conducted on 22 November 2010. This experiment was typical, with an initial VOC:NO X ratio of around 50 and continuous addition of NO during photochemical aging.

Figure 5: Estimation of NO X regime. (a) Calculated branching ratio between nitrate and peroxide reactions (orange squares) in the smog chamber during aging of emissions from an idling 2S scooter and (b) measured concentrations of ozone (O 3 ) (purple circles) and nitrogen monoxide (NO) (grey circles) as well as calculated peroxy radical (HO 2 +RO 2 ) (blue circles) concentrations. Full size image

The concentration of NO as a function of time t is given by:

Where j NO2 is the photolysis rate of NO 2 in the smog chamber (0.01 s−1) and k 1 (1.8 × 10−14 cm3 molecule−1 s−1) and k 2 (7.7 × 10−14 cm3 molecule−1 s−1) are the reaction rate constants for NO with O 3 and peroxides (CH 3 O 2 ) at 298 K, respectively. Assuming a steady state of NO (only an approximation, Fig. 5 indicates this point is not reached until around 15 h of OH exposure (at OH=106 molecules cm−3)), equation (2) can be written:

Equation (3) suggests NO concentrations at least an order of magnitude higher than RO 2 +HO 2 (for example, 14 times higher at OH=10 × 106 molecules cm−3 h) during the experiment, based on concentrations measured inside the chamber (Fig. 5).

The branching ratio r between the RO 2 /HO 2 reactions with NO versus other with peroxides (Fig. 5a) is determined using

where k 3 is the reaction rate constant between HO 2 and CH 3 O 2 (7.7 × 10−12 cm3 molecule−1 s−1) at 298 K. We assume that the concentration of HO 2 and RO 2 is the same.

Figure 5a illustrates that the NO pathway is dominant, by at least a factor of 20 until OH=10 × 106 molecules cm−3 h, and by initially thousands of times higher, over the peroxide pathway. Since r >>1 we consider these experiments ‘high NO X ’.

An estimation of r during the tests on driving cycle emissions is complicated by the lack of an accurate NO X instrument (that is, one equipped with a blue light converter). Furthermore, NO was only continuously added during the experiment where emissions were sampled from the cold phase (which featured the highest VOC:NO X ratio of any experiment). However, given that the driving cycle tests generally produced higher NO emissions than the idling tests, and given that most fall on the yield curve in Fig. 2, we also assume r>>1, although we can not rule out that during some experiments the conditions change from high to low NO X . Although the VOC:NO X ratios were high (around 50), our best estimate suggests that idling experiments were high NO X throughout. Figure 5b shows the measured concentrations of NO and O 3 , during the experiment.

Idling scooter experiments

Emissions were introduced into the 27 m3 Paul Scherrer Institute Teflon environmental chamber24. The external temperature of the scooter exhaust was monitored (Thermocouple type K, Messelemente) and after an initial warming period of several minutes (consisting of idling or applying low power) the emissions were injected only when the external exhaust temperature was stable at idle or at simulated low power. Supplementary Table 2 provides the operating conditions, smog chamber OA concentrations and aerosol EFs of this study used in Fig. 1.

OA was monitored with a high-resolution time-of-flight aerosol mass spectrometer (Aerodyne). Unity collection efficiency is assumed, since emitted particles likely consist of spherical oil-like droplets with low bounce. After an initial spike in the OA concentration following sample injection, a time of at least 20 min was allowed for equilibration. The concentration of OA after this point was taken as the initial POA emission. A battery of 80 × 100 W ultraviolet black lights (ErgoLine ‘Cleo Performance’, Solarium lights), was used to initiate photo-oxidation and SOA formation. Experiments were carried out with a steady injection of NO (<20 ml min−1) whereby NO was maintained at around 2–3 ppb(v). Relative humidity inside the smog chamber was between 40–60% for all experiments, and temperature was maintained at 25 °C.

OA was corrected for wall losses using

where OA WLC (t) and OA MEAS (t) are the wall-loss corrected and measured organic matter concentrations, respectively, as a function of time t, and k is the first order mass-loss rate constant determined from an exponential fit of BC data.

VOCs inside the smog chamber were quantified with a quadrupole proton transfer reaction mass spectrometer, while carbon monoxide was quantified with a dedicated CO monitor (Aerolaser, CO-Monitor AL5002) and total gas-phase hydrocarbons were measured from the chamber using a flame ionization detector (FID, J.U.M model VE 7). Additional measurements at the tailpipe were performed by transferring emissions through a heated line (191 °C) to a Fourier transformed infrared spectrometer (MKS Multigas analyser 2030) for online measurements (at 1 Hz) of small hydrocarbons, nitrogen containing species (NO, NO 2 , N 2 O, NH 3 and HCN) and other oxygenated small organics (formaldehyde, acetaldehyde), as well as CO and CO 2 .

Online reactive oxygen species measurements

Online particle-bound ROS analysis utilized the fluorescence probe 2,7-dichlorofluorescein in solution. Particles were collected and continuously extracted on a wetted hydrophilic filter. The particle collector samples air at 5 l min−1 and collects particles larger than aerodynamic diameter 50 nm with greater than 95% efficiency. Particles are collected and extracted in an aqueous solution of horseradish peroxidase (0.5 U ml−1) allowing immediate reaction of ROS on collection. The concentration of ROS is characterized following subsequent reaction of the oxidized horseradish peroxidase with 2,7-dichlorofluorescein (5 μM) for 10 min at 40 °C, yielding the fluorescent product DCF in the continuous flow set up. The concentration of 2,7-dichlorofluorescein is measured using fluorescence spectroscopy in a flow-through cell and calibrated to ROS concentration with hydrogen peroxide. ROS data in Fig. 3 are normalized to the total carbon m−3, determined from high-resolution fitting of aerosol mass spectrometer data, and presented as a percentage.

Driving cycle scooter experiments

The Paul Scherrer Institute mobile smog chamber3 was deployed, and experiments conducted in a certified chassis dynamometer test cell (Vehicle Emissions Laboratories, Joint Research Centre of the European Commission, JRC-Ispra, Italy)28,29. Emissions from 2S scooters were sampled at the tailpipe during full ECE47 driving cycles, during Ph1 only of the ECE47 (first four modules of the driving cycle, Ph1), and during Ph2 only of the ECE47 (final four modules of the driving cycle, Ph2). The emissions were transferred to the smog chamber via a heated inlet system (150 °C) and Dekati ejector dilutor. Ultraviolet lights were switched on after several minutes to initiate photochemistry. OA concentrations were measured with a high-resolution time-of-flight aerosol mass spectrometer (Aerodyne), while black carbon was quantified with an aethalometer (AE33, Aerosol d.o.o.). The exponential decay rate of black carbon k was used in equation (5) to correct for particle losses to the walls. Gas-phase compounds were monitored with a proton transfer reaction time-of-flight mass spectrometer (Ionicon), while CO 2 and CO were measured using a cavity ring down spectrometer (Picarro, G2401) and total hydrocarbons were measured with a flame ionization detector (Horiba, THC Monitor APHA-370).

Emission factor determination

EFs from both measurement campaigns (EF, g C kg−1 fuel), (see also Supplementary Table 3), were calculated using a carbon mass balance:

where C denotes carbon mass, and the subscripts CO 2 , CO, HC, carbon dioxide, carbon monoxide and hydrocarbon, respectively. W c is the fuel carbon content (0.847 for gasoline).

For the idling scooter experiments, C CO and C CO2 were measured at the tailpipe using the Fourier transformed infrared spectrometer. C HC was measured from the smog chamber and scaled-up to the tailpipe concentration using the dilution ratio CO tailpipe /CO smog chamber . Meanwhile, for the driving cycles all concentrations were determined at the smog chamber.

Emission factors from the literature

Figure 1 and Supplementary Table 3 show EFs calculated from the literature. When available, EFs are given as reported. OA EFs measured in tunnels/roadside are assumed to consist purely of POA and are converted to EFs in units of g kg−1 fuel using an organic matter to organic carbon ratio (OM:OC) of 1.2 (ref. 30). EFs given in units of g km−1 are converted using the following fuel consumptions (km kg−1): Asia LDVs: 16.43; US LDVs: 14.93; EU LDVs: 18.20; Heavy-duty vehicles: 2.85. EFs measured during the Kansas City vehicle study are estimated by inserting per km EFs into equation (6). SOA in Supplementary Table 3 (g C kg−1 fuel) is converted from units of g kg−1 fuel using an OM:OC ratio of 2.0 (ref. 30). SOA formation from 2S scooters is not available in the literature, but was estimated from emissions of aromatic hydrocarbons using a yield (see Supplementary Note 1 and Fig. 2) of 8.4% (suspended OA concentration 50 μg m−3). Further notes to individual studies are also provided in Supplementary Table 3.