Despite being high emitters of primary PM, regulations for scooters are generally less stringent than for other vehicles, for example, in Europe having reached Euro 5/V (a fifth tranche of regulations), for passenger cars and trucks, versus only Euro 2 for scooters… Accordingly, a scientific report to the European Commission suggests that scooters will emit more volatile organic compounds (VOCs) than all other vehicles combined in Europe by 2020. Furthermore, high PM levels and toxic aromatic hydrocarbons, important SOA precursors, have been observed in many cities, especially in Asia. Globally, organic aerosol (OA) dominates PM, with SOA accounting for the largest fraction.

Here we show that 2S scooters emit significant amounts of primary organic aerosol (POA), aromatic VOCs and also produce significant SOA [secondary organic aerosol]. … Chemical analysis of the emissions shows that SOA is mainly produced via photo-oxidation of aromatic VOCs, present in gasoline, from the exhaust. This shows that the known issue of incomplete fuel combustion during the 2S cycle is also responsible for SOA formation. Finally, we present the first online measurements of aged exhaust showing that SOA formation also produces reactive oxygen species (ROS) with potentially detrimental effects on our lungs.

—Platt et al.

For the study, the team combined results from two measurement campaigns where 2S scooter exhaust was injected through a heated inlet into smog chambers 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 cycles.

They also used ambient data are from roadside/tunnel measurements in the US, EU and Asia for light-duty and heavy-duty vehicles, and data from Indian in-use 2S autorickshaws for comparison to the European scooters of this study.

Emission factors (EFs) from scooters and other vehicles. EFs plotted as box-and-whiskers (median line, red; 25th and 75th percentile, box; 10th and 90th percentile, whiskers) of (a) POA, (b) aged OA (POA + SOA formation), (c) benzene and (d) light aromatics (benzene, toluene and C2–C4 alkylated benzenes). Points shown next to the box-and-whiskers are the individual data points, colored depending on measurement region for ambient data. 2S scooters were run in idle or during driving cycles (ECE47). Data on the other vehicles shown are from the literature for light-duty and heavy-duty vehicles (LDVs and HDVs). LDVs data are further divided between vehicles meeting Euro 5 and those not meeting Euro 5, labelled <Euro 5 in parenthesis. Ambient data are split according to a contribution of HDVs to the data of higher than or lower than 50%. Many of the higher ambient values are from older vehicle studies. Platt et al. Click to enlarge.

They found that the oxidation of VOCs in 2S scooter emissions produces significant SOA (g carbon (C) kg^-1 fuel), with total OA on average 2.9 and 2.4 times higher than POA after aging for idling and driving 2S scooters, respectively. In addition, substantial toxic aromatic emissions (up to ~40% of emitted VOC volume for the scooters of this study) of benzene, toluene and C₂–C₄ alkylated benzenes, which are recognized SOA precursors, were present in the exhaust.

Among the aromatics, benzene is of particular concern due to its carcinogenicity, they noted. Levels in the raw 2S scooter exhaust were as high as 300,000 mg m^-3 or 146 ppm (v) from idling. The EU annual mean limit for the protection of human health is 5 mg m^-3 (ref. 13), while the US National Institute for Occupational Safety and Health (NIOSH) recommends that workers wear special breathing equipment when exposed to benzene at levels exceeding 1 ppm for 15 min. Waiting in traffic behind an idling 2S scooter may therefore be highly deleterious to health, the authors said.

In general, ambient EFs [emission factors] from Asian vehicles are in the same range as European and US vehicles, while emissions from in-use 2S rickshaws are slightly higher than from the European scooters of this study. POA emissions from 2S scooters are on average around 20 (maximum 2,780) times higher than ambient (light-duty dominant) values, and aged OA emissions on an average 53–771 times higher than laboratory studies on other vehicle types.

It should be noted that absolute aerosol concentrations can influence EFs: higher measurement concentrations would lead to higher EFs. SOA formation is most significant from idling scooter emissions, while smaller at higher engine loads. However, POA emissions are higher under the latter conditions, and the aggregate POA + SOA emission at high load is comparable with that from idling.

—Platt et al.

The team suggested several reasons for these relatively large OA and aromatic emissions from 2S scooters.:

1. 2S engines, unlike four-stroke (4S), require addition of lubricant oils to the fuel, some of which is emitted in the exhaust.

2. During the 2S engine cycle some of the fresh fuel/air mixture passes directly through the engine, increasing VOC emissions, which may explain the high SOA formation.

3. Scooters generally utilize rich combustion (low-air/fuel ratio), improving drivability while producing higher CO, VOC and PM emissions (but lower NO_x). Accordingly, the VOC emissions measured—in particular aromatics as found in raw gasoline—are also on average 124 and 11 times higher from idling and driving 2S scooters, respectively, compared with those from other vehicles.

4. Scooter after-treatment systems are inherently inefficient due to their relatively small size and longer light-off times.

Despite the difficulty of making a precise estimation of a relative contribution to vehicular PM and aromatics from 2S scooters, the researchers suggested that many 2S scooters likely fall into the super-polluting category, especially as a considerable number of scooters are in operation in some regions without any form of emissions control. In contrast, the scooters presented in the study all were equipped with two-way oxidation catalysts, which reduce emissions of carbon monoxide and VOCs). Further, real-world emissions may be further exacerbated by poor maintenance and tampering, rife for scooters.

Using the average 2S scooter EF (ECE47 driving cycle) … suggests that 2S scooters contribute to around 60% of roadside POA in Bangkok, where they account for 10% of fuel consumption. In a more extreme case (comparing the 75^th percentile for scooters and 25^th percentile for ambient light-duty dominated), 2S scooters would contribute over 96% to roadside POA. Note that these values are based on the European scooters of this study. …Emissions from some in-use Asian 2S vehicles may be higher, by a factor of three.

… Our data suggest that 2S scooters are a significant, and in many cities the largest, source of vehicular PM and toxic SOA and aromatic hydrocarbons, despite being a relatively small fraction of the total fleet. Therefore, given the alternative technologies available, restrictions on 2S scooters, already implemented in China, could improve air quality in many cities around the globe.

—Platt et al.

Resources

• S.M. Platt et al. (2014) “Two-stroke scooters are a dominant source of air pollution in many cities,” Nature Communications 5, Article number: 3749 doi: 10.1038/ncomms4749