That’s 1,000,000,000,000,000,000,000,000 exhaust emissions particles that are due to be emitted in the United States that don’t need to be. How? A large majority of European and Chinese cars are now sold with tailpipe particle filters, known as gasoline particulate filters (GPFs) or diesel particulate filters (DPFs) in the industry, but this is not the case in the US. Focus on small, ‘ultrafine’ particles in the European regulations has driven gasoline exhaust filter adoption in a way that has not happened in the US, where particulate mass measurement has primacy, and effectively ignores these ultrafines.
A gasoline vehicle with a filter emits around 0.9 x 1011 particles per mile (90 billion), but without a filter it is about 6.3 x 1011 (630 billion), according to testing conducted by Emissions Analytics. With about 300 million internal combustion engine (ICE) vehicles likely to be on the road on average over the next ten years, each driving an average of 10,000 miles per year, that put the total number of ‘unnecessary’ particles emitted to US air to be 1.6 x 1024, or 1.6 septillion particles. If each particle were a dollar bill, the whole of the US could be carpeted over half a mile deep in money.
It is important, however, not to think of these particles as creating visible plumes of black smoke from the tailpipe. That is the clichéd image of exhaust particles, but that applies to older diesel vehicles without a particle filter. For gasoline vehicles, and those with a filter, the concern is for very small particles, down to 10 nm in size, which are invisible, numerous and highly mobile.
These ultrafine particles are of concern for human health, as their small size means that when inhaled they can be drawn deep into the lungs and cross the blood-brain barrier, with potential effects on cognitive development. The direct link to negative health outcomes is not generally considered as proven yet, but Europe has nevertheless regulated due to the weight of evidence and as a precaution. European particle-number-based standards have led to the widespread adoption of GPFs, which are effective at trapping these small particles. This was quickly adopted by China and India.
Even if only particle mass is concerned, on average a gasoline vehicle without a filter emits around 0.070 mg/mile, compared to a vehicle with a filter at 0.027 mg/mile. The unnecessary particle mass to be emitted in the US is therefore forecast to be over 100 tonnes over ten years. Not only does this have the understood impacts on air quality in urban areas, but observations suggest that it eventually migrates to the North and South Poles, settles on the ice, and helps accelerate climate change because of the high ‘black carbon’ content in soot, which is darker and a known global warming agent.
Some would say that this is minor or irrelevant as the light-duty fleet moves to full electrification. However, even on the most aggressive scenarios, new ICE vehicles will be sold in the US for over ten years. Those vehicles are likely to have a lifespan of around twelve years, so these vehicles could be on the road easily for 25 years from today. This does not reflect a lack of ambition in decarbonisation, but a practical reality. In the transition, there is a strong argument that we should do what we can to make ICE vehicles as clean as possible. Emissions Analytics has written extensively before that hybridisation is a better route to quick decarbonations – see a previous newsletter – and, if this position comes to pass, it could mean that hybrid vehicles are sold long after pure ICE vehicles are phased out, and these hybrids emit ultrafine particles in a similar way.
The emissions values described above are the result of programme run by Emissions Analytics in 2021 that tested apparently similar pairs of vehicles between the US (without GPFs) and Europe (with GPFs). The US vehicles were tested in Michigan, and the European vehicles in the UK. Four pairs of gasoline vehicles were tested. First, the three-litre BMW X5 was a 2019 model year in the US and 2020 in Europe. Second, there were two 2020 Jeep Wranglers with a 2.0 litre engine. Third, the full hybrid Toyota RAV4s were 2019 model year with the 2.5 litre engine. Finally, the 2020 1.5 litre Ford engine was tested in the Escape in the US and its equivalent Kuga model in Europe. These vehicles were drawn from different manufacturers and where the models were near-identical in technical specification and model year between the two regions. That said, in each case, the Europe vehicle was equipped with a GPF, while the US vehicle was not.
Each vehicle was then tested on similar on-road routes in the two countries to allow the comparison of their particle mass and number emissions. The gas emissions and particle number were measured using a regulatory-approved portable emissions measurement system (PEMS) from Sensors, Inc, together with a diffusion charger for particle mass measurement. Similar on-road routes were designed in both locations, which, despite the inevitably different traffic and ambient conditions, were shown to have similar dynamics overall. The average emissions across the four pairs of vehicles are shown in the charts below.
The results were highly consistent between the four vehicles. They were also consistent between different types of driving, from urban cold start to highway warm start. The table shows that the average reduction in particle number was 86% across the different vehicles and driving. The variation was from 76% to 96% – in all cases, the reduction was impressive. As the vehicle specifications were near-identical in other respects, it is very likely that the improvement is due to the GPF. While there were some differences in the chemical composition of the standard pump fuel used in both locations, this would not account for a significant proportion of this reduction.
The equivalent overall particle mass reduction was lower but still impressive at 55%, with 81% reduction on the highway. Despite this positive performance, the question remains as to whether the benefit in improved air quality is worth the extra cost. A GPF system typically costs less than $200, which is about the same cost as the optional carpet mats on the BMW X5. Therefore, the cost for removing every billion particles emitted into the air is just 0.0004 cents. In addition to this, there is the further potential benefit of removing particles from the air that have come from other sources. As Emissions Analytics has shown in another newsletter, diesel particulate filters (DPFs) can ‘clean’ the air as the vehicle drives. GPFs are now achieving filtration efficiency rates – typically 80%, with the next generation of GPFs for Euro 7 regulations likely to exceed 95% – that are getting close to those of DPFs, and so the same net-cleaning effect can be expected. Moreover, the filtration efficiency tends to improve with the age of the filter, so the positive effect, if anything, grows.
The septillion particles mentioned above may even underestimate the potential for emissions reduction. The numbers are based on a rapid transition to full battery electric vehicles (BEVs), with no tailpipe emissions. However, this transition may not be as fast as anticipated, which would mean more ICEs for longer. Widespread installation of GPFs would, therefore, generate greater benefits over a longer period. But even as BEVs take off, there is growing evidence that they emit more tire wear emissions that ICE vehicles due to their increased weight. Emissions Analytics’ tests showed a 21% increase in tire particle mass emissions for 500 kg extra vehicle mass – roughly equivalent to the mass of a large battery pack – although this may be partially offset by the effect of regeneration breaking. More generally, there is an on-going trend towards heavier vehicles of all types, which increases tire emissions. Even though only 10-20% of tire wear emissions hang in the air, GPFs could have the added benefit of removing these particles from the air. In other words, ‘legacy’ ICE vehicles could help clean up emissions from BEVs.
Put another way, GPFs bring direct value, but also have an important ‘insurance’ effect that ensures there is benefit however the vehicle car parc evolves.
With filters on gasoline cars in Europe and China, and even coming soon to India, it is surprising that most new car sales in the world's second largest car market are not equipped with the best available technology for emissions reduction. There are signs that this might change, however. Even if US regulators do not embrace particle number as a measure that should be regulated, the potential reduction of the particle mass limit to 1 mg/mile at normal and cold temperatures (20°F or -7°C) might enable cleaner air in the US with widespread deployment of GPFs.
Few things in emissions control are a no-brainer. Mostly, awkward trade-offs between cost, vehicle utility and emissions reduction have to be resolved. In this case, the benefits appear high, costs relatively low, and risk minimal.
Credits: BMW VX images from BMW and www.thecarconnection.com.