If you can’t measure it,you can’t regulate it…

Once upon a time, measuring vehicle emissions was easy…

 

Once upon a time, measuring vehicle emissions was easy1.  Pollutant chemicals came out of the tailpipe in large volumes, and we knew what they were and how to measure them.  This was bad for the environment, but good for metrology.  Now, relatively, very little comes out of the tailpipe, but does that mean that future vehicle testing is easier or regulation no longer necessary?  Quite the opposite.  Pollution from vehicles now comes from small amounts of substances of concern, often carcinogenic and hard-to-measure, from multiple sources including tyres and cabin materials.  We may loosely describe these as ‘toxic’.

Low-concentration, toxic chemicals from vehicles matter because there are just so many vehicles in operation in the world: approximately 1.4 billion2, which could easily be drivien ten trillion kilometres each year.  If a toxic chemical were only being emitted at a one microgram per kilometre, that would still be ten tonnes released in aggregate per year.  Further, now that tailpipe emissions are generally low, non-exhaust sources now account for a substantial and growing portion of total emissions.  These other sources are harder to measure as they don’t issue from a convenient metal tube.

Of course, tailpipe emissions were never actually that easy to measure, especially when it came to on-road testing, even for the most abundantly emitted gases.  The development of Portable Emissions Measurement Systems (PEMS) in the late 1990s had to overcome many challenges, not least real-time exhaust flow measurement to be able to calculate total mass emissions accurately.  Had this technology been available earlier, many of the ‘defeat device’ scandals may never have happened.

To maintain the valuable contribution of on-road testing to regulation and surveillance, it is therefore necessary to develop new methods for real-world sampling and measurement, rather than retreating into the laboratory again.  So, this sets up the challenge of capturing emissions samples during normal driving and then being able to analyse them with sufficiently high sensitivity to pick up even low concentration toxic compounds.  

This question is particularly timely and relevant with the ongoing discussions around the proposed Euro 7 regulation.  In addition to widening the regulated types of driving, including for dynamics and ambient temperature, the number of pollutants covered will probably grow.  Most likely is the inclusion of a count of particles between 10 and 23 nanometres, together with the explicit regulation of methane and ammonia.  All three can be measured with existing test equipment.  Beyond these, it has been suggested that formaldehyde, acetaldehyde and other volatile organic compounds (VOCs) be measured.  These provide a greater challenge because of their volatility, low concentrations and the difficulty in separating individual species of VOCs from the hundreds, if not thousands, present in tailpipe gas.

The temptation will be to drop these pollutants from Euro 7 due to measurement difficulties, but this would be a mistake.  The proposed method for measuring these is a laser-based, Fourier transform infrared spectroscopy (FTIR) device.  However, there are challenges around the robustness of the kit in real-world driving, the sensitivity at low concentrations and cost.  It would be a mistake to ignore these pollutants, however, because VOCs are a precursor to ground-level ozone formation, a key component in smog – which has potentially worse human health effects when inhaled than the nitrogen dioxide (NO2) that has gained so much focus with Dieselgate.  Further still, as the usage of biofuels, e-fuels and other low-carbon fuels increases, the production of VOCs under combustion may change significantly, with commensurate effects on air quality and health.

Indeed, according to the European Environment Agency, “Concentrations of every air pollutant in Europe have declined since the year 2000, with the exception of ozone”3.  Due to the complex set of chemical reactions in the air, there will become a point beyond which further reductions in NOx may well increase ozone concentrations – as was seen in many cities during lockdown.  As sunlight cannot be eliminated, the only viable course to improve air quality further will be to reduce VOC emissions.  Hence, it would be a mistake to drop these from regulation under Euro 7.

And, it is not necessary to drop them, as alternative measurement technologies are already available.  Under the US Code of Federal Regulations Part 10654, gas chromatography is permitted for similar exhaust gas testing, but the challenge is to adapt the technology for real-world usage.  Emissions Analytics has done this by coupling it with sample collection on short metal tubes containing a VOC ‘sponge’5, known as thermal desorption tubes.  Put this together with a proportional, constant-volume sampling system, and it is possible to measure the mass emissions of compounds at concentrations as low as parts per trillion.  By using a two-dimensional gas chromatography system, it is further possible to separate out the thousands of different VOCs, and identify and measure them separately.

Overall, this approach can measure many more compounds than FTIR and with higher sensitivity.  Further, sample collection is more robust to vibrations during on-road testing.  It is also lower cost overall.
  Once upon a time, measuring vehicle emissions was easy1.  Pollutant chemicals came out of the tailpipe in large volumes, and we knew what they were and how to measure them.  This was bad for the environment, but good for metrology.  Now, relatively, very little comes out of the tailpipe, but does that mean that future vehicle testing is easier or regulation no longer necessary?  Quite the opposite.  Pollution from vehicles now comes from small amounts of substances of concern, often carcinogenic and hard-to-measure, from multiple sources including tyres and cabin materials.  We may loosely describe these as ‘toxic’.

Low-concentration, toxic chemicals from vehicles matter because there are just so many vehicles in operation in the world: approximately 1.4 billion2, which could easily be drivien ten trillion kilometres each year.  If a toxic chemical were only being emitted at a one microgram per kilometre, that would still be ten tonnes released in aggregate per year.  Further, now that tailpipe emissions are generally low, non-exhaust sources now account for a substantial and growing portion of total emissions.  These other sources are harder to measure as they don’t issue from a convenient metal tube.     Of course, tailpipe emissions were never actually that easy to measure, especially when it came to on-road testing, even for the most abundantly emitted gases.  The development of Portable Emissions Measurement Systems (PEMS) in the late 1990s had to overcome many challenges, not least real-time exhaust flow measurement to be able to calculate total mass emissions accurately.  Had this technology been available earlier, many of the ‘defeat device’ scandals may never have happened.

To maintain the valuable contribution of on-road testing to regulation and surveillance, it is therefore necessary to develop new methods for real-world sampling and measurement, rather than retreating into the laboratory again.  So, this sets up the challenge of capturing emissions samples during normal driving and then being able to analyse them with sufficiently high sensitivity to pick up even low concentration toxic compounds.  

This question is particularly timely and relevant with the ongoing discussions around the proposed Euro 7 regulation.  In addition to widening the regulated types of driving, including for dynamics and ambient temperature, the number of pollutants covered will probably grow.  Most likely is the inclusion of a count of particles between 10 and 23 nanometres, together with the explicit regulation of methane and ammonia.  All three can be measured with existing test equipment.  Beyond these, it has been suggested that formaldehyde, acetaldehyde and other volatile organic compounds (VOCs) be measured.  These provide a greater challenge because of their volatility, low concentrations and the difficulty in separating individual species of VOCs from the hundreds, if not thousands, present in tailpipe gas.

The temptation will be to drop these pollutants from Euro 7 due to measurement difficulties, but this would be a mistake.  The proposed method for measuring these is a laser-based, Fourier transform infrared spectroscopy (FTIR) device.  However, there are challenges around the robustness of the kit in real-world driving, the sensitivity at low concentrations and cost.  It would be a mistake to ignore these pollutants, however, because VOCs are a precursor to ground-level ozone formation, a key component in smog – which has potentially worse human health effects when inhaled than the nitrogen dioxide (NO2) that has gained so much focus with Dieselgate.  Further still, as the usage of biofuels, e-fuels and other low-carbon fuels increases, the production of VOCs under combustion may change significantly, with commensurate effects on air quality and health.    
Indeed, according to the European Environment Agency, “Concentrations of every air pollutant in Europe have declined since the year 2000, with the exception of ozone”3.  Due to the complex set of chemical reactions in the air, there will become a point beyond which further reductions in NOx may well increase ozone concentrations – as was seen in many cities during lockdown.  As sunlight cannot be eliminated, the only viable course to improve air quality further will be to reduce VOC emissions.  Hence, it would be a mistake to drop these from regulation under Euro 7.

And, it is not necessary to drop them, as alternative measurement technologies are already available.  Under the US Code of Federal Regulations Part 10654, gas chromatography is permitted for similar exhaust gas testing, but the challenge is to adapt the technology for real-world usage.  Emissions Analytics has done this by coupling it with sample collection on short metal tubes containing a VOC ‘sponge’5, known as thermal desorption tubes.  Put this together with a proportional, constant-volume sampling system, and it is possible to measure the mass emissions of compounds at concentrations as low as parts per trillion.  By using a two-dimensional gas chromatography system, it is further possible to separate out the thousands of different VOCs, and identify and measure them separately.

Overall, this approach can measure many more compounds than FTIR and with higher sensitivity.  Further, sample collection is more robust to vibrations during on-road testing.  It is also lower cost overall.     The compromise is that you do not get real-time measurements.  However, by using automatic geofencing, sampling can switch between different tubes in different areas to achieve separate measurements for urban, rural and motorway driving, for instance, as well as cold start.  Overall, the compromises are small, the advantages great, and the technology is here today to tackle the growing contribution of VOCs to urban air quality.

The importance of geo-spatial separation of emissions can be seen if we look at recent VOC results from Emissions Analytics’ testing of four gasoline vehicles, as shown below.     Due to the effectiveness of the three-way catalytic converter on modern gasoline vehicles, the emissions when the vehicle is fully warmed up – ‘warm start’ – are relatively low.  However, for the first two minutes from switching on a vehicle from cold, there are significantly elevated emissions.  The overall results shown are based on a 93 km test cycle.  On this basis, 98% of total VOC emissions arise from the cold start phase.

In reality, the average journey length in Europe is about 5 km.  Therefore, real-world VOC emissions may be as high as 4.5 mg/km.  There is no relevant official limit to compare this to as the regulated value of total hydrocarbons includes the more abundant methane, not included here as it is primarily a greenhouse gas.

One of the benefits of this gas chromatography approach is that these VOCs can be broken down into individual compounds and groups of species, as shown in the table below.  While the variability in results between vehicles and route type is interesting, it is the fact such measurements, including drill-down to individual compounds within each group, are possible.     The aromatics group includes aldehydes (including acetaldehyde) and ketones, which are sometimes carcinogenic.  The polycyclic aromatic hydrocarbons (PAHs) and nitrogen-containing compounds are more virulent carcinogens.  The alkane group also includes alkenes, alkynes and cycloalkanes – these are more commonly associated with diseases of the lungs, liver, kidneys and brain.  All the results are from the warm start phase, hence the unit being nanograms, or one billionth of a gram.  This demonstrates the ability of this technique to measure successfully down to low concentrations.

Beyond the tailpipe, VOCs are also being understood as an increasing risk to human health and the environment in the vehicle cabin and via tyre wear emissions.  The interior materials of vehicles such as plastics, leathers, carpets and glues – especially when new – create a potentially toxic soup in the cabin, particularly on hot days.  Manufacturers are increasingly aiming to specify lower-emitting materials for this reason.  Tyre wear emissions, which are in total between one and three orders of magnitude greater than the particle mass from a modern exhaust pipe, contain a large number of toxic VOCs, including PAHs.  Very similar compound sampling, separation, identification and quantification can be used for these as with the tailpipe, using sample tubes, gas chromatography and mass spectrometry.  Complicated though it sounds, it is an economic and well-established technique, available today.  Furthermore, equipment and testing service providers need as much as clarity as possible, as soon as possible, as to the direction Euro 7 will take, otherwise we might find regulatory ambition with no viable equipment to deliver it.

Standing back, perhaps we are seeing automotive emissions regulation become less of a special case.  Rather than high concentrations of a small number of easy-to-measure pollutants, such as nitrogen oxides, the impact of vehicles is becoming more about low concentrations of more toxic compounds.  As with industrial emissions and contaminants in consumer products, it may be more about volatile organic emissions from polymers, elastomers and combustion products.  And these may prove the key to reducing the direct toxic effects on humans, animals and the biosphere, as well as the indirect effect on ozone, smog and secondary particulate formulation.  Let’s grab the opportunity of Euro 7 to tackle this in what could be the most ambitious automotive regulation in the world.     Footnotes: It wasn’t, but please read on… Wards Intelligence, cited at: https://www.carsguide.com.au/car-advice/how-many-cars-are-there-in-the-world-70629 https://www.eea.europa.eu/publications/soer-2020/chapter-08_soer2020-air-pollution https://www.ecfr.gov/current/title-40/chapter-I/subchapter-U/part-1065#1065.15 More strictly, it is a special adsorbent material designed to trap VOCs.

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