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U Birmingham study finds DPFs have limited impact on ultrafine particle emissions

A study by researchers at the University of Birmingham in the UK has found that while Diesel Oxidation Catalysts (DOCs) and Diesel Particle Filters (DPFs) has been very effective in controlling the tailpipe emissions of solid particles, there has been little change in the liquid mode (<30 nm) particles, and that concentrations of ultrafine particles (<100 nm) still well exceed the threshold for “high” concentrations (>104 cm−3/24-hour mean) defined by WHO.

An open-access paper on their work is published in the journal Environment International

While the World Health Organization has not yet set a guideline for safe levels of ultrafine particles, it recognizes that particulate pollution overall is associated with negative impacts on cardiovascular and respiratory health. Air quality guidelines published by WHO in 2021 also outline concerns over ultrafine particles and their ability to be transported around the body.

High concentrations of ultrafine particles are likely to be a widespread and persistent phenomenon. In order to meet WHO guidelines we are likely to need a much higher uptake of electric vehicles, as well as additional measures to reduce emissions from diesel vehicles.

—Professor Roy Harrison, corresponding author

The team used data collected in from a monitoring station in Marylebone Road, in London. Air quality sampling at this site has produced the most comprehensive, long-term dataset in the world, containing data for particle mass and number dating back to 2010.

The data showed a steep decline in larger particles. Black carbon, for example, declined by 81% between 2014 and 2021. This is a clear indication that there has been a positive impact from the introduction of exhaust filters.

In contrast, however, the number of ultrafine particles reduced by only 26%. The smallest group of particles, measuring less than 30 nanometers, did not reduce at all, giving a clear indication that filters are not effective against these types of particle.

WHO guidelines define concentrations of ultrafine particles above 10,000 per cubic cm as “high”; concentrations measured at the Marylebone Road site were around twice this level.


  • Seny Damayanti, Roy M. Harrison, Francis Pope, David C.S. Beddows (2023) “Limited impact of diesel particle filters on road traffic emissions of ultrafine particles,” Environment International doi: 10.1016/j.envint.2023.107888



I think this is an important study which potentially swings the balance for hybrid versus BEV vehicles.

I have previously been critical of the failure to take sufficient account of factors like non exhaust emissions in evaluating BEVs, where weight is important.

But the main thing IMO is not to get tied into a particular path, so that new data is downrated and effectively ignored.

These ultra fine particles may be significant, so as a long term critic of what in my view has been a somewhat one-eyed advocacy of BEVs, it behoves me to not simply ignore or attempt to downgrade this information.

We should try to be as objective as possible, and open to new insight, not marry a position, for better or worse until death etc!


This study shouldn't be dismissed out-of-hand, but after reading through the full paper, it doesn't appear that the case that nanoparticles come mostly from diesel vehicles even with DPF is well supported.

There are many previous studies which measured nanoparticles down to 4 nm to be very low with DPF.

Also, the assumption in this paper that tire and brake wear produce only "small quantities" of nanoparticles is in conflict with Emissions Analytics' measurements of nanoparticles (6 nm - 23 nm) from tires as being >100 times greater than those measured from the exhaust of diesel with DPF.


Thanks for the insight, Carl.

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