By Dr. Yuhsuan Tsai

Dr Tsai is a Lecturer in Organic Chemistry at the University of Cardiff. In 2018, he undertook a BSA Media Fellowship with BBC Wales, sponsored by the Royal Society of Chemistry. Here, he reports from the BSA Chemistry Section's Presidential Address at the British Science Festival.

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Air pollution is closely associate to our lives. It can have huge impact on human health, affecting many of our organs including lungs, brain, and heart . A recent report from the Royal College of Physicians revealed that, in the UK, around 40 000 deaths a year are attributed to air pollution; and that health-related problems caused by air pollution cost the country over £20 billion a year. So, it is not surprising there are dozens of campaigns for clean air in the UK.

Is air fresher now then? There have been significant drops in all monitored pollutants since the 1970s. However, if we look at the statistics in the last 10 years, we notice a peculiar phenomenon: the level of nitrogen oxides remains largely unchanged, while the level of all other pollutants shows clear drops. This phenomenon intrigued Dr Jacqui Hamilton, a Reader in Atmospheric Chemistry at the University of York. She had expected to see the same trend, because nitrogen oxides and the other monitored pollutants share the same major source: combustion vehicles. 

Jacqui soon realised that although both petrol and diesel engines produce nitrogen oxides, these two types of engines have very different emission profiles otherwise. A petrol engine mainly emits small volatile organic compounds (VOCs) which can be measured easily and are recorded in the emissions inventories by the government and research organisations. On the other hand, a diesel engine emits VOCs that are much larger in size and difficult to measure. To address this problem, Jacqui and her colleagues have developed an instrument that can measure VOCs of all sizes, and the instrument is portable so that they can record roadside emissions as well.

Usually the large VOCs are not monitored in standard emission tests, and there is no record of their levels in the government emissions inventories. Jacqui called these “missing emissions”. So, the reason that all other monitored pollutants, i.e. small VOCs, decreases but nitrogen oxides do not is likely due to the increases number of diesel vehicles, as most pollutants from them, i.e. large VOCs, are not monitored.  

This explanation fits well with the Vehicle Licensing Statistics from the Department for Transport. From 2007 to 2017, the number of petrol vehicles decreased by 15% from 21 to 18 million, while diesel vehicles increased by 85% from 6.7 to 12 million. In addition, during this period, petrol cars have become cleaner due to technology advance but there is little change in pollution produced by diesel cars despite stricter EU regulation. In fact, 9 out of 10 new diesel cars exceed EU pollution limit.

So, what should we do? Jacqui thinks that “it's important that large VOCs are measured in lab studies and included in emissions inventories” so these emissions are no longer missed in the records. This will give a more faithful picture about air pollution. Particularly, these large VOCs are normally more reactive and can form other organic aerosols, which are bad for our health. And if you want to improve the air quality but still can’t give up your private car, switching to an electric model would be the most ideal. Otherwise, hybrid or even petrol would still be better than a state-of-the-art diesel car.