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European estimates of nitrogen dioxide (NO2) directly emitted from vehicles may have been overestimated, according to new analysis of public data by scientists at the University of York and National Centre for Atmospheric Science.

Many European Union countries, including the UK, are struggling to comply with the legal limits for roadside levels of toxic nitrogen dioxide, a problem attributed largely to the increasing use of diesel vehicles across the continent.

But according to scientists who have re-examined 130 million hourly measurements from 61 European cities, the future projections of roadside NO2 air quality may be “overly pessimistic.”

traffic roadside emissions picture 700px

The team say the way vehicles are tested could be a factor in the findings as there is currently a “gap in evidence” on emissions as vehicles age.

Dr David Carslaw, from York's Department of Chemistry, said: “Most projections of air quality are based on estimates of the total amount of NOx emitted from vehicle exhaust, but these do not correctly take into account the split between nitrogen oxide (NO) and nitrogen dioxide (NO2).

He said: “The forecasts are likely pessimistic because they overstate the current fraction of emissions that is directly released as NO2. This can have a large impact on NO2 measured roadside, which is where the air quality standards are currently breached in many towns and cities.

“NO2 coming from the tailpipe grew during the 2000s and was projected to stay high up to 2030, but actually around 2010 this trend reversed and the amount of direct NO2 coming out of vehicles is now only around half the value used in policy predictions.”

“The implication is that European cities may become compliant with NO2 targets, probably ahead of schedule,” added Dr Carslaw.

Professor Alastair Lewis, from the National Centre for Atmospheric Science at York, and co-author, said the data had implications for the way we currently test vehicles.

“Only new cars are tested for emissions and that is just for the total amount of NOx. Even the new Europe drive cycle test brought in post-VW doesn’t measure how much of the NOx is NO2 and how much is NO.

“A new car probably gives the peak fraction of exhaust as NO2 you will ever get, but as the car and exhaust system ages direct NO2 potentially reduces over time.

“We need to test cars not just on the day they roll off the production line, but through the whole of their lifespan.”

The study, which is published in the journal Nature Geoscience was supported by a Wild Fund Scholarship at the University of York, the National Centre for Atmospheric Science and the Natural Environment Research Council (NERC).

For copies of the paper, interviews, or further information, contact:

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