The fumes belched out by road vehicles are deadly – but levels of one harmful component, nitrogen dioxide, may already have peaked in Europe.
“It’s not air quality solved or anything, but it is positive,” says lead author at the University of York, UK.
Nitrogen dioxide (NO2) is a polluting chemical released by petrol and diesel vehicles. Each molecule contains one nitrogen atom and two oxygen atoms. NO2 is blamed for tens of thousands of deaths in Europe each year. A second variant, nitric oxide (NO), contains only one oxygen atom per molecule – but over time it tends to change into NO2. Collectively, the two are known as NOX.
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Grange and his colleagues analysed data from nearly 500 roadside monitoring stations across 61 urban areas in Europe. They focused on how much of the total NOX in the air at the roadside was in the form of NO2. This is important because people’s health is thought to be most affected near roads where concentrations of the gas are high.
Between 1995 and 2010, the proportion of NOX in the form NO2 dramatically grew, probably because of an increase in the number of diesel vehicles – which governments supported because they release smaller quantities of greenhouse gases than petrol ones. However, since 2010, NO2 levels have either stabilised or begun falling. Although the measurements don’t tell us why, it may be partly thanks to the gradual influx of new less-polluting cars. Also, older vehicles generally emit less NO2 as they age.
published in 2014 both predicted that NO2 levels would be higher today than the team observed, and would not yet have peaked. Grange’s results suggest those predictions were too pessimistic.
Some countries – including the UK – are seeing a marked , so things could keep getting better, says Grange.
Dirty air
That doesn’t mean that Europe doesn’t still have a big air pollution problem. The average NO2 concentration in the air at roadsides remains at least 70 micrograms per cubic metre. That is well above the , and of 40 micrograms per cubic metre.
What’s more, a small number of cities bucked the trend: their ratio of NO2/NOX increased. These included Dublin, Rotterdam and Helsinki.
The research team used newly available data to assess emissions on a Europe-wide scale. That is a “powerful” approach, says at the University of Birmingham, UK. “Our own analyses, using a slightly different method, suggest a nitrogen dioxide emission ratio in the UK of around 14 per cent for 2016, very consistent with the results in this paper.”
Roadside monitoring, even at hundreds of sites, may not give the full picture. Nick Molden at in the UK says his firm has measured relatively high proportions of NO2 coming from the tailpipes of new vehicles, which were built to .
However, overall NOx output is falling, and older vehicles won’t put out as much NO2. Grange’s data hints at the impact this is having.
“What you can conclude is that the rise has ceased but no downward trend is conclusively proven,” says Molden.
There are also other forms of air pollution: fine particulate matter, sulphur dioxide and ozone.
Journal reference: Nature Geoscience, DOI:
