Targets for reducing illegal levels of NO2 pollution from vehicles will be achieved more quickly than the government expects, a study says.
Researchers say government projections of future NO2 are too pessimistic, because they ignore the latest real world data.
They say cities may achieve roadside emissions standards several years earlier than ministers expect.
The research is published in the journal Nature Geoscience.
The authors at the University of York warn that despite the adjustment, the government is still lagging badly on NO2 targets.
And they say the air in the UK's big cities remains dangerously polluted from many different compounds.
But how did the government miscalculate NO2?
The authors say it's because they relied on projections of future emissions, rather than on real world roadside emissions data at the European Environment Agency (EEA).
The government's figures are based on tailpipe readings from vehicles with new particle filters. They projected these figures forward to estimate future pollution levels.
But that projection doesn't take into account that as a filter ages it fortuitously creates less NO2.
It seems that the government has been either unwilling or unable to update its estimates by harnessing the real world road site data collected in recent years.
Prof Ally Lewis, from the University of York, is one of the paper's authors.
He told BBC News: "Our research has involved downloading all data on air pollution from the European data base.
"It's very hard to use… it's a place that data goes to die. We've tidied it up and recreated in York so you can ask questions of it.
"We can throw some clever PhD students at the problem to work out smart ways of analysing the data. I guess government doesn't have that capacity."
The researchers say the government should now track the lifetime emissions of new vehicles to see how catalytic convertors perform over their whole lifetime.
The government appears to have been surprised by the nugget of good NO2 news.
It hasn't yet been unable so far to respond to the specific suggestion that its figures may be too pessimistic.
A spokesman told BBC News: "Defra uses a combination of monitoring and modelling to assess national-level air quality in the UK.
"This modelling is continually kept under review and is updated to take account of the latest evidence."
But if the York paper is right, this wasn't the case for NO2.
Prof Paul Monks from Leicester University told BBC News: "As a car ages the fraction of primary emissions of NO2 decreases and this is not been accounted for in emission inventories that 'predict' the future.
"It is pure luck… but is positive in terms of the way it impacts air pollution."
The episode illustrates the complexity of the air pollution issue. NO2 from vehicles has made the recent headlines, but many other pollutants also put health at risk.
We breathe in toxins from construction machines, planes, boats, gas boilers, farming, burning wood, paints, barbecues and cooking indoors.
Particulates also come from the abrasive friction of tyres on the road and brakes being worn to dust.
This means electric vehicles will help tackle pollution, but won't solve it.
Then there's the complexity of the pollutants themselves. Prof Lewis says a single pollution particle can contain 100,000 chemicals.
Diesel alone, he says, contains 25,000 chemicals. "The composition of particles is wildly complicated. When we are monitoring emissions we just weigh them! We can't attempt to discern which chemicals are most problematic.
"Even so, what we know about particles is way ahead of how they are regulated."
Prof Lewis suggested that the government might choose to submit new data into their long – and so far unsuccessful – court battle with the pressure group ClientEarth over the failure to meet NO2 targets.
ClientEarth's lawyer Alan Andrews told BBC News: "This study shows that we need more evidence to know exactly how much older cars are polluting. We strongly support that.
"It not only highlights a gap in the evidence but also how the government has failed to get a grip on the problem over the years.
"To lower ambition now would be hoping for the best and have real implications for people who are suffering from the health impacts of air pollution today. It would also be illegal in the face of the High Court's ruling."
The government added: "Air pollution has improved significantly since 2010, but we recognise there is more to do which is why we have put in place a £3.5bn plan to improve air quality and reduce harmful emissions.
"We will also end the sale of conventional new diesel and petrol cars and vans by 2040, and next year we will publish a comprehensive clean air strategy which will set out further steps to tackle air pollution."
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