The proportion of nitrogen dioxide (NO2) in European roadside nitrogen oxides (NOx) emissions could be two times smaller than assumed in policy projections, concludes a study published online in Nature Geoscience this week. The study therefore suggests that European roadside air quality standards - which regulate NO2 concentrations - may be achieved sooner than expected.

In Europe, emissions standards for new vehicles regulate nitrogen oxides (NOx), which encompasses NO2 and nitric oxide (NO). These emissions have declined in Europe since about 1990. By contrast, air quality standards set limits for the concentration of NO2 only, and roadside concentrations of NO2 have declined much less than expected, which has been largely attributed to increasing use of diesel vehicles in Europe and more direct tail-pipe emissions of NO2.

Stuart Grange and colleagues analysed 130 million hourly measurements of NOx, NO2 and ozone (O3) from roadside monitoring stations across 61 European cities. They find that that the proportion of NO2 in NOx increased from 1995 to around 2010, but stabilized or decreased thereafter in most regions, and remained at a level that is lower than assumed in air quality standards. The authors suggest that policy projections of air quality that use too high a value for the proportion of NO2 in NOx will predict higher concentrations of roadside NO2 than may actually occur for the same total amount of NOx emitted.

In an accompanying News & Views article, Drew Gentner writes: “Going forward, the findings are important for informing air-quality policies in diesel-vehicle-heavy developing regions, which typically inherit used vehicles and have emissions standards that lag slightly behind the US and Europe."