Lower vehicular primary emissions of NO\(_2\) in Europe than assumed in policy projections
Lower vehicular primary emissions of NO\(_2\) in Europe than assumed in policy projections
Nature Geoscience, 2017
Many European countries do not meet legal air quality standards for ambient nitrogen dioxide (NO\(_2\)) near roads; a problem that has been forecasted to persist to 2030. Although European air quality standards regulate NO\(_2\) concentrations, emissions standards for new vehicles instead set limits for NO\(_x\)—the combination of nitric oxide (NO) and NO\(_2\). From around 1990 onwards, the total emissions of NO\(_x\) declined significantly in Europe, but roadside concentrations of NO\(_2\)—a regulated species—declined much less than expected. This discrepancy has been attributed largely to the increasing usage of diesel vehicles in Europe and more directly emitted tailpipe NO\(_2\). Here we apply a data-filtering technique to 130 million hourly measurements of NO\(_x\), NO\(_2\) and ozone (O\(_3\)) from roadside monitoring stations across 61 urban areas in Europe over the period 1990–2015 to estimate the continent-wide trends of directly emitted NO\(_2\). We find that the ratio of NO\(_2\) to NO\(_x\) emissions increased from 1995 to around 2010 but has since stabilized at a level that is substantially lower than is assumed in some key emissions inventories. The proportion of NO\(_x\) now being emitted directly from road transport as NO\(_2\) is up to a factor of two smaller than the estimates used in policy projections. We therefore conclude that there may be a faster attainment of roadside NO\(_2\) air quality standards across Europe than is currently expected.