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Lower vehicular primary emissions of NO2 in Europe than assumed in policy projections

Nature Geoscience volume 10, pages 914–918 (2017)Cite this article

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Abstract

Many European countries do not meet legal air quality standards for ambient nitrogen dioxide (NO2) near roads; a problem that has been forecasted to persist to 2030. Although European air quality standards regulate NO2 concentrations, emissions standards for new vehicles instead set limits for NO x —the combination of nitric oxide (NO) and NO2. From around 1990 onwards, the total emissions of NO x declined significantly in Europe, but roadside concentrations of NO2—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 NO2. Here we apply a data-filtering technique to 130 million hourly measurements of NO x , NO2 and ozone (O3) from roadside monitoring stations across 61 urban areas in Europe over the period 1990–2015 to estimate the continent-wide trends of directly emitted NO2. We find that the ratio of NO2 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 NO2 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 NO2 air quality standards across Europe than is currently expected.

Since the mid-1990s the European vehicle fleet has undergone considerable dieselization1,2,3,4 with incentivization over other fuels and technologies on the basis of predicted fuel efficiency, lower CO2 emissions and increased driving performance5,6,7. By 2014 diesel vehicles accounted for an average of 53% of new European passenger vehicle sales compared with 14% in 1990, in contrast to little increase in their adoption into US fleets3, 4. The proportion of diesel-powered vehicles across Europe has contributed to widely published problems where legal ambient air quality standards are breached, usually near roads. Of particular concern in recent years is NO2, although particulate matter (PM) is also important8. Many European Union (EU) member states are struggling to comply with the 2008/50/EC Air Quality Directive, which sets legal limits for hourly and annual average NO2 concentrations8,9,10. While the total national emissions of NO x (NO + NO2) have shown reductions in Europe, urban concentrations of NO2 have decreased less than expected and this has been attributed to the growth in diesel-fuelled vehicles11,12,13,14,15,16,17,18,19.

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Fig. 1: Mean NO2/NO x ratio for all roadside monitoring sites for the 61 European urban areas analysed between 1990 and 2015.
Fig. 2: Mean NO x and NO2 concentrations after the filtering method was applied.
Fig. 3: The change in the NO2/NO x ratio for each urban area for two time periods.
Fig. 4: Comparison of three methods used to estimate roadside primary NO2.

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Acknowledgements

The authors thank A. Wild for the provision of the Wild Fund Scholarship. This work was also partially funded by the 2016 Natural Environment Research Council (NERC) air quality studentships programme (Grant no. NE/N007115/1). A.C.L. is supported by the NCAS national capability programme and S.J.M. acknowledges the receipt of a NERC KE Fellowship. C. Stovell and his team are thanked for setting-up and maintaining a PostgreSQL database server.

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Authors and Affiliations

  1. Wolfson Atmospheric Chemistry Laboratories, University of York, York, UK

    Stuart K. Grange, Alastair C. Lewis, Sarah J. Moller & David C. Carslaw

  2. National Centre for Atmospheric Science, University of York, Heslington, York, UK

    Alastair C. Lewis & Sarah J. Moller

  3. Ricardo Energy & Environment, Harwell, UK

    David C. Carslaw

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Contributions

D.C.C. designed the research questions, and with S.K.G. developed and evaluated the appropriate methods. S.K.G. processed the European air quality data and conducted the data analysis with D.C.C. S.K.G., D.C.C., A.C.L. and S.J.M. wrote the paper.

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Correspondence to Stuart K. Grange.

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Grange, S.K., Lewis, A.C., Moller, S.J. et al. Lower vehicular primary emissions of NO2 in Europe than assumed in policy projections. Nature Geosci 10, 914–918 (2017). https://doi.org/10.1038/s41561-017-0009-0

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