A Dedicated European Emission Inventory for Road Transport with special focus on Ultrafine Particles and Semi- and Intermediate-Volatile Organic Compounds

Despite significant reductions in emissions from road transport driven by air quality regulations, this sector continues to rank among the largest contributors to air pollution globally and within Europe, with profound implications for human health. For certain pollutants, including ultrafine particles (UFP) and semi- and intermediate-volatility organic compounds (S/IVOCs), emission control regulations are lagging behind. In the case of particle number, solid particles are regulated by the Euro standards but the contribution to volatile particle formation is not. This is partly due to insufficient understanding of their emission levels and health impacts. S/IVOCs, which occupy a volatility spectrum between non-methane volatile organic compounds (NMVOCs) and particulate matter (PM), are particularly concerning as they are efficient precursors to secondary organic aerosols (SOAs).

In this study, we present the development and refinement of a high-resolution (6×6 km) road transport emission inventory for Europe, undertaken as part of the EASVOLEE and RI-Urbans projects, by combining information on vehicle mileages by country and emission factors for all major air pollutants, including newly derived factors for total particle number (TPN) and S/IVOCs.

Through a comprehensive literature review, we derived updated and more detailed TPN emission factors that consider fuel injection technologies and particle filters for petrol vehicles and the effect of regeneration for diesel particle filters. To improve the robustness of these factors, they will be further refined using results from measurement campaigns conducted under the EASVOLEE project, including real-world driving and lab measurements. Cold starts and non-exhaust emissions were explicitly modelled for all pollutants. For S/IVOCs, emission profiles were introduced to specify the total SVOC and IVOC mass fractions as a proportion of NMVOC emissions. Furthermore, the spatial distribution of emissions was refined to achieve a more representative allocation of road transport emissions.

Our initial findings highlight that, while PM2.5 emissions from exhaust road transport are less significant compared to other sources, TPN emissions remain a substantial contributor, second only to shipping. Another notable distinction is the strong contribution of non-exhaust sources to PM mass compared to their marginal influence on TPN. This trend aligns with existing research, as non-exhaust emissions predominantly consist of larger particles. Furthermore, cold starts, while varying by pollutant, were found to contribute to roughly 10% of total emissions, emerging as a key consideration for emission inventories, considering that these emissions predominantly occur in urban areas. For the S/IVOC profiles, IVOCs account for approximately 50% of NMVOC emissions from diesel vehicles and 5% from petrol vehicles, while SVOCs contribute 9% and 1.5%, respectively.

These advancements directly support the efforts of modellers to improve the quantification of particulate number concentrations and SOA formation. By enhancing the accuracy of emission data, this work underpins the development of robust policies in line with the EU’s new Ambient Air Quality Directive, which tightens pollutant limit values and drives progress toward cleaner air and improved public health.