Unaccounted precursors of SOA in Europe: IVOCs emitted from on-road vehicles

On-road transport is a significant source of organic aerosol, especially in urban environments across Europe. Over the past decades, the implementation of European air-quality policies has successfully led to the reduction of on-road primary organic aerosol (POA). Despite these advances, the total organic aerosol burden from on-road transport is still considerable, as gas-phase species emitted by vehicles act as precursors and form secondary organic aerosol (SOA). Among these precursors, intermediate volatility organic compounds (IVOCs) represent a significant yet poorly constrained source of on-road SOA. IVOCs are organic gases with an effective saturation concentration (C^*) between 10³ and 10⁶ μg m^-3 at 298 K, and in the majority of chemical transport models (CTMs), they are either highly parameterized or even neglected.  The aim of this study is to assess both the overall contribution of on-road IVOCs to SOA formation and the contribution of specific compounds in the IVOC range over Europe. To achieve this, the results of the EASVOLEE (Effects on Air Quality of Semi-VOLatile Engine Emissions) emission characterization campaigns were used to update the emissions of IVOCs and the corresponding SOA-iv formation parametrizations in PMCAMx, a three-dimensional chemical transport model. The model was used to simulate one winter and one summer month (January and July 2019) in Europe. The contribution of the IVOCs emitted by different vehicle types to SOA formation was quantified, including the role of specific IVOCs.