Spatiotemporal assessment and evaluation of aromatic VOC emissions: a case study for Spain

Volatile organic compounds (VOCs) significantly contribute to air pollution, pose serious health hazards to humans, and influence ozone formation and secondary organic aerosol production. Anthropogenic sources include various human-driven activities, such as solvent use, traffic and fuel evaporation, industrial emissions, and biomass burning. Despite their importance, the uncertainties associated with representing VOCs in atmospheric emission inventories are considerably higher than other reported air pollutants. This work presents a spatiotemporal assessment and evaluation of benzene, toluene, and xylene (BTX) emissions and concentrations in Spain. We run the High-Elective Resolution Modelling Emission System (HERMESv3) model to produce gridded bottom-up emissions of BTX and use it as input in the Multiscale Online Nonhydrostatic AtmospheRe CHemistry model (MONARCH) chemical transport model to simulate surface concentrations across Spain. The modelling results were then evaluated against official ground-based observation data for the year 2019. The intercomparison between modelled and officially reported observed levels allows for identifying sources of uncertainty in the anthropogenic emission inputs, which we further explored through specific sensitivity test runs. The largest levels of observed benzene and xylene were found in industrial sites near coke ovens, refineries and car manufacturing facilities, where the air quality modelling results show large underestimations. Official emissions reported for these facilities were replaced by alternative estimates, allowing heterogeneous improvement of the model's performance and highlighting that uncertainties representing industrial emission processes remain. For toluene, consistent overestimations in background stations were mainly related to uncertainties in the spatial disaggregation of emissions from industrial use solvent activities, mainly from wood paint applications. Observed benzene levels in Barcelona's urban traffic areas were five times larger than the ones observed in Madrid. MONARCH failed to reproduce the observed gradient between the two cities due to uncertainties in estimating emissions from motorcycles and mopeds. Our results are constrained by the spatial and temporal coverage of available BTX observations, posing a key challenge in evaluating the spatial distribution of modelled levels and associated emissions.