From direct emission factors to inverse and indirect impact factors of road traffic: Influence of emerging and unregulated pollutants

Abstract. Even though emissions regulations have successfully reduced concentrations of some road traffic-originated air pollutants, road traffic is still an important source of many unregulated emerging pollutants such as semi-volatile ultrafine particles, non-exhaust emissions and volatile organic compounds (VOCs) in urban areas. In addition, road traffic can have season- and location-dependent inverse and indirect effects on the urban aerosol which are not well understood. In our study, we investigate wintertime vehicle fleet emission factors (EF) of road traffic in a street canyon environment in central Helsinki, Finland, by using CO₂ as a tracer for fuel consumption. We report EFs for particle number and size distribution, black carbon, PM₁₀, PM_2.5, NO_x, NO₂, NO, CO as well as for organic and inorganic chemical components of particulate matter, and VOCs, including aromatic hydrocarbons, alkanes, and polycyclic aromatic hydrocarbons. The obtained fleet EFs of PM and NO_x were considerably higher than required for new vehicles in Europe since 2014, showing the major role of older vehicles and heavy-duty traffic on the urban aerosol. In addition to direct EFs, we show that CO₂-based EF determination enables detection of inverse and indirect effects of road traffic. For example, road traffic had an inverse effect, i.e., negative EF, on O₃ and small ion (< 2 nm) concentrations. Also, we found that road traffic contributes to compounds that are not necessarily considered to be traffic-derived such as terpenoid VOCs. We introduce a term impact factor (IF) to describe the found inverse and indirect effects of road traffic.