Variation in Polycyclic Aromatic Compound (PAC) Concentrations in a Norway Spruce Stand Close to a Major Traffic Route—Influence of Distance and Season

Air concentrations of polycyclic aromatic compounds (PACs) were measured in a Norway spruce ( Picea abies ) stand at four different distances from a major traffic route near Gothenburg, Sweden, during summer and winter 2019, using passive samplers. In total, the concentrations of 32 polycyclic aromatic hydrocarbons (PAHs) and six dibenzothiophenes (DBTs) and NO 2 were analysed. PAC air concentrations were on average 2.5 times higher during the winter compared to the summer. All investigated PAH categories and compounds as well as dibenzothiophene declined linearly with the logarithmic distance from the traffic route. This indicates that the length of pollutant pathways through vegetation is a valid proxy to assess health risks of traffic-related PAC pollutants. Concentrations of heavier PAHs declined faster (concentration at site furthest away from the road was 6% and 33% of concentration at site closest to road, for summer and winter respectively) with increasing distance from traffic compared to lighter PAHs (63% and 68% for summer and winter respectively). As a result, the mix of PAHs changed with the depth into the forest stand (i.e. distance from the road) with potential effects on the toxicity as well as on diagnostic ratios used for source attribution. This is likely caused by different deposition rates among the PAH compounds to the vegetation surfaces, although the exact mechanism needs to be further investigated. This study provides new information that can improve air quality risk assessment and limit the exposure of the population to toxic air pollutants such as PACs.