Estimation of the potential BVOC emissions by the different tree species in Malmö

Abstract: BVOC emissions from trees contribute to ozone and secondary aerosol formation and therefore have an impact on air quality. The two most abundant BVOCs emitted from trees are isoprene and monoterpenes. In urban areas, air pollution levels are already elevated and high rates of isoprene and monoterpene emissions from trees will potentially contribute to even higher levels of ozone and aerosols in the atmosphere. In Malmö, trees are planted along streets, in parks and in private gardens. In this study, aerial images were used to determine the overall tree cover, the species composition and their potential contribution to BVOC levels in the atmosphere, with the help of literature sources for standardized emission rates. The results showed that 14% of the study area were covered by trees, and for a smaller section of the study area, it was determined that 62% of that tree cover resulted from unknown and unregistered trees. For the whole study area, Tilia (linden) trees proved to be the dominant genus, making up over 44% of the known tree cover. The second most abundant tree genus is Aesculus (horse chestnut) with 8.2% of the tree cover and Platanus (plane tree) with 8.1% of the tree cover. The emission potential was calculated for each species using literature values and multiplying them with the area that each tree covered. The results showed that Quercus robur (European oak), Platanus x hispanica (London plane) and Quercus rubra (northern red oak) have the highest isoprene emission impact on atmospheric chemistry by having standardized emission rates of around 19 to 67 μgC gdw-1 h-1 and therefore falling into the categories of moderate to highest emitters of the emission categories used in this study. The tree species with the highest monoterpene emission impact levels are Aesculus hippocastanum (horse chestnut), Fagus sylvatica (European beech) and Platanus x hispanica (London plane) with standardized emission rates of around 4 to 12 μgC gdw-1 h-1 and being in the categories of high and highest emitters. The most abundant genus, Tilia, is a low emitter and therefore does not have the highest emission contribution, despite its high occurrence. The methodology proved to be appropriate to give an estimate of emission impact for a large area but came with many limitations and uncertainties and would not be appropriate to calculate the emission impact on an individual tree level.