Significant impact of urban-tree biogenic emissions on air quality estimated by a bottom-up inventory and chemistry-transport modeling

Abstract. Biogenic Volatile Organic Compounds (BVOCs) are emitted by vegetation and react with other compounds to form ozone and secondary organic matter (OM). In regional air-quality models, biogenic emissions are often calculated using a Plant Functional Type approach, which depends on the land-use category. However, over cities, the land-use is urban, so trees and their emissions are not represented. Here, we develop a bottom-up inventory of urban-tree biogenic emissions, in which the location of trees and their characteristics are derived from the tree database of the Paris city combined with allometric equations. Biogenic emissions are then computed for each tree based on their leaf dry biomass, tree-species dependent emission factors and activity factors representing the effects of light and temperature. Emissions are integrated in WRF-CHIMERE air-quality simulations performed over June–July 2022. Over Paris city, the urban tree emissions have a significant impact on OM, inducing an average increase of OM of about 5 %, reaching 14 % locally during the heatwaves. Ozone concentrations increase by 1.0 % on average, by 2.4 % during heatwaves with local increase of up to 6 %. The concentration increase remains spatially localized over Paris, extending to the Paris suburbs in the case of ozone during heatwaves. The inclusion of urban-tree emissions improves the estimation of OM concentrations compared to in situ measurements, but they are still underestimated as trees are still missing from the inventory. OM concentrations are sensitive to terpene emissions, highlighting the importance of favoring urban tree species with low terpene emissions.