The Interplay Between Ozone and Urban Vegetation-BVOC Emissions, Ozone Deposition, and Tree Ecophysiology

Tropospheric ozone (O-3) is one of the most prominent air pollution problems in Europe and other countries worldwide. Human health is affected by O-3 via the respiratory as well the cardiovascular systems. Even though trees are present in relatively low numbers in urban areas, they can be a dominant factor in the regulation of urban O-3 concentrations. Trees affect the O-3 concentration via emission of biogenic volatile organic compounds (BVOC), which can act as a precursor of O-3, and by O-3 deposition on leaves. The role of urban trees with regard to O-3 will gain further importance as NOx concentrations continue declining and climate warming is progressing-rendering especially the urban ozone chemistry more sensitive to BVOC emissions. However, the role of urban vegetation on the local regulation of tropospheric O-3 concentrations is complex and largely influenced by species-specific emission rates of BVOCs and O-3 deposition rates, both highly modified by tree physiological status. In this review, we shed light on processes related to trees that affect tropospheric ozone concentrations in metropolitan areas from rural settings to urban centers, and discuss their importance under present and future conditions. After a brief overview on the mechanisms regulating O-3 concentrations in urban settings, we focus on effects of tree identity and tree physiological status, as affected by multiple stressors, influencing both BVOC emission and O-3 deposition rates. In addition, we highlight differences along the rural-urban gradient affecting tropospheric O-3 concentrations and current knowledge gaps with the potential to improve future models on tropospheric O-3 formation in metropolitan areas.