The role of urban forest composition in modulating summertime air quality in Los Angeles

In this period of rapid global urbanization and climate change, the public health value of urban trees is evident. Trees provide shade, sequester carbon dioxide, and remove air pollutants through deposition. However, some trees emit volatile organic compounds (VOCs) which can react to form fine particulate matter (PM_2.5) and ozone (O₃). Because VOC emissions vary by tree species both in magnitude and composition, and pollutant removal capacity also varies depending on a tree's physical characteristics, discerning net air quality impacts (AQIs) must be done on a species-specific basis. Previous studies have attempted to quantify the AQI of different species at the individual tree level, and we expand on that body of work by considering the comprehensive influence of a collective urban forest. We undertake such an analysis modeling tree functionality in the city of Los Angeles (LA) utilizing literature-based parameterizations. We use novel surveys of the current distribution of LA tree species to extrapolate tree-driven production and removal rates of PM_2.5 and O₃ at the city-wide scale. We then consider how a theoretical tree planting scheme might be employed to gradually replace high VOC-emitting trees with non-emitters as the former age out of the population. Doing so, we estimate that it would take on the order of 40 years to transform the distribution of LA tree species to become net air pollutant sinks rather than sources.