Quantifying biogenic carbon dioxide fluxes in an urban area

Urban areas constitute complex and highly heterogeneous mosaics of CO₂ sources and sinks. Anthropogenic emissions - mainly from fuel combustion due to vehicle traffic, building heating, energy production and other industrial activities - are producing high amounts of CO₂, dominating the urban CO₂ flux. The biogenic fluxes (i.e. photosynthesis, autotrophic-heterotrophic respiration) are usually smaller than the anthropogenic fluxes in urban areas, however they potentially affect the seasonal and spatial variability of urban emissions according to green area cover fraction and seasonal climate variability. Quantifying the urban biogenic fluxes would help in discriminating human emissions from natural fluxes, recognizing the seasonal and interannual CO₂ emission variability and trends, enhance our current understanding on urban metabolism and function, and eventually improve the current urban emission inventories. Urban biogenic flux dynamics are expected to differ significantly from the rural ecosystems due to the extreme variability of urban climate in micro and local scales, urban-related stressors and diverse management practices. The Urban Heat Island (UHI) phenomenon is one of the factors that would potentially alter the urban biogenic CO₂ balance, since it affects both soil and air temperature which are important environmental drivers of the biogenic CO₂ flux processes. A relevant scientific question is if urban green tends to behave as carbon sink or source in the long term, which is still a matter of controversy in today's literature. In the framework of diFUME project (https://mcr.unibas.ch/difume/), the spatial and temporal variability of CO₂ flux by the anthropogenic and biogenic sources and sinks in Basel city centre is modelled and monitored. The approach involves the development of mechanistic models of photosynthetic uptake, plant respiration and soil respiration, dedicated to urban environment, according to meteorological observations, spatial representation of urban structure and EO monitoring of vegetation dynamics. An extended urban sensor network in the study area is used to monitor air temperature, soil temperature and soil moisture variability. The spatial variability of solar radiation is modelled according to the 3-dimensional architecture of the urban canopy. A high-resolution aerial Lidar dataset of the study area is used to extract building and tree morphology, as well as tree Leaf Area Index (LAI). The multiple radiation interactions between buildings and urban vegetation are considered in a multilayer modelling approach of radiation intercepted by plant canopies, taking into account horizontal and vertical distribution of LAI and building structures. The biogenic flux models are calibrated during an extended field campaign of microscale in-situ CO₂ flux measurements on urban trees and soils of Basel city centre during the summer of 2020. This study presents the developed modelling approaches for the three biogenic fluxes, the first results from the field measurement campaign and initial estimations of the spatial and temporal variability the urban biogenic CO₂ fluxes.