High-Resolution Quantification of Biogenic CO2 Fluxes over a Metropolitan Area

Cities are hot spots on greenhouse gas (GHG) emissions, yet green infrastructure (GI) such as green spaces and parks provides potential solution for reducing urban carbon footprints through photosynthetic uptake and carbon sequestration. Studies have shown that the offset of urban vegetation uptake on local anthropogenic CO₂ emissions varies between 2% and 100%, underscoring the complexity associated with this solution. Quantifying CO₂ capture by GI is challenging due to the interplay of photosynthetic uptake and respiration, seasonal variability, the heterogeneous distribution of GI, urban climate, and soil conditions. While biosphere models have been used to quantify carbon exchange processes, they are often employed at the ecosystem level and at coarse spatial resolutions(10-100km), making them insufficient for capturing biospheric signals at the urban scale(10m-1km). Therefore, high-resolution quantification of biogenic CO₂ fluxes is essential for understanding their role on urban GHG budget.

This study estimates biogenic CO₂ fluxes for 2023 in the Metropolitan Area of Barcelona (AMB) at a 10 m resolution using the Vegetation Photosynthesis and Respiration Model (VPRM). Our approach integrates vegetation indices derived from Sentinel-2, a detailed vegetation land cover dataset constructed by merging local land cover and tree maps, and meteorological inputs (temperature and shortwave radiation) from the Weather Research and Forecasting (WRF) model coupled with an urban canopy scheme that better represents atmosphere exchanges inside the urban canyons. A sensitivity analysis is conducted comparing different VPRM configurations including flux parameterization, input satellite-derived vegetation indices and modifications to land cover map. To constrain the modelled biogenic CO₂ emissions and determine their uncertainties, the estimated biogenic fluxes are evaluated with atmospheric CO₂ mixing ratios observations from the AMB GHG monitoring network using an atmospheric transport model (WRF-Chem) in a passive tracer approach. This research presents an improved method to estimate the urban biogenic CO₂ fluxes and provides guidance for improving and creating more robust ways of accounting for the contribution of urban green to aid policy and urban planners in the design and implementation of GI.