Direct CO2 emissions and uptake at neighbourhood-scale across the urban area of Beijing

Urban areas are significant contributors to global carbon dioxide (CO${}_2$) emissions, highlighting the need to comprehend CO${}_2$ flux dynamics within cities for effective climate change mitigation. Neighbourhood-scale assessments of land-atmosphere CO${}_2$ exchange are needed due to the intricate interactions between human activities, infrastructure, and vegetation. In this study, surface CO${}_2$ flux (Scope 1 direct emissions) was modelled over the urban area of the megacity Beijing, China, in 2016 at 500-m resolution to examine the relative contributions of the different local sources and their dependencies on different Local Climate Zones (LCZs). The model considered direct CO${}_2$ emissions from on-road traffic, local fuel combustion within buildings, human metabolism, soil and vegetation respiration, and CO${}_2$ uptake by vegetation photosynthesis.

The results showed that the spatial average of anthropogenic CO${}_2$ emission was 4.5 kg C m⁻² yr⁻¹. Traffic and local building emissions contribute 38% and 37%, respectively, of total CO${}_2$ emissions, followed by human metabolism with 13%. Vegetation uptake offset only 4% of emissions, playing a minor role in climate mitigation due to limited areal coverage. CO${}_2$ fluxes showed high heterogeneity, with hot spots resulting primarily from traffic emissions. Net CO${}_2$ flux increased and then decreased with distance from the city centre, following the pattern in the impervious surface fraction and population density. LCZs helped explain patterns in biogenic and building-related CO${}_2$ fluxes, but they were less effective at capturing the complexity of traffic-related emissions. Simulating both anthropogenic and biogenic fluxes provides insight into their relative magnitudes on the neighbourhood scale and helps to identify the areas where emission reductions would be most critical to be made and nature-based solutions are most urgently needed.