Natascha Kljun

Climate change is primarily driven by anthropogenic emissions of greenhouse gases. Reducing these emissions globally requires a massive effort at the individual, city, and national scales. Urban areas are hotspots of anthropogenic emissions, given the high density of human-based activities, and clarity on the variations of emissions in these areas will enable effective targeted reduction plans. However, efforts to do so are hampered by a lack of direct measurements of temporal and spatial trends of the emissions. This study combines emission inventories with flux observations and footprint modeling in three pilot sites for urban emission studies (Zurich, Munich, and Paris) with a focus on carbon dioxide (CO₂), carbon monoxide (CO), and methane (CH₄). Results indicate that the sectors contributing most significantly to CO₂ fluxes, stationary combustion and road transport, are consistent across the cities and require future reduction plans to target winter months and daytime hours (05:00–17:00 UTC). The sectors contributing to CO and CH₄ fluxes vary by city and do not always have consistent seasonal or diurnal patterns. Results also provide a basis for improving emission inventories and temporal scaling factors across sites and species in order to achieve better agreement with observations.