Toward quantification and source sector identification of fossil fuel CO₂ emissions from an urban area: Results from the INFLUX experiment

The Indianapolis Flux Experiment (INFLUX) aims to develop and assess methods for quantifying urban greenhouse gas emissions. Here we use CO₂, 14CO₂, and CO measurements from tall towers around Indianapolis, USA, to determine urban total CO₂, the fossil fuel derived CO₂ component (CO₂ff), and CO enhancements relative to background measurements. When a local background directly upwind of the urban area is used, the wintertime total CO₂enhancement over Indianapolis can be entirely explained by urban CO₂ff emissions. Conversely, when a continental background is used, CO₂ff enhancements are larger and account for only half the total CO₂enhancement, effectively representing the combined CO₂ff enhancement from Indianapolis and the wider region. In summer, we find that diurnal variability in both background CO₂ mole fraction and covarying vertical mixing makes it difficult to use a simple upwind-downwind difference for a reliable determination of total CO₂urban enhancement. We use characteristic CO₂ff source sector CO:CO₂ff emission ratios to examine the contribution of the CO₂ff source sectors to total CO₂ff emissions. This method is strongly sensitive to the mobile sector, which produces most CO. We show that the inventory-based emission product (“bottom up”) and atmospheric observations (“top down”) can be directly compared throughout the diurnal cycle using this ratio method. For Indianapolis, the top-down observations are consistent with the bottom-up Hestia data product emission sector patterns for most of the diurnal cycle but disagree during the nighttime hours. Further examination of both the top-down and bottom-up assumptions is needed to assess the exact cause of the discrepancy.