Partitioning of Urban CO2ff Emissions By Source Sector: Results from the Influx Project

Abstract:

Urban areas contribute ~75% of fossil fuel CO₂ (CO₂ff) emissions, and city governments are often leading the way in emission reduction efforts. As emissions are regulated and assigned a price, there is an increasing need to independently evaluate the reported bottom-up emissions and to attribute them to specific source sectors (e.g. electricity production, industry, vehicles). We demonstrate how multispecies atmospheric observations can be used to achieve this.

The Indianapolis Flux Experiment (INFLUX) aims to develop and evaluate methods for detection and attribution of urban GHG fluxes. The INFLUX observation network includes twelve towers measuring in situ CO₂ and CO and flask measurements of another 50 species. ¹⁴CO₂ measurements have shown that in winter, the total CO₂ enhancement over Indianapolis approximates the CO₂ff added. This somewhat surprising result allows us to use the wintertime in situ total CO₂ and CO measurements to determine the observed CO:CO₂ff ratio (R_CO) at high resolution. First, we demonstrate that the USEPA CO inventory for Indianapolis overestimates CO emissions by a factor of about 2.5. Then we use the Hestia bottom-up CO₂ff data product and revised characteristic R_CO values for each CO₂ff source sector to predict the diurnal cycle in R_CO for Indianapolis. The tower observations and bottom-up R_CO estimates are consistent during the daytime, but the observed R_CO is significantly higher than the bottom-up estimate during the night. We show how the bottom-up and top-down methods can be used together to determine the cause of this discrepancy and improve CO₂ff estimates from both methods.