Quantification of Urban Fossil Fuel CO2 Emissions from the Indianapolis Flux Project (INFLUX)
J. Turnbull1, M.O. Cambaliza2, C. Sweeney3, A. Karion3, T. Newberger3, P. Tans4, S. Lehman5, K. Davis6, N. Miles6, S. Richardson6, T. Lauvaux6, P. Shepson2, K. Gurney7, Y. Song7 and I. Razlivanov7
1GNS Science and Cooperative Institute for Reseach in Environmental Sciences, University of Colorado, Boulder, CO 80309; 644-570-4726, E-mail: firstname.lastname@example.org
2Purdue University, Department of Chemistry, West Lafayette, IN 47907
3Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309
4NOAA Earth System Research Laboratory, Boulder, CO 80305
5Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, Boulder, CO 80309
6Pennsylvania State University, Department of Meteorology, University Park, PA 16802
7Arizona State University, Tempe, AZ 85287
Quantification of fossil fuel CO2 (CO2ff) emissions is vital to improving our understanding of the global and regional carbon cycle, and independent evaluation of reported emissions is essential to the success of any emission reduction efforts. The urban scale is of particular interest, because ~75% CO2ff is emitted from urban regions. Measurements of 14CO2 can be used to partition CO2ff from total CO2 in flask samples, but it is difficult to obtain sufficient 14CO2 measurements to infer the urban emission flux.
In the Indianapolis Flux Project (INFLUX), we make flask measurements of 14CO2 and ~50 trace gases from a network of towers and light aircraft. CO2ff and other anthropogenic trace gases are consistently enhanced at tower sites downwind of the city and in the urban plume sampled from the aircraft. The choice of background is critical in isolating the urban CO2ff component from biospheric CO2, and we examine this in detail.
Previous studies have used the strong and consistent relationship between CO2ff and carbon monoxide (CO) to quantify CO2ff. In Indianapolis, we find only weak and variable correlations between CO2ff and CO, likely due to the mix of CO2ff sources in Indianapolis. Yet in winter, total CO2 enhancement is consistent with CO2ff, and we therefore use total CO2 enhancement from the high resolution in situ total CO2 measurements from the aircraft in a simple mass balance model to estimate the urban CO2ff emissions. An initial comparison shows a ~20% difference between the top-down and bottom-up methods from aircraft samples.
Figure 1. Correlation between fossil fuel CO2 (CO2ff) and total CO2 enhancement (ΔCO2) at INFLUX downwind tower in winter, using upwind background values from INFLUX Tower 1 (red) or using a continental background from the LEF Site in Wisconsin (blue). When the local upwind tower is used, ΔCO2 is entirely explained by CO2ff. When the LEF continental background is used, CO2ff and ΔCO2 are both larger, and only about half of ΔCO2 is explained by CO2ff. Choice of background is therefore critical in isolating the urban CO2ff signal.