Atmospheric Observations of Carbon Monoxide (CO) and Fossil Fuel CO2 Emissions from a Medium Sized City: Sacramento, California
J. Turnbull1, A. Karion2, M. Fischer3, C. Sweeney2, I. Faloona4, T. Guilderson5, S. Lehman2, J. Miller2 and P. Tans1
1NOAA Earth System Research Laboratory, 325 Broadway, Boulder, CO 80305; 303-497-4836, E-mail: Jocelyn.turnbull@noaa.gov
2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309
3Lawrence Berkeley National Laboratory, Berkeley, CA 94720
4University of California at Davis, Davis, CA 95616
5Lawrence Livermore National Laboratory, Livermore, CA 94550
Accurate measurement of recently added fossil fuel carbon dioxide (CO2ff) in the atmosphere is needed not only to quantify CO2ff emissions, but also aids in understanding emissions of other anthropogenic trace gases. We use measurements of the radiocarbon content of atmospheric CO2 (Δ14CO2) to determine recently added CO2ff in flask samples taken by aircraft over and downwind of Sacramento, California on February 27 and March 6, 2009. We determine the CO to CO2ff emission ratio from the flask measurements as 14 ± 1 ppbCO/ppmCO2ff (figure 1). Our results support the bottom-up inventory estimate of 15.4 ppbCO/ppmCO2ff obtained from the CEPAM (CO) and Vulcan (CO2ff) databases for the Sacramento region. However, if total CO2 enhancement is assumed to represent CO2ff in the urban plume, the CO:CO2 ratio is much lower (figure 1), and would have suggested lower overall CO emissions. We use the 14C-derived CO:CO2ff emission ratio to obtain a high resolution CO2ff record from continuous CO observations made on the February 27, 2009, flight. Comparison with total CO2 mole fraction, also measured continuously in the same flight, shows that while CO2ff emissions dominate the CO2 variability in the Sacramento plume, they are not sufficient to explain all of the CO2 variability, indicating some contribution from biospheric CO2 (CO2bio) exchange. Furthermore, within the Sacramento plume, a positive CO2bio flux is observed, indicating net respiration and/or biofuel use. Conversely, CO2bio is negative outside the urban plume, indicating net photosynthetic uptake in the rural Sacramento Valley. In a second flight, on March 6, 2009, the same type of analysis, using only the flask samples, shows that CO2bio within the city was negative.
Figure 1. Relationship between CO2ff (green symbols, bottom axis) and CO, and between total CO2 (blue symbols, top axis) and CO. Flask measurements are the large circles, with the different shades indicating the two flights. The small, light blue circles are continuous measurements made on February 27, 2009. The lines are the best fit to the combined datasets, and the slopes are the obtained CO:CO2ff or CO:CO2total emission ratio.