Acknowledge DATA

DATA comes from the following:
Gurney, K.R., D. Mendoza, Y. Zhou, M. Fischer, C. Miller, S. Geethakumar, S. de la Rue du (2009) The Vulcan Project: High resolution fossil fuel combustion CO2 emissions fluxes for the United States, Environ. Sci. Technol, 43, doi:10.1021/es900,806c