Seasonally varying contributions to urban CO2 in the Chicago, Illinois, USA region: Insights from a high-resolution CO2 concentration and δ13C record

Understanding urban carbon cycling is essential given that cities sustain 54% of the global population and contribute 70% of anthropogenic CO2 emissions. When combined with CO2 concentration measurements ([CO2]), stable carbon isotope analyses (delta C-13) can differentiate sources of CO2, including ecosystem respiration and combustion of fossil fuels, such as petroleum and natural gas. In this study, we used a wavelength scanned-cavity ringdown spectrometer to collect similar to 2x10(6) paired measurements for [CO2] and delta C-13 values in Evanston, IL for August 2011 through February 2012. Evanston is located immediately north of Chicago, IL, the third largest city in the United States. The measurements represent one of the longest records of urban [CO2] and delta C-13 values thus far reported. We also compiled local meteorological information, as well as complementary [CO2] and delta C-13 data for background sites in Park Falls, WI and Mauna Loa, HI. We use the dataset to examine how ecosystem processes, fossil fuel usage, wind speed, and wind direction control local atmospheric [CO2] and delta C-13 in a midcontinent urban setting on a seasonal to daily basis. On average, [CO2] and delta C-13 values in Evanston were 16-23 ppm higher and 0.97- 1.13% lower than the background sites. While seasonal [CO2] and delta C-13 values generally followed broader northern hemisphere trends, the difference between Evanston and the background sites was larger in winter versus summer. Mixing calculations suggest that ecosystem respiration and petroleum combustion equally contributed CO2 in excess of background during the summer and that natural gas combustion contributed 80%-94% of the excess CO2 in winter. Wind speed and direction strongly influenced [CO2] and delta C-13 values on an hourly time scale. The highest [CO2] and lowest delta C-13 values occurred at wind speeds <3 m s(-1) and when winds blew from the northwest, west, and south over densely populated neighborhoods.