FINDING THE RHYTHM OF CHICAGO'S URBAN METABOLISM

To understand carbon cycling in an urban ecosystem, we are conducting high-resolution, continuous monitoring of atmospheric CO₂ concentrations ([CO₂]) and δ¹³C values in Evanston, IL. Evanston is located on the western shore of Lake Michigan, 12 miles north of downtown Chicago. δ¹³C is used to fingerprint local CO₂ sources (e.g., petroleum or methane combustion, ecosystem respiration) and calculate the quantity of locally-sourced anthropogenic CO₂.

Measured [CO₂] and δ¹³C fluctuated seasonally and varied with meteorological conditions, particularly wind speed and direction. Preliminary median [CO₂] and δ¹³C were 391.1 ppm and –9.2‰ in Aug. 2011 versus 415.7 ppm and –9.8‰ in Dec. 2011. In Aug., the diurnal photosynthesis-respiration cycle resulted in variable [CO₂] ranging from 414.8 ppm between 6 and 7:00 am to 386.1 ppm between 3 and 4:00 pm with anti-correlated δ¹³C values. In contrast, in Dec. the morning high [CO₂] period was evident, though subdued, but the afternoon low [CO₂] period was not observed.

In Aug. and Dec., [CO₂] decreased as wind speeds increased. When wind speeds were >10 mph, [CO₂] and δ¹³C approached global average values. In Aug., [CO₂] was higher when winds blew from the west (~180˚ to 360˚) over highly populated areas than when easterly winds blew off Lake Michigan. In Dec., the highest [CO₂] corresponded with southerly winds that had blown over Chicago. [CO₂] was lower with westerly winds, still lower with northerly-northwesterly winds, and were again the lowest with easterly winds.

Aug. high [CO₂] resulted from a combination of anthropogenic and ecosystem sources. Dec. δ¹³C values indicate that high [CO₂] periods resulted from methane combustion for building heating, and during those periods, 15-20% of CO₂ came from local anthropogenic sources. Measured [CO₂] and δ¹³C patterns in Evanston reflected local CO₂ cycling from anthropogenic and natural sources, which was superimposed on the overall northern hemisphere seasonal cycle.