CARBON CYCLING IN AN URBAN ENVIRONMENT

As a result of the global reliance on fossil fuels, the rate at which humans emit carbon into the atmosphere is increasing, which has significant implications for global climate. Urban centers emit the majority of CO₂ produced by fossil fuel combustion. The goal of this study is to use the concentration and carbon isotope composition of atmospheric CO₂ ([CO₂] and δ¹³C, respectively) to gain insight into local carbon cycling on a daily and seasonal basis within the airshed of Chicago, IL, the third largest metropolitan area in the US. The distinct isotope signals of CO₂ sources (atmospheric global average δ¹³C ≈ –8‰, plant (C₃) respiration ≈ –26‰, petroleum combustion ≈ –28‰, and methane combustion ≈ –41‰) along with meteorological information allow us to fingerprint local CO₂ sources and calculate the amount of CO₂ added to, or removed from, the local atmosphere by photosynthesis, natural respiration, or anthropogenic fossil fuel combustion.

Using Wavelength Scanned Cavity Ring Down Spectroscopy, we continuously measured [CO₂] and δ¹³C over several months in Evanston, IL, twelve miles north of downtown Chicago. The focus of this study is two eight-week periods, one in the summer and the other in the winter. During both periods, wind speeds >8-12 mph caused significant atmospheric mixing and thus local [CO₂] within 20 ppm of the global average (~390 to 410 ppm). When wind speeds were <8 mph, [CO₂] and δ¹³C reflected local carbon cycling processes. During the summer, photosynthetic uptake of CO₂ reduced local [CO₂]. During the winter, the lack of photosynthetic activity allowed local [CO₂] to increase. Additionally, during the winter, δ¹³C revealed CO₂ contributions from natural gas combustion for home heating. For both seasons, between 5 and 8:00 am, spikes in [CO₂] coincided with more negative δ¹³C ratios reflecting a local CO₂ contribution, which at least in part, came from petroleum combustion.