Why are you analyzing the "urban metabolism" of Chicago?

Where is the measurement site located?

What geographic area do the measurements represent?

How are the measurements made?

How are carbon isotope ratios reported?

What is the significance of measuring the carbon isotope composition of CO₂?

How are the graphs on the data page interpreted?

Why are you analyzing the "urban metabolism" of Chicago?

The term "urban metabolism" illustrates that cities have unique matter and energy flows compared to nature. The connection between urban metabolism and global warming is clear. Carbon dioxide (CO₂) is a major byproduct of urban metabolism. Carbon dioxide is also a greenhouse gas whose continued accumulation in the atmosphere is expected to increase Earth's surface temperature during the 21^st century[1]. While records of atmospheric CO₂ variations in the global atmosphere are well established, few such records exist for urban areas. Fossil fuel consumption at the level of individual citizens combines to elevate urban CO₂ concentrations. When integrated over the total number of densely populated cities, urban CO₂ forms a substantial input to the global atmosphere. In the U.S., the residential and transportation sectors contribute approximately 50% of all CO₂ emissions[2], implying that individual behavior has global consequences. In 2005, Chicago's greenhouse gas emissions totaled 36.2 MMTCO₂e (million metric tons of CO₂ equivalents)[3]. This equates to 12.5 tons for each of Chicago's 2.9 million residents[3]. Electricity usage provided most of the CO₂ (44%), followed by natural gas combustion (27%) and transportation (20%)[3]. Of the 44% attributed to electricity usage, Chicago's two coal-fired power plants emitted 35% of the CO₂ directly into the local atmosphere[3]. Against the backdrop of these carbon footprint metrics, NUCO₂ aims to measure Chicago's atmospheric CO₂ concentrations and fingerprint its sources. Increased understanding is a critical step toward improving and implementing local sustainability initiatives having global implications.
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Where is the measurement site located?

Instrumentation is located on the roof of Scott Hall on the Northwestern University campus in Evanston, IL. The air intake is located approximately 15 m, or about 50 ft, above ground level. Our long-term goal is to install additional stations throughout Chicago.

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What geographic area do the measurements represent?

The geographic coverage of the station is a complicated function of numerous variables, including, but not limited to, height above the ground surface, wind speed, wind direction, proximity to local CO₂ sources, and roughness elements that affect air flow, such as trees and buildings. Equations considering these and other factors suggest our station samples over a minimum area of 6 km², or about 1500 acres. Excursions in the dataset partly reflect the station's proximity to a busy intersection (Sheridan Rd. and Chicago Ave.). Nonetheless, mobile sampling conducted prior to permanent installation shows that average background CO₂ concentrations in Chicago's atmosphere are consistent over large distances, well beyond the minimum estimated sampling area.
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How are the measurements made?

We are employing a novel, laser-based technique called Wavelength Scanned Cavity Ring Down Spectroscopy (WS-CRDS). Specifically, we are using a Picarro G1101-i. Click here for a detailed explanation about WS-CRDS. The G1101-i measures concentrations of ¹²CO₂ and ¹³CO₂ in parts per million units by volume (ppm). The concentration ratio of these isotopologues yields the carbon isotope composition of CO₂, reported in delta notation (δ). The instrument makes a measurement every 7–8 seconds yielding about 11,400 data points per day. Carbon dioxide concentrations reported on this website are not corrected for water vapor, but incoming air is dried to <0.15% by volume. Correcting for this small amount of water vapor negligibly increases reported CO₂ concentrations. We calibrated the G1101-i using CO₂ standards prepared by the World Meteorological Organization (WMO) Central Calibration Laboratory (CCL) in the Earth System Research Laboratory (ESRL) of the Global Monitoring Division (GMD) at the National Oceanic and Atmospheric Administration (NOAA). To monitor drift and maintain quality control, we analyze two conventional CO₂ standards for 8 minutes each twice per day. The concentrations of these standards are certified to within 1%, and we measured the carbon isotope composition of the standards using a gas source isotope ratio mass spectrometer. Thus far, repeated analyses of these conventional standards demonstrates an external reproducibility of ±0.1 ppm (1σ) for CO₂ concentrations and ±0.35‰ (1σ) for δ values. The schematic below shows the design for our sampling station.

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How are carbon isotope ratios reported?

Because carbon isotope abundance variations are typically very small, occurring at or beyond the third significant figure of the isotope ratio (¹³C/¹²C), the convention in stable isotope geochemistry is to express isotopic variations using a differential notation called delta notation (δ). The equation is:

,      (1)

where R_smp is the ¹³C/¹²C ratio of the sample, and R_std is the ¹³C/¹²C ratio of a standard reference material with an internationally agreed upon value, in our case, Vienna Peedee Belemnite (VPDB). Equation (1) expresses the relative difference between the sample and standard in parts per thousand units (ppt). The corresponding symbol for ppt, ‰, is called "permil", and is analogous to the percent symbol, %. For example, in 2010, global atmospheric CO₂ had an average δ¹³C value of –8.37‰, or –8.37 permil.
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What is the significance of measuring the carbon isotope composition of CO₂?

Carbon naturally occurs as two stable isotopes, ¹²C (98.93%) and ¹³C (1.07%). Plants have relatively low ¹³C/¹²C ratios (or relatively negative δ¹³C values) because they preferentially incorporate ¹²C during growth. Because fossil fuels, such as coal, petroleum, and natural gas, originate from the partial decomposition of ancient plants, fossil fuels have roughly the same carbon isotope composition as those plants (typically -23.5 to -26.8‰)[4], with variations among different fossil fuels partly reflecting the different pathways that created them (typically -25.7 to -31.7‰ for petroleum and -36.3 to -46.4‰ for natural gas) [5][6][7]. In turn, fossil fuel combustion yields CO₂ that has the same isotope composition as its source. The carbon isotope composition of fossil fuel CO₂ is distinct from that of CO₂ in the global atmosphere, which has more positive δ¹³C values than plants and fossil fuels (-8.37‰ in 2010) [8]. Fossil fuel combustion over the past 150 – 200 years has decreased the carbon isotope composition of atmospheric CO₂ relative to pre-industrial values. Before contributing to global trends, fossil fuel combustion locally impacts the atmosphere where it is burned. The carbon isotope composition of CO₂ therefore allows us to quantitatively "fingerprint" the origin of CO₂ in Chicago's atmosphere.

Natural processes, such as photosynthesis and respiration, also impact the concentration and carbon isotope composition of atmospheric CO₂. Photosynthesis decreases CO₂ concentrations but increases δ¹³C values because as mentioned above, photosynthesis preferentially utilizes ¹²CO₂. Similar to fossil fuel combustion, respiration increases CO₂ concentrations but decreases δ¹³C values because the CO₂ originates from plants. The effects of photosynthesis and respiration are most evident during summer. During daytime, photosynthesis may decrease CO₂ concentrations and increase δ¹³C values to levels lower and higher, respectively, than CO₂ in the globally well-mixed atmosphere. During nighttime, respiration may increase CO₂ concentrations and decrease δ¹³C values. Respiration and petroleum combustion yield roughly similar trends, but the former tends to dominate during nighttime, whereas the latter tends to dominate during daytime (e.g., rush hour). It is worth noting that photosynthesis withdraws CO₂ representing a mixture of CO₂ from the global atmosphere, respiration, and fossil fuel combustion. Thus, in other words, plants growing in Chicago partly subsist on fossil fuel CO₂ emitted locally by human activity. During winter, when photosynthesis and respiration are less influential, we observe that fossil fuel usage contributes up to ~25% of the CO₂ in Chicago’s atmosphere. Seasonal differences in our dataset can be viewed with the user-defined filter.
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How are the graphs on the data page interpreted?

When two CO₂ sources mix in varying proportions, the concentration and carbon isotope composition of CO₂ in the resulting mixture is a function of the concentration and isotope composition of CO₂ in the two end-member sources. Plotting the δ¹³C value of CO₂ versus the reciprocal of its concentration provides a useful test for two-component mixing. This plot is often called a "Keeling plot" after Charles David Keeling (also known for the "Keeling Curve"), but the principle applies to all isotopic mixtures. The figure above shows two-component mixing lines connecting CO₂ in the global atmosphere with two fossil fuel sources, namely petroleum and natural gas. During winter, when photosynthesis and respiration are not important, these mixing lines bracket data for CO₂ in Chicago's atmosphere. In other words, the measured data actually represent three-component mixing. Photosynthesis and respiration influence the pattern during summer, but the overall mixing trends hold. We can use the following equation to calculate the relative proportion (x) of CO₂ in Chicago's atmosphere from fossil fuels versus the global background:

,      (2)

where chi, gbl, and ff respectively refer to Chicago, the global background, and fossil fuels. Using end-member values for the global background (2010 average CO₂ = 389.8 ppm, δ¹³C = –8.37‰)[8], petroleum combustion (CO₂ = 32,300 ppm, δ¹³C = –28.7‰)[5], and natural gas combustion (CO₂ = 7760 ppm, δ¹³C = –40.7‰)[5], we calculated values for x noted on the diagram above.
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References

1. Solomon, S. et al. (eds.) Climate Change 2007 : The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. (Cambridge University Press, 2007). (back)
2. Emissions of Greenhouse Gases in the United States 2008, Report #: DOE/EIA-0573(2008), U.S. Energy Information Administration, Office of Integrated Analysis and Forecasting, U.S. Department of Energy. (back)
3. McGraw, J., Haas, P., Young, L. & Evans, A. Greenhouse gas emissions in Chicago: Emissions inventories and reduction strategies for Chicago and its metropolitan region. Journal of Great Lakes Research 36, 106-114 (2009). (back)
4. F. A. Smith (McInerney), J. W. C. White, Modern calibration of phytolith carbon isotope signatures for C₃/C₄ paleograssland reconstruction. Palaeogeography, Palaeoclimatology, Palaeoecology 207, 277 (2004). (back)
5. Widory, D. & Javoy, M. The carbon isotope composition of atmospheric CO₂ in Paris. Earth and Planetary Science Letters 215, 289-298 (2003). (back)
6. S. E. Bush, D. E. Pataki, J. R. Ehleringer, Sources of variation in δ¹³C of fossil fuel emissions in Salt Lake City, USA. Applied Geochemistry 22, 715 (2007). (back)
7. Measured values for CO₂ from natural gas in Chicago (-46.4‰) (back)
8. NOAA. ESRL/GMD FTP Data Finder. (back)