NUCO2 Group, Northwestern University, Department of Earth and Planetary Sciencies

The purpose of this program is to measure the "urban metabolism" of Chicago. This is part of a growing trend towards experimentation and inquiry into how cities have unique matter and energy flows as compared to those 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 21st century. 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, implying that individual behavior has global consequences. 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.

We have placed our measurement system 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.  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. 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.

For measurements we have deployed a Picarro G1101-i to obtain concentrations two isotopes of CO₂ (12CO₂ and 13CO₂) in parts per million units by volume (ppm). The instrument makes a measurement every 7–8 seconds yielding about 11,400 data points per day. 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. 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. NUCO₂ is financially supported by the Initiative for Sustainability and Energy at Northwestern (ISEN), The David and Lucile Packard Foundation, the Weinberg College of Arts and Sciences, the McCormick School of Engineering, and the Northwestern University Office for Research. Dr. Joel Moore (Earth and Planetary Sciences post-doctoral fellow, 2008 - 2011) designed, calibrated, and installed the WS-CRDS measurement system. Several Northwestern University undergraduate students contributed to NUCO₂.

Using a Picarro gives us a unique combination of high precision and ease of use. We can run samples unattended all day without worrying about drift or other problems. The analyzer is simple to operate and can be run by undergrads with no problem. The data it produces is excellent. We hope to standardize on Picarro analyzers as we build out our measurement network across Chicago.

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