Initial Measurements from Chicago's First Long-Term Urban Flux Site

Assumptions underlying traditional flux measurement techniques, such as spatial homogeneity and unobstructed upwind fetch area, typically do not hold true in urban environments. Installing a flux measurement system above the urban canopy is not practically feasible in large and dense cities such as Chicago, and is further hampered by strict regulations in the city. This makes urban flux measurements particularly challenging and hence not commonly attempted yet for long-term data collection. The U.S. Department of Energy’s Community Research on Climate and Urban Science (CROCUS) project, led by Argonne National Laboratory, seeks to understand urban climate change and guide climate adaptation and mitigation strategies. As part of the CROCUS project, Chicago's first long-term urban flux site was installed at the University of Illinois Chicago (UIC) and aims to further understand the challenges of urban flux measurements. A 10-meter tower equipped with METEK Sonic anemometer, measuring 3D winds, and LI-COR infrared gas analyzer, measuring water vapor (H₂O) and carbon dioxide (CO₂) concentrations, were installed to measure exchanges of these gasses between the city and the atmosphere. A plug-in software for real-time flux data collection using the Waggle platform was developed. Preliminary data provides insights into the dynamics of H₂O, CO₂ fluxes along with key turbulent characteristics, and sensible heat flux in Chicago to better understand the interactions between the built environment and atmospheric processes under various weather conditions such as lake breezes and extreme temperatures, potentially helping the refinement and parameterizations of urban climate models. The long-term dataset will be useful in characterizing the impacts of applying the eddy covariance method in an urban setting, where theoretical assumptions of the method are violated, and elucidating pathways for improving and refining the application of the method for urban flux measurements. Data will be openly available for the researchers through the CROCUS repository at Environmental System Science Data Infrastructure for a Virtual Ecosystem (ESS-DIVE, https://data.ess-dive.lbl.gov/portals/crocus) data portal, to support the development of precise climate models for the CROCUS project, which focuses on urban climate impacts and addressing environmental justice in Chicago. This study will also aid collaborations with other Urban Integrated Field Laboratories (UIFLs) to create an integrated urban flux network in the US.