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Urban CO2 Dome (Salt Lake City, UT, USA) -- Summary
Anthropogenic activities are generally recognized as being responsible for the historic increase in the air's CO2 content. Hence, it is only logical that where people congregate there will be an increase in atmospheric CO2 concentration. This being the case, one might expect large population centers to exhibit significantly elevated near-surface atmospheric CO2 concentrations; and a number of studies substantiate this expectation, as we indicate here for one of them (Salt Lake City, Utah, USA), where the phenomenon has been studied in significant detail.

Pataki et al. (2003) measured atmospheric CO2 concentrations continuously - and carbon and oxygen isotopic composition weekly - 18 meters above the ground for a period of one year, after which the seasonal cycles of δ13C and δ18O were used to assess the relative contributions of natural gas combustion, gasoline combustion and biogenic respiration to the urban CO2 concentration in excess of the rural background concentration at different times of the year. This work revealed that urban CO2 concentrations were highest in the winter, with maximum nighttime values approaching 600 ppm. Nighttime average values, however, were considerably lower, ranging from approximately 390-480 ppm in the winter and from 375-400 ppm in the spring and summer. Afternoon values, on the other hand, were typically within 5 ppm of the rural background value of 372 ppm. As for the isotopic measurements, they revealed that approximately 60% of the winter urban-rural CO2 differential came from natural gas combustion, while 40% was derived from the burning of gasoline. This latter component remained a large portion of the urban CO2 dome in summer, while the natural gas component vanished and biogenic plant and soil respiration had their largest effect in the spring and late summer.

Pataki et al. (2005) measured the air's CO2 concentration and its isotopic (δ13C) composition at four locations in the Salt Lake Valley during a persistent "cold pool" event in the winter of 2004, when the air in the valley was trapped beneath an inversion that formed on the fifth of January and did not "mix out" until 20 days later. In doing so, they found that mean daily (24-hour) CO2 concentrations at the tops of four- and five-story buildings and at the tops of 4.5- and 9-meter-tall towers ranged from 382 to 527 ppm during the close-to-three-week measurement period. In addition, the δ13C data indicated that the major source of the cold-pool CO2 was the local combustion of gasoline and natural gas. Also, they found that the air's CO2 concentration was generally well correlated with its particulate matter concentration. As a result, they concluded that atmospheric CO2 concentrations, which are not commonly monitored in most urban areas at present, "can provide useful information regarding atmospheric transport and mixing in complex terrain such as mountain basins."

Between 15 Dec 2004 and 20 Jan 2005, Pataki et al. (2006) measured atmospheric CO2 concentration and its stable carbon isotope (δ13C) composition at a height of 18 meters above the ground on the campus of the University of Utah in Salt Lake City by means of tunable diode laser absorption spectrometry, conventional isotope ratio mass spectrometry, and infrared gas analysis. In terms of maximum CO2 concentrations observed, toward the end of the measurement period values as high as 600 ppm were recorded, coinciding with a major inversion event. On a diurnal basis, there was a pattern of "relatively larger contributions of natural gas combustion in early morning, pre-dawn hours representing about 60-70% of total fossil fuel-derived CO2, and smaller contributions of about 30-40% during late afternoon and evening rush hour," which findings, according to Pataki et al., are "consistent with greater natural gas use during cold nighttime hours and increased gasoline combustion during evening rush hour." In addition, they report there was a pattern of "decreasing relative contributions of natural gas combustion over [a] week-long measurement period that corresponded to increasing ambient air temperature," which the researchers say is likely due to "reduced natural gas usage for residential heating during a warming period."

This study demonstrated "for the first time," in Pataki et al.'s words, that "atmospheric measurements may be used to infer patterns of energy and fuel usage on hourly to daily time scales," and that they can provide "insight into urban energy use patterns and drivers." In addition, they shed further light on the origin of the urban CO2 dome, highlighting the major roles played by the heating of homes and other buildings by the burning of natural gas and the powering of cars and other vehicles by the burning of gasoline.

Last of all, Pataki et al. (2007) made spatially and temporally intensive measurements of atmospheric CO2 concentration across an urban-to-rural CO2 gradient comprised of Salt Lake City's downtown business district, a residential neighborhood, and a non-urbanized rural location within the Salt Lake Valley from 2004 to 2006, as well as measurements of CO2 and water isotopic composition at the same locations over a one-year period. This effort indicated that the highest CO2 concentrations, occasionally exceeding 500 ppm, were recorded during wintertime inversions, with the downtown site showing the highest CO2 concentrations at night, but exhibiting similar values to the neighborhood site in the daytime, while the rural site, according to the four researchers, "showed consistently lower and relatively constant values."

This pattern was also observed in the summer, although they found that "absolute CO2 concentrations were lower at all three sites, particularly during afternoon, well-mixed atmospheric conditions." In addition, they report that the "inverse analysis of CO2 sources and the O isotope composition of ecosystem respiration showed large contributions (>50%) of natural gas combustion to atmospheric CO2 in the wintertime, particularly at the city center, and large contributions (>60%) of biogenic respiration to atmospheric CO2 during the growing season, particularly at the rural site." Pataki et al. say these results demonstrate that "spatial and temporal patterns of urban CO2 concentrations and isotopic composition can be used to infer patterns of energy use by urban residents as well as plant and soil processes in urban areas," concluding that "these measurements have great potential for inferring patterns of human activities and biological respiration in complex, human-dominated ecosystems."

Pataki, D.E., Bowling, D.R. and Ehleringer, J.R. 2003. Seasonal cycle of carbon dioxide and its isotopic composition in an urban atmosphere: Anthropogenic and biogenic effects. Journal of Geophysical Research 108: 10.1029/2003JD003865.

Pataki, D.E., Bowling, D.R., Ehleringer, J.R. and Zobitz, J.M. 2006. High resolution atmospheric monitoring of urban carbon dioxide sources. Geophysical Research Letters 33: 10.1029/2005GL024822.

Pataki, D.E., Tyler, B.J., Peterson, R.E., Nair, A.P., Steenburgh, W.J. and Pardyjak, E.R. 2005. Can carbon dioxide be used as a tracer of urban atmospheric transport? Journal of Geophysical Research 110: 10.1029/2004JD005723.

Pataki, D.E., Xu, T., Luo, Y.Q. and Ehleringer, J.R. 2007. Inferring biogenic and anthropogenic carbon dioxide sources across an urban to rural gradient. Oecologia 152: 307-322.

Last updated 3 October 2007