First Studies to Document Urban CO2 Dome
Central to the CO2-induced global warming hypothesis is the notion that man is responsible for nearly all of the documented increase in atmospheric carbon dioxide concentration over the last two centuries, due to the accelerated burning of fossil fuels that accompanied the human population explosion over that time period. This being the case, one might expect that large population centers would exhibit elevated levels of CO2. Two recent studies provide evidence that substantiates this hypothesis.
In the first of these studies, Idso et al. (1998a) measured air temperatures, relative humidities, and atmospheric CO2 concentrations at a height of 2 m at approximate 1.6-km (1.0-mile) intervals prior to sunrise and in the middle of the afternoon across four transect routes through the metropolitan area of Phoenix, Arizona during a five-day period in January of 1998. The data revealed the presence of what the authors called an "urban CO2 dome," where concentrations in the city center reached as high as 555 ppm. Concentrations diminished from the city center toward the outlying rural areas, where they were around 370 ppm. Pre-dawn CO2 values inside the dome were found to be considerably higher than mid-afternoon values, and temperature and relative humidity were found to have little influence on the magnitude or location of the CO2 dome.
In the second study, Idso et al. (1998b) repeated the measurement program described above during the hottest period of the year in an effort to determine whether or not the winter relationship between urban and rural near-surface atmospheric CO2 concentrations would apply to Phoenix in the summer. 
Although maximum and minimum daily air temperatures were 24°C warmer, and wind speeds nearly two times greater than they were in the winter, near-surface atmospheric CO2 concentrations varied but little between the two times of year. Fine-scale measurements of atmospheric CO2 concentration measured every three seconds while traveling from the eastern edge of the city through the center of the metropolitan area to the western edge of the city (see accompanying figure) reveal the complexity of the urban CO2 dome with its several localized peaks and valleys.
These two pioneering studies are the first to document a large and substantial enhancement in urban CO2 concentrations, with peak concentrations at the city center fully 50% higher than the surrounding rural mean, and localized spikes over 70% greater. These findings also suggest that cities may well serve as analogues of the world as a whole within the context of impending global change; for in addition to the analogy between global warming and the urban heat island (Changnon, 1992), there now appears to be a similar relationship between anticipated increases in the global background level of atmospheric carbon dioxide and the urban CO2 dome.
In terms of direct impact, the urban CO2 dome should enhance the robustness of urban vegetation, given the well-documented fact that atmospheric CO2 enrichment tends to enhance plant growth rates and increase the efficiency with which plants utilize water to produce organic matter (see Growth Response to CO2 in our Subject Index). In addition, elevated urban CO2 levels should reduce the deleterious effects of airborne pollutants on plant health (Allen, 1990) by reducing the apertures of the stomatal openings by which pollutants gain entry into plant leaves (Pallas, 1965; Kimball and Idso, 1983). As a result, within an urban CO2 dome, some of the positive effects of one of the major end products (CO2) of urban combustion processes tend to counteract one of the negative effects of some of the minor by-products (air pollutants) of those same processes.
Because the urban CO2 dome is a newly recognized phenomenon, there is much that remains to be learned about it. For example, our Fact Sheet is presently unable to broach the subject of how large it is in other cities, or whether or not it is a well-defined function of population. In addition, we do not know whether it is different for first and third world cities, nor is its vertical extent defined. As additional studies on the phenomenon make their way into the literature, we will update the contents of this Fact Sheet.
References
Allen, L.H. Jr. 1990. Plant responses to rising carbon dioxide and potential interactions with air pollutants. Journal of Environmental Quality 19: 15-34.
Changnon, S.A. 1992. Inadvertent weather modification in urban areas: lessons for global climate change. Bulletin of the American Meteorological Society 73: 619-627.
Idso, C.D., Idso, S.B. and Balling, R.C., Jr. 1998. The urban CO2 dome of Phoenix, Arizona. Physical Geography 19: 95-108.
Idso, C.D., Idso, S.B., Idso, K.E., Brooks, T., and Balling, R.C. 1998. Spatial and temporal characteristics of the urban CO2 dome over Phoenix, Arizona. Preprint volume of the 23rd Conference on Agricultural & Forest Meteorology, 13th Conference on Biometeorology and Aerobiology, and 2nd Urban Environment Symposium, pp. 46-48. American Meteorological Society, Boston, MA.
Kimball, B.A. and Idso, S.B. 1983. Increasing atmospheric CO2: Effects on crop yield, water use and climate. Agricultural Water Management 7: 55-72.
Pallas, J.E. 1965. Transpiration and stomatal opening with changes in carbon dioxide content of the air. Science 147: 171-173.