What was done
The authors measured atmospheric CO2 concentration, temperature and wind speed two meters above the ground every minute of most of an entire year (315 days of the year 2000) in a residential neighborhood of a suburb of Phoenix, after which they computed the implications of their results for CO2 concentrations in the center of the city on the basis of prior two-meter-high CO2 measurements made on various automobile transects of the metropolitan area (see Urban CO2 Dome in our Subject Index).

What was learned
The results suggested a mean cold-season maximum atmospheric CO2 concentration at the center of the city of approximately 620 ppm, which is 67% greater than the rural background mean of that time of year.  At the residential site, however, the urban-induced elevation of the near-surface atmospheric CO2 concentration was only 33%; and averaged over the entire night, this enhancement dropped to 25% in the cold season and 11% in the warm season, while over the complete daylight period it averaged just over 10% in both seasons.  CO2 concentrations were greater on weekdays than on weekends from 0415 to 0830 in the warm season and from 0445 to 1045 in the cold season.  The maximum weekday-weekend CO2 differential was 40 ppm in the cold season and 22 ppm in the warm season.

It was also learned, in the words of the authors, that "the primary controlling factors of the strength of the CO2 dome are (1) the presence of air temperature inversions at night and in the early morning, which inversions trap vehicular-generated CO2 near the ground, increasing its concentration there, and (2) solar-induced convective mixing during the mid-day period, which greatly dilutes the air's CO2 content near the ground."  Secondary controlling factors identified in the study were (a) wind speed and (b) wind direction, particularly in relation to the location of primary sources of CO2 (freeways and major roads) and areas of pristine rural air.

What it means
Near-surface atmospheric CO2 concentrations in large cities may often be much higher than those of surrounding rural areas; but they tend to drop off rapidly with distance from the city-center and major roadways.  Nevertheless, CO2 enrichment of the air in many urban and suburban locations may be large enough to provide plants that grow there with significant protection against the debilitating effects of local air pollution (see, for example, Deepak and Agrawal (2001) and Ozone (Effects on Plants).

Reference
Deepak, S.S. and Agrawal, M.  2001.  Influence of elevated CO2 on the sensitivity of two soybean cultivars to sulphur dioxide.  Environmental and Experimental Botany 46: 81-91.