Reference
Takahashi, H.A., Konohira, E., Hiyama, T., Minami, M., Nakamura, T. and Yoshida, N. 2002. Diurnal variation of CO2 concentration, Δ14C and δ13C in an urban forest: estimate of the anthropogenic and biogenic CO2 contributions. Tellus B 54: 97-109.
What was done
Working in an "urban forest" within Japan's fourth largest city (Nagoya, population in excess of two million people), the authors measured atmospheric CO2 concentrations over a 24-hour period on 9 February 1999 at heights of 1.55, 5.60, 9.10, 14.75, 19.00 and 23.75 meters at intervals of three hours at night and two hours during the day, while they measured the air's carbon isotopic composition (Δ14C and δ13C) at the highest level in the atmosphere and in soil-respired CO2 coming from the surface of the ground.
What was learned
As best we can determine from the graphical representations of Takahashi et al.'s data, the air's CO2 concentration was essentially the same (393 ppm) at all measurement levels at the time (1400) of their lowest daily concentrations, while it ranged from a low of 455 ppm (at 20.75 m) to a high of 464 ppm (at 1.55 m) at the time (2100) of the highest daily concentrations. In addition, the six researchers report that "the respective contributions of fossil-fuel burning and soil respiration to the total atmospheric CO2 ranged from 0.4 to 15.5% (2-70 ppm) and from 2.4 to 8.2% (13-32 ppm) at 23.75 m (just above the forest canopy)."
What it means
Noting that "the diurnal variation of the anthropogenic CO2 was the major cause of the total atmospheric CO2 variation, while the biogenic CO2 remained relatively constant throughout the day," Takahashi et al. confirmed what has been revealed by several other studies, conducted both before and after theirs (see Urban CO2 Dome in our Subject Index).