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Winter Night CO2 Concentrations in a Suburb of Tokyo
Reference
Moriwaki, R., Kanda, M. and Nitta, H. 2006. Carbon dioxide build-up within a suburban canopy layer in winter night. Atmospheric Environment 40: 1394-1407.

What was done
The authors investigated temporal changes in vertical profiles of wind, temperature and atmospheric CO2 concentration on 60 winter days within a low-storied (mean house height of 7.3 m) residential sector of Kugahara, Tokyo, Japan, via measurements made at various levels along a tower erected in the backyard of one of the sector's homes, starting at 0.7 m above ground level (agl) and ending at 29 m agl.

What was learned
On a diurnal basis, Moriwaki et al. report that the average CO2 concentration at 29 m agl ranged from 406 to 444 ppm, with the minimum value occurring at approximately 1400 hours and the maximum occurring around midnight, which values are to be compared to a background rural value of 380 ppm. During periods of calm and stably stratified conditions, however, midnight maximum concentrations sometimes exceeded 500 ppm. In addition, there was a small morning peak in CO2 concentration at about 0800 hours, which they attributed to "the increase of fossil fuel consumption in houses and traffic."

With respect to the vertical profile of CO2, the researchers report that above what they call the "suburban canopy," the air's CO2 concentration decreased rapidly with height. Within the canopy, however, they say that the CO2 concentration varied but little with height, "which indicates that the CO2 emitted from the houses accumulated within the canopy." More specifically, and noting that indoor-air CO2 concentrations sometimes exceed 1000 ppm, they opine that a nighttime cold-air subsidence flow from nocturnally-cooling rooftops brought down air parcels with high CO2 concentrations coming from the outlets of building ventilating fans located in the middle to upper parts of the canopy.

What it means
In order to correctly understand the origins of the urban CO2 dome in different situations, Moriwaki et al. suggest that "urban microclimate models should include both the three-dimensional turbulent flow around [buildings] and the source distributions to accurately describe the dynamical behavior and diffusion processes of the scalars within and above urban canopies."

Reviewed 4 October 2006