Carbon dioxide build-up within a suburban canopy layer in winter night [An article from: Atmospheric Environment]

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Description:
This study concerns the build-up of carbon dioxide, hereafter CO"2, within a suburban canopy under nocturnal stably stratified conditions. The results were derived from wintertime field measurements of the vertical profiles of CO"2, air temperature and turbulent exchange processes, in a residential area with a mean canopy height of 7.3m in Tokyo, Japan. In the daytime, especially under very windy conditions, the CO"2 concentration measured at a reference height of 29m a.g.l. was almost the same as the background CO"2 concentration (380ppmv). In contrast, during night-time the CO"2 concentration significantly increased under stably stratified conditions. We therefore examined the ensemble mean vertical profile of CO"2 concentration using the bulk Richardson number (Rb) as a stability index. Under stably stratified conditions (Rb>5), the CO"2 concentration above the canopy decreased with height such that the difference between within and above the canopy was as high as 40ppmv. In contrast, the CO"2 concentration within the canopy stayed at almost the same level, which indicates that the CO"2 emitted from the houses accumulated within the canopy. Such behavior was not found in the H"2O profile. We argue that these concentration profiles of CO"2 and H"2O are influenced by subsiding cold air from the rooftop, thus drawing the high CO"2 levels from ventilation fans down towards the ground.