Carbon dioxide build-up within a suburban canopy layer in winter night

This study concerns the build-up of carbon dioxide, hereafter CO2, within a suburban canopy under nocturnal stably stratified conditions. The results were derived from wintertime field measurements of the vertical profiles of CO2, air temperature and turbulent exchange processes, in a residential area with a mean canopy height of 7.3 m in Tokyo, Japan. In the daytime, especially under very windy conditions, the CO2 concentration measured at a reference height of 29 m a.g.l. was almost the same as the background CO2 concentration (380 ppmv). In contrast, during night-time the CO2 concentration significantly increased under stably stratified conditions. We therefore examined the ensemble mean vertical profile of CO2 concentration using the bulk Richardson number (Rb) as a stability index. Under stably stratified conditions (Rb>5), the CO2 concentration above the canopy decreased with height such that the difference between within and above the canopy was as high as 40 ppmv. In contrast, the CO2 concentration within the canopy stayed at almost the same level, which indicates that the CO2 emitted from the houses accumulated within the canopy. Such behavior was not found in the H2O profile. We argue that these concentration profiles of CO2 and H2O are influenced by subsiding cold air from the rooftop, thus drawing the high CO2 levels from ventilation fans down towards the ground.