Atmospheric carbon dioxide concentration within a narrow valley in a forested catchment area

In order to investigate atmospheric carbon dioxide (CO2) accumulation as a cool air pool in hilly terrain, two-dimensional cross-sectional profile of CO2 concentration and air temperature was measured for 20 days in summer in a 0.8-ha forested catchment. CO2 concentration was vertically stratified within the valley, with a slight increase near valley sidewalls compared to the valley center, except during the nights under calm or weak ambient wind conditions. In the daytime, the constantly higher CO2 concentration at the valley bottom was associated with temperature inversion below the canopy, which could suppress vertical CO2 dissipation and leaded to vertical stratification, and with limited CO2 consumption by photosynthesis due to insufficient radiation. In the nighttime, the vertical CO2 concentration difference within the valley was related to strength of temperature inversion, which was correlated with ambient wind speed. On windy nights, relatively warmer ambient wind covered the cooler valley, to form a temperature inversion, thus confining CO2 to the lower part of the valley. On calm nights, thermal lapse not the inversion layer resulted from radiative cooling of the canopy, and homogeneous CO2 concentration within the valley was observed. A large CO2 storage change within the valley was calculated compared with that of forests on flat terrain. In particular, the vertical change of CO2 storage calculated with only the center of the valley was much larger than the cross-sectional change of CO2 storage with the entire valley due to excluding of side parts of the valley with lower CO2 concentration.