CO<sub>2</sub> flux estimation for a valley terrain using the atmospheric boundary layer method

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  • CO₂ flux estimation for a valley terrain using the atmospheric boundary layer method

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Abstract

For this study, we conducted nighttime upper-air observations in a complex valley terrain to test the applicability of the atmospheric boundary layer (ABL) method for CO2 flux estimation, comparing the obtained flux with that observed using the eddy covariance technique. Three different definitions for the determination of the nocturnal boundary layer height did not strongly affect the calculation of CO2 flux using the ABL method, which implies that the change in CO2 concentration near the surface strongly affects flux evaluations using the ABL method. The CO2 flux calculated using the ABL method was generally 2-5 times greater than the eddy CO2 flux at<0.5 m s-1 in the nighttime average horizontal wind velocity, which indicates that the advection from a 2-5 times broader surrounding area caused CO2 accumulation near the surface of the valley bottom, if a slight site-to-site variation in ecosystem respiration within the source area is assumed for the ABL observation. To incorporate advection terms, the equation for the ABL method was modified using the advection factor (AF), where AF was expressed as a linear function of the nighttime average horizontal wind velocity. The modified CO2 flux agreed well with the eddy CO2 flux, but the function of the AF itself is likely to have site-to-site variation. It must be normalized in future studies by consideration of other environmental factors, such as temperature and topographical features.

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