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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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We examined sensitivity and offset changes of a fast-response open-path gas analyzer in an Arctic environment. Output voltage from the gas analyzer was compared to water vapor density calculated from a slow-response thermometer and hygrometer, and then sensitivity and offset were determined. This procedure serves as an effective calibration of the gas analyzer. In this study, changes in sensitivity caused over- or underestimation of up to 10% in water vapor flux. An effective strategy was established for calibrating a gas analyzer in the field in the Arctic environment. The strategy comprises the following main points: 1) Parallel observation with a stable slow-response sensor is needed to calibrate a fast-response gas analyzer. The slow-response sensor should be calibrated as necessary. 2) The appropriate period length for applying the effective calibration should be determined, considering the data availability, the stability of sensors used, and the local environment, to obtain a statistically robust sensitivity and offset. 3) Rain events may cause a step change in offset, but not in sensitivity. Periods for applying effective calibration should be carefully determined in order to account for possible step changes in the sensitivity and offset. Additionally, this methodology may be applied to other environments in principle.

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