Measuring nighttime CO2 flux over terrestrial ecosystems using eddy covariance and nocturnal boundary layer methods

The respiration from plants and soil is an important component of the carbon balance of ecosystems. Its measurement is challenging due to the relatively small size of the carbon dioxide fluxes and because these fluxes occur under environmental conditions that are frequently unfavorable for flux measurements. Micrometeorological techniques based on turbulent transfer frequently underestimate CO2 fluxes during nighttime conditions. An approach based on calculating the CO2 budget in the nocturnal boundary layer (NBL) might be an alternative during light wind conditions to estimate nighttime CO2 fluxes. This study presents typical net CO2 efflux observations measured at night over agricultural crops for several years near Ottawa, Ont. and over an old black spruce stand near Candle Lake, Sask. during the intensive field campaigns of the Boreal Ecosystem Atmosphere Study (BOREAS). We used the eddy covariance technique for windy nights and the NBL budget approach for calm nights. Criteria for screening data into either windy or calm conditions were made using the friction velocity (u∗) and the standard deviation of the vertical wind speed (σW). The threshold at which the 30 min turbulent CO2 flux observations were independent of u∗ or σW and had limited scatter were determined to be in the range 0.075–0.1 m s−1 for u∗ and σW, and about 1.5 m s−1 for horizontal wind speed (U) for multiple years of corn and soybean data. σW≥0.4 m s−1 was shown to be a good screening threshold over the black spruce canopy. Current methods for handling nocturnal CO2 data involve systematically replacing data during calm conditions where eddy covariance is deficient with those from windy conditions. This can lead to an overestimation of the nocturnal CO2 flux. We suggest a variation in this procedure which, through the screening of entire nights, allows the retaining of an acceptable proportion of calm periods within predominantly windy nights. The NBL budget method requires calm nights for measuring the respiration unless other budget terms are quantified. A good agreement was found between CO2 flux measured using the NBL approach when the NBL was well developed and the eddy covariance technique, when restricted to windy nights. The NBL profiles integrate a larger area than eddy covariance, which means that high-emission spots can be included with this kind of approach.