Abstract

CO₂ transport processes relevant for estimating net ecosystem exchange (NEE) at the Niwot Ridge AmeriFlux site in the front range of the Rocky Mountains, Colorado, USA, were investigated during a pilot experiment. We found that cold, moist, and CO₂-rich air was transported downslope at night and ... Show moreCO₂ transport processes relevant for estimating net ecosystem exchange (NEE) at the Niwot Ridge AmeriFlux site in the front range of the Rocky Mountains, Colorado, USA, were investigated during a pilot experiment. We found that cold, moist, and CO₂-rich air was transported downslope at night and upslope in the early morning at this forest site situated on a ~5% east-facing slope. We found that CO₂ advection dominated the total CO₂ transport in the NEE estimate at night although there are large uncertainties because of partial cancellation of horizontal and vertical advection. The horizontal CO₂ advection captured not only the CO₂ loss at night, but also the CO₂ uptake during daytime. We found that horizontal CO₂ advection was significant even during daytime especially when turbulent mixing was not significant, such as in early morning and evening transition periods and within the canopy. Similar processes can occur anywhere regardless of whether flow is generated by orography, synoptic pressure gradients, or surface heterogeneity as long as CO₂ concentration is not well mixed by turbulence. The long-term net effect of all the CO₂ budget terms on estimates of NEE needs to be investigated. Show less