Soil CO2 Flux Measurements -- Atmospheric CO2 Profile Measurements Using the LI-8100/8150 System
The LI-8100 Automated Soil CO2 Flux System has been designed to help answer the spatial and/or temporal variability questions that arise when performing soil CO2 flux measurements. When using the LI-8150 Multiplexer with the LI-8100 Analyzer Control Unit, atmospheric CO2 profile measurements can be performed in addition to soil CO2 flux measurements, with minimal added cost. This application note provides a brief theory on CO2 profiling measurements and explains the LI-8100 hardware requirements and setup as well as software configuration.
Eddy covariance and other micrometeorological techniques measure the flux of CO2 (FCO2) across a horizontal plane at the instrument height, while the research community is interested in understanding the net ecosystem CO2 exchange (NEE) of the underlying surface. When the boundary layer is well mixed, the flux from the surface is equal to the flux measured at the instrument level and we have the following: NEE = F CO 2 (1)
However, during periods with reduced exchange between canopy and the atmosphere (low winds, thermally stable conditions, transition periods, etc.) the total mass of CO2 in the air column beneath the instrument is not constant. For example, during low winds at night, CO2 can build up beneath the measurement level to be released later with a gust of wind or due to a change in thermal stratification from stable to unstable in the morning. Under such conditions, a CO2 storage term must be accounted for in order to estimate NEE from the eddy covariance flux measurements (Baldocchi et al., 2001).