A Comparison of Infrared Gas Analyzers Above a Subalpine Forest

Abstract:

Infrared gas analyzers (IRGAs) are a key component in the
eddy-covariance measurement of water vapor and carbon dioxide exchange
between the surface and atmosphere. Historically, closed-path IRGAs
have been used for the fast (> 10 Hz) measurement of atmospheric H2O
and CO2. In order to use them in the field, these IRGAs were typically
housed in temperature-controlled enclosures or buildings that were
tens of meters away from the measurement location. This necessitated
the use of long tubing and pumps to bring the air sample to the IRGA
cell. Attenuation of H2O and CO2 fluctuations within the tubing was a
persistent problem with such a setup, especially for H2O. As an
alternative, open-path IRGAs have frequently been utilized, but the
key trade-offs with the open-path design are: (i) precipitation and
dew-related data gaps, and (ii) the need to account for WPL density
effects. Over the past five years a new type of closed-path IRGA has
emerged which is weather-proof, compact, and low-maintenance. Because
of its small size, short intake tubing can be used, which places the
sampling cell close to the sonic anemometer and reduces high frequency
signal loss. Two such IRGAs are the LI-COR LI-7200 and the Campbell
Scientific EC155, which is part of the CPEC200 eddy covariance system.

The Niwot Ridge AmeriFlux tower has used a LI-COR LI-6262 IRGA to
measure CO2 fluxes above a subalpine forest since November, 1998.
Starting in summer 2013, a LI-7200 (along with an open-path LI-7500)
were deployed at 21.5 m on the AmeriFlux tower. In Fall 2013, a
EC155/CPEC200 was added so that a side-by-side comparison between all
four IRGAs was possible. The preliminary results presented in our
study compare the CO2 and H2O mean and variance measured by each IRGA,
the vertical wind statistics from three side-by-side sonic
anemometers, as well as the corresponding spectra and cospectra from
these sensors as well as other important aspects of system
performance.