Do open-path Eddy Covariance CO2 Analyzers Need a Spectroscopic
Correction for Fast Temperature Fluctuations in the Optical Path?
Abstract
Carbon dioxide is a greenhouse gas that has a strong absorption in the
4.2– 4.3 micrometers region of the infrared spectrum. Consequently,
non-dispersive infrared (NDIR) spectroscopy using interference optical
filters tuned in this spectral band can be utilized to provide reliable,
high resolution and fast response measurements of atmospheric CO2
concentrations. As part of eddy covariance systems, open-path gas
analyzers based on this principle are widely used in remote locations
around the world because of their low-power consumption, fast frequency
response, and ease of operation. One of the challenges of the open-path
design is that the in-situ optical beam is exposed to the rapid
fluctuations in ambient temperature. Besides gas composition and
pressure being the two major spectroscopic line broadening mechanisms
that affect the absorption of infrared light, air temperature also can
influence the broadened half-width and the intensity of the spectral
lines. Consequently, the fast temperature fluctuations of the air
parcels in the optical path of such a sensor can produce changes in the
amount of absorbed light and cause errors in the gas concentration
measurement that can propagate into the flux calculations. The
temperature dependence of the infrared absorption has not been
quantified in the context of the CO2 NDIR gas analyzer methodology. The
study will evaluate the temperature effects on absorption spectra of
CO2-air-mixtures across the 4.2–4.3 micrometers infrared active region,
typically used by NDIR gas analyzers. Infrared spectra will be modeled
line-by-line from spectral-line parameters obtained from the
high-resolution transmission molecular spectroscopic database (HITRAN).
HITRAN-predicted molecular cross sections, the product of component
spectral line intensity and spectral line shape at different wavelength,
will be used to generate absorption spectra of CO2 air mixtures at
ambient pressure using different concentrations and temperatures. The
temperature dependence of CO2 absorption will be inferred from the
integrated area under the absorptivity curve. Results interpreted in the
context of the Beer-Lambert law will further characterize the
temperature related spectroscopic effects on CO2 concentration
calculations.