Automated Photoacoustic Spectrometer for High Precision Measurement of Ambient Carbon Dioxide

Zachary D. Reed, Brent Sperling, Keith A. Gillis, Joseph T. Hodges, Roger D. van Zee, and James R. Whetstone


The increasing demand for higher spatial and temporal resolution in the greenhouse gas monitoring network motivates the development of accurate and reliable sensors that can be operated with minimal intervention. We have developed an automated sensor for high precision measurement of ambient carbon dioxide mixing ratios utilizing photo-acoustic spectroscopy. The sensor employs an intensity-modulated, fiber-amplified distributed feedback laser (DFB) operating near 1.57 m. The laser frequency is stepwise-tuned across a single CO2 spectral line and the absorption signal at each point is detected by a photoacoustic spectrometer. Carbon dioxide concentrations are determined by fitting the area of the resulting absorption profile using an automated Labview routine and comparing to the fitted area of a NIST-calibrated reference gas mixture. After initial calibration, no further reference gas is required. Precision of better than 0.24 ppm, or 1 part in 1500 of the nominal concentration in the atmosphere, has been demonstrated.

Field studies of ambient air are being conducted from Building 101 at NIST-Gaithersburg. Ambient air is dried by a Nafion counterflow drier, which reduces water concentrations to ~200 ppm and permits reporting of the dry-gas CO2 concentration. Initial studies of ambient air mixing ratios are reported.