O2 A-Band Photoacoustic Spectroscopy

The goal of the oxygen A-band project is to provide high accuracy spectroscopic parameters based on fundamental collisional physics for the Orbiting Carbon Observatory-2 (OCO-2), a NASA satellite mission measuring global sources and sinks of carbon dioxide. Atmospheric carbon dioxide levels have been increasing exponentially since the Industrial Revolution leading to elevated surface temperatures due to the greenhouse effect. However, on average only about half of the emitted CO2 accumulates in the atmosphere each year. Understanding the sources and sinks of carbon dioxide is critical for understanding how the Earth will respond to future increases in CO2.

OCO-2 is equipped with three grating spectrometers that use reflected sunlight to measure two carbon dioxide bands (1.61 µm and 2.06 µm) as well the oxygen A-band (762 nm). Since oxygen is uniformly distributed throughout the atmosphere, the oxygen A-band measurements are used for determining air mass, solar pathlength and surface pressure, so errors in oxygen spectroscopy propagate through to the retrieved column CO2 values. The goal of OCO-2 is to retrieve the dry column CO2 mole fraction with 0.25% accuracy; currently, the largest spectroscopic uncertainties are line mixing (LM) and collision-induced absorption (CIA) in the A-band. Line mixing is the collisional transfer of energy between closely spaced energy levels and results in the transfer of intensity between absorption lines. Collision-induced absorption arises from the short-lived dipoles induced during a collision which can allow for the absorption of radiation. These collisional effects impact the spectra in the wings and baseline and become more prominent at elevated pressures. 

To improve these spectroscopic uncertainties in line mixing and collision-induced absorption, the Okumura group uses photoacoustic spectroscopy. The large dynamic range and zero-background advantages of this technique allow us to to measure unsaturated, high SNR spectra over a wide pressure range to quantify LM and CIA. The spectrometer is capable of measuring both the P and R-branches of the A-band up to J'=28 with a signal-to-noise ratio of 30,000 for pressures of 50-4,000 Torr. A temperature control system has also been implemented to allow for measurements over the range of atmospherically relevant temperatures. This project involves instrument development, data collection, and spectral fitting as well as collaborations with JPL and NIST.

References:

Eldering et al., The Orbiting Carbon Observatory-2: first 18 months of science data products, Science, 358 (6360), 2017: https://amt.copernicus.org/articles/10/549/2017/

E.M. Lunny, High-Resolution Photoacoustic Spectroscopy of the Oxygen A-Band, 2020: https://resolver.caltech.edu/CaltechTHESIS:06082020-132244698

Drouin et al., Multispectrum analysis of the oxygen A-band, J Quant Spectrosc Radiat Transf., 186 (118-138), 2017: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5103325/