High Resolution Broadband Photoacoustic Spectroscopy of the Oxygen A-band as a Key to Characterizing the Concentration of Carbon Dioxide in the Atmosphere

Anthropogenic carbon emissions result in increasingly elevation of atmospheric carbon dioxide(CO2) since the industrial revolution which result in a net increase in global temperatures. In the last decade NASA missions OCO-2 and OCO-3 have spectroscopically investigated the global sources and sinks of carbon dioxide with unprecedented spatial and temporal resolution. These missions take advantage of the near infrared (NIR) solar radiation that is reflected from Earth to the satellite, to quantify the dry air mole fraction of CO2. A high-resolution broad-band photoacoustic spectrometer (PAS) was developed in order to quantify the full absorption spectra of the oxygen A-Band as a function of pressure and temperature, with the precision to meet the needs of the OCO missions. The PAS has the advantage of zero background with a large dynamic range and high sensitivity. As a consequence, it allows for accurate measurements of both lineshapes and weak photo-physical effects line-mixing and collision-induced absorption (CIA). Results from spectral fitting of the photoacoustic data provide the most accurate A-band pressure shift coefficients for both oxygen and air measured to date. Measurements over a wide pressure range allow us to accurately characterize the effects of line mixing and collision-induced absorption. We have made the most precise measurements of the pressure shift coefficients and report on progress towards characterizing line-mixing and CIA in the A-Band, and compare to previous measurements and theoretical calculations.