The goal of the A-band project is to provide high accuracy spectroscopic parameters based on fundamental collisional physics for 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 observing reflected sunlight with the goal of retrieving the dry column CO2 mole fraction with 0.25% accuracy. The oxygen A-band is used for determination of air mass, solar pathlength and surface pressure due to the uniform concentration of oxygen throughout the atmosphere; therefore, errors in oxygen spectroscopy propagate through to the retrieved column CO2 values. Currently, the largest spectroscopic uncertainties are line mixing (LM) and collision induced absorption (CIA) in the A-band. These collisional effects impact the spectra in the wings and baseline and become more prominent at elevated pressures. Therefore, we leverage the large dynamic range and zero-background advantages of photoacoustic spectroscopy to measure unsaturated, high SNR spectra over a wide pressure range to quantify LM and CIA.