High-Resolution Photoacoustic Spectroscopy of the Oxygen A-Band

There have been many advances in recent years in remote sensing and ground-based measurement technologies utilizing optical detection to identify and quantify species in the atmosphere. Many of these instruments record high signal-to-noise spectra requiring sophisticated spectral modeling beyond the Voigt profile. In order to properly quantify the higher order spectral effects, high-resolution laboratory data measuring samples of known composition under carefully controlled conditions are required. The oxygen A-band is used in a number of atmospheric composition measurements due to the uniform, well-known concentration of oxygen throughout the atmosphere. Previous laboratory A-band measurements using cavity ring-down spectroscopy and Fourier transform spectroscopy have greatly improved the understanding of spectral parameters. However, current spectral models are insufficient to fit some high quality remote sensing data, such as the OCO missions. The largest spectroscopic uncertainties in modeling result from characterization of line mixing and collision-induced absorption. These collisional effects, resulting in small absorption changes in the baseline and wings, which become more prominent at elevated pressures can be accurately measured with photoacoustic spectroscopy, a background free measurement with a large dynamic range producing high signal-to-noise spectra. A novel high-resolution photoacoustic spectrometer was designed and constructed to improve the understanding of A-band spectral parameters to meet the OCO mission goals. 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 was also implemented to allow for measurements over the range of atmospherically relevant temperatures. Results from spectral fitting of data from the newly developed spectrometer provide the most accurate A-band pressure shift coefficients for both oxygen and air measured to date. The data also indicates the importance of lineshape profile choice for resonant absorption in order to accurately characterize line mixing and collision-induced absorption; the speed-dependent Nelkin-Ghatak profile is required for the current data set. Finally, preliminary fitting of line mixing and collision-induced absorption suggests the photoacoustic data achieves the required sensitivity to provide improved understanding of line mixing and collision-induced absorption based on fundamental physical principles.</p

PHOTOACOUSTIC SPECTROSCOPY OF THE O2 A-BAND IN SUPPORT OF REMOTE SENSING

Accurate spectroscopic models are required for remote sensing missions that use spectroscopic methods to interrogate atmospheric composition. The oxygen A-band (762 nm) is utilized for determination of air mass, solar pathlength and surface pressure in remote sensing applications due to the uniform concentration of molecular oxygen throughout the atmosphere and the spectral isolation of the band. NASA’s OCO-2 satellite seeks to retrieve atmospheric carbon dioxide concentrations with an accuracy of 0.25\%, placing stringent demands on our knowledge of the A-band spectral parameters. Current limitations in the A-band spectroscopic models, primarily from the treatment of line mixing (LM) and collision induced absorption (CIA), remain a significant source of error in carbon dioxide column retrievals. LM is manifested as an intensity exchange due to collisional population transfer between closely spaced energy levels while CIA appears as a broad, weak continuum absorption feature arising from transient dipoles induced by molecular collisions. Photoacoustic spectroscopy, a zero-background technique with a large dynamic range, is an ideal method to observe these effects which become increasingly prominent at elevated pressures. We have developed a high precision (SNR 10,000), broadband photoacoustic spectrometer for recording full A-band spectra at room temperature over a wide range of pressures (300-3000 Torr). Intensity exchange due to LM is observed in these unsaturated, high SNR spectra, and the weak baseline CIA profile can be extracted without interferences from instrumental background effects. Results from multispectrum fits of this data with non-Voigt line shapes showing insufficiencies in current A-band models will be presented

PHOTOACOUSTIC SPECTROSCOPY OF THE OXYGEN A-BAND

The oxygen A-band (760 nm) is used in a number of remote sensing applications due to the precisely known, uniform distribution of molecular oxygen throughout the atmosphere and the spectral isolation of the band. The A-band is used to determine the pathlength of solar radiation for OCO-2, a current NASA mission which seeks to measure the global sources and sinks of carbon dioxide at unprecedented spatial and temporal resolution. The goal of measuring atmospheric carbon dioxide concentrations with a precision of 0.25% requires a precise knowledge of line shape parameters. Currently, the most significant uncertainties in A-band spectroscopy result from line mixing and collision induced absorption, which become more prominent at elevated pressures. Photoacoustic spectroscopy is ideal to observe these phenomena due to the large dynamic range and zero-background advantages of the technique. Photoacoustic spectra of the oxygen A-band over a range of pressures will be presented in addition to line shape parameters extracted from multispectrum fits of the data

Measured in-situ mass absorption spectra for nine forms of highly-absorbing carbonaceous aerosol

Mass absorption coefficient spectra were measured between λ = 500 nm and 840 nm for nine forms of highly-absorbing carbonaceous aerosol: five samples generated from gas-, liquid- and solid-fueled flames; spark-discharge fullerene soot; graphene and reduced graphene oxide (rGO) crumpled nanosheets; and fullerene (C_(60)) assemblies. Aerosol absorption spectra were measured for size- and mass-selected particles and found to be dependent on fuel type and formative conditions. Flame-generated particles had morphologies consistent with freshly emitted black carbon (BC) with mass absorption coefficients (MAC) ranging between 3.8 m^2 g^(−1) and 8.6 m^2 g^(−1) at λ = 550 nm. Absorption Ångström exponents (AAE) – i.e. MAC spectral dependence – ranged between 1.0 and 1.3 for flame-generated particles and up to 7.5 for C_(60). The dependence of MAC and AAE on mobility diameter and particle morphology was also investigated. Lastly, the current data were compared to all previously published MAC measurements of highly-absorbing carbonaceous aerosol

Absorption Coefficient (ABSCO) Tables for the Orbiting Carbon Observatories: Version 5.1

The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO₂), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to oxygen (O₂) absorption coefficients (ABSCO) for the 0.76 μm A-band and the water vapor (H₂O) continuum model within the 1.6 μm and 2.06 μm CO₂ bands used within the Orbiting Carbon Observatory (OCO-2 and OCO-3) algorithm are described here. Updates in the O₂ A-band involve the inclusion of new laboratory measurements within multispectrum fits to improve relative consistency between O₂ line shapes and collision-induced absorption (CIA). The H₂O continuum model has been updated to MTCKD v3.2, which has benefited from information from a range of laboratory studies relative to the model utilized in the previous ABSCO version. Impacts of these spectroscopy updates have been evaluated against ground-based atmospheric spectra from the Total Carbon Column Observing Network (TCCON) and within the framework of the OCO-2 algorithm, using OCO-2 soundings covering a range of atmospheric and surface conditions. The updated absorption coefficients (ABSCO version 5.1) are found to offer improved fitting residuals and reduced biases in retrieved surface pressure relative to the previous version (ABSCO v5.0) used within B8 and B9 of the OCO-2 retrieval algorithm and have been adopted for the OCO B10 Level 2 algorithm

Absorption Coefficient (ABSCO) Tables for the Orbiting Carbon Observatories: Version 5.1

The accuracy of atmospheric trace gas retrievals depends directly on the accuracy of the molecular absorption model used within the retrieval algorithm. For remote sensing of well-mixed gases, such as carbon dioxide (CO₂), where the atmospheric variability is small compared to the background, the quality of the molecular absorption model is key. Recent updates to oxygen (O₂) absorption coefficients (ABSCO) for the 0.76 μm A-band and the water vapor (H₂O) continuum model within the 1.6 μm and 2.06 μm CO₂ bands used within the Orbiting Carbon Observatory (OCO-2 and OCO-3) algorithm are described here. Updates in the O₂ A-band involve the inclusion of new laboratory measurements within multispectrum fits to improve relative consistency between O₂ line shapes and collision-induced absorption (CIA). The H₂O continuum model has been updated to MTCKD v3.2, which has benefited from information from a range of laboratory studies relative to the model utilized in the previous ABSCO version. Impacts of these spectroscopy updates have been evaluated against ground-based atmospheric spectra from the Total Carbon Column Observing Network (TCCON) and within the framework of the OCO-2 algorithm, using OCO-2 soundings covering a range of atmospheric and surface conditions. The updated absorption coefficients (ABSCO version 5.1) are found to offer improved fitting residuals and reduced biases in retrieved surface pressure relative to the previous version (ABSCO v5.0) used within B8 and B9 of the OCO-2 retrieval algorithm and have been adopted for the OCO B10 Level 2 algorithm