Spectroscopic measurements of a CO2 absorption line in an open vertical path using an airborne lidar

Abstract

We used an airborne pulsed integrated path differential absorption (IPDA) lidar to make spectroscopic measurements of the pressure-induced line broadening and line center shift of atmospheric carbon dioxide (CO 2 ) at the 1572.335 nm absorption line. We scanned the lidar wavelength over 13 GHz (110 pm) and measured the absorption lineshape at 30 discrete wavelengths in the vertical column between the aircraft and ground. A comparison of our measured absorption lineshape to calculations based on HITRAN shows excellent agreement with the peak optical depth accurate to within 0.3%. Additionally, we measure changes in the line center position to within 5.2 MHz of calculations, and the absorption linewidth to within 0.6% of calculations. These measurements highlight the high precision of our technique, which can be applied to suitable absorption lines of any atmospheric gas.Optical remote sensing of trace gases in the atmosphere is widely used to understand terrestrial processes 1-4 . However, the accuracy of such remote sensing measurements can be limited by the accuracy of the spectroscopy of the absorption lines 5 . Lab-based precision laser spectroscopy measurements are difficult to validate in the open atmosphere with passive remote sensing instruments 4 owing to complications arising from clouds and aeorosols 6,7 and uncertainties in the air mass factor 8 . In this letter, we report laser absorption spectroscopy measurements of carbon dioxide (CO 2 ) made in an open vertical atmospheric path. Using an aircraftmounted pulsed lidar, we study the effect of pressure on the 1572.335 nm (6359.967 cm −1 ) line of the CO 2 vibrational-rotational band.The absorption spectra of atmospheric CO 2 has been studied by satellite 9,10 and ground-based passive spectrometers 11 , as a means to obtain the CO 2 distribution. IPDA measurements from aircraft have also been demonstrated 12-15 as part of efforts to develop spaceborne lidars. Converting absorption measurements to CO 2 concentrations requires precise spectroscopic knowledge, which is typically derived from controlled laboratory experiments [16][17][18][19] . Our present work extends spectroscopic investigations to conditions closer to those of atmospheric remote sensing.The lidar instrument, primarily designed for measuring the tropospheric CO 2 concentration in the column beneath the aircraft 15,20 , was flown aboard the NASA DC-8 during the ASCENDS 21 (Active Sensing of CO 2 Emissions over Nights, Days and Seasons) campaign in 2011, at altitudes ranging from 2 to 13 km above sea level. The 1572.335 nm absorption line was chosen based on its optimal line strength and temperature sensitivity 22 . A schematic of our lidar instrument is shown in FIG. 1(a) and the relevant system parameters tabulated (table I). Our lidar source was a distributed feedback diode laser amplified by an erbium-doped fiber amplifier (EDFA) operating in a master oscillator power amplifier configuration. The laser was gated with an acousto-optic modulator (AOM) to transmit a train of discrete puls...

Frequency-comb-based remote sensing of greenhouse gases over kilometer air paths

“…Absorption spectroscopy over open paths provides a means of remotely sensing changes in greenhouse gas mole fractionsa critical need for climate change science 1 . It is implemented in satellite instruments, upward-looking Fourier Transform Spectrometers (FTS), and ground-based FTS and laser spectrometers [2][3][4][5][6][7][8][9] . Ideally, these systems should detect the dry-air mole fractions (which correct for variable dilution by water vapor) of multiple gases over long paths with high precision and reproducibility to enable mapping of small gradients in both space and time.…”

Airborne measurements of CO<sub>2</sub> column concentrations made with a pulsed IPDA lidar using a multiple-wavelength-locked laser and HgCdTe APD detector

“…This provides several benefits including extracting line shape and some information on the vertical CO 2 distribution in the retrievals. It also allows solving for useful spectroscopic information, such as line center wavelengths, line widths and errors in the fits (Ramanathan et al, 2013). This approach also provides information that allows solving for several different measurement environmental variables and instrument parameters, such as Dopplershift and wavelength offsets, baseline tilts and wavelengthdependent instrument transmission.…”

Airborne Measurements of CO2 Column Concentration and Range Using a Pulsed Direct-Detection IPDA Lidar

“…Our work has shown this information is quite important to minimize instrumental biases in retrievals. It also allows solving for useful spectroscopic information, such as line center wavelengths, line widths and errors in the fits [30].…”