Atmospheric CO<sub>2</sub> measurements with a 2 μm airborne laser absorption spectrometer employing coherent detection

Gary D. Spiers; Robert T. Menzies; Joseph Jacob; Lance E. Christensen; Mark W. Phillips; Yonghoon Choi; Edward V. Browell

doi:10.1364/AO.50.002098

Applied Optics
Vol. 50,
Issue 14,
pp. 2098-2111
(2011)
•https://doi.org/10.1364/AO.50.002098

Atmospheric ${CO}_{2}$ measurements with a $2 μm$ airborne laser absorption spectrometer employing coherent detection

Gary D. Spiers, Robert T. Menzies, Joseph Jacob, Lance E. Christensen, Mark W. Phillips, Yonghoon Choi, and Edward V. Browell

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Gary D. Spiers, Robert T. Menzies, Joseph Jacob, Lance E. Christensen, Mark W. Phillips, Yonghoon Choi, and Edward V. Browell, "Atmospheric CO₂ measurements with a 2 μm airborne laser absorption spectrometer employing coherent detection," Appl. Opt. 50, 2098-2111 (2011)

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Abstract

We report airborne measurements of ${CO}_{2}$ column abundance conducted during two 2009 campaigns using a $2.05 μm$ laser absorption spectrometer. The two flight campaigns took place in the California Mojave desert and in Oklahoma. The integrated path differential absorption (IPDA) method is used for the ${CO}_{2}$ column mixing ratio retrievals. This instrument and the data analysis methodology provide insight into the capabilities of the IPDA method for both airborne measurements and future global-scale ${CO}_{2}$ measurements from low Earth orbit pertinent to the NASA Active Sensing of ${CO}_{2}$ Emissions over Nights, Days, and Seasons mission. The use of a favorable absorption line in the ${CO}_{2}$ $2 μm$ band allows the on-line frequency to be displaced two (surface pressure) half-widths from line center, providing high sensitivity to the lower tropospheric ${CO}_{2}$ . The measurement repeatability and measurement precision are in good agreement with predicted estimates. We also report comparisons with airborne in situ measurements conducted during the Oklahoma campaign.

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Parameter	Value
${CO}_{2}$ line center frequency	$4875.749 {cm}^{- 1}$
JPL LAS ON frequency JPL LAS OFF frequency	$4875.882 {cm}^{- 1}$ $4875.225 {cm}^{- 1}$
Laser output power	$100 mW$
Transmit/receive telescope apertures	$10 cm$ diameter
Receiver FOV (diffraction-limited)	$60 μrad$
Photomixer type	InGaAs
Receiver heterodyne frequency window	$10 - 20 MHz$
Signal digitization	$14 bits / 50 MHz$

Error Source	Uncertainty/Assumption	Measurement Error (% full column DAOD)	Measurement Error (ppmv ${CO}_{2}$ )	Comments
Atmosphere
Surface pressure	$1 hPa$	0.15	0.6	Eliminate by measuring DAOD in layers bounded by multiple-level overflights
Pressure at aircraft altitudes	$1.5 hPa$	0.22	0.9
Temperature profile	$2 K$	0.11	0.4	Applicable to $R (30)$ line
Water vapor mixing ratio: (1) conversion to dry ${CO}_{2}$ (2) $H_{2} O$ line interference	10% in “tropical” atmosphere (worst case)	0.1 0.1	0.4 0.4	Inferred ${CO}_{2}$ error can be $\sim 3$ times less for LEO than for low flying aircraft measurements, due to water vapor vertical profile
Aerosol and thin cloud spectral scattering/extinction		negligible	negligible	Negligible spectral variability of scattering within $Δ λ \sim 0.3 nm$
Surface
Surface elevation	$3 m$	0.1	0.4	Eliminate by measuring DAOD in layers bounded by multiple-level overflights
Aircraft Position
Aircraft altitude (GPS)	$3 m$	0.1	0.4	Relative error depends on layer thickness
Laser Transmitter
Power transmitted	0.5%	0.75	3	Use validator to monitor drifts
Frequency knowledge/control	$2 MHz$	0.08	0.3	On-line frequency offset from line center
System Gain Ratio Stability	0.5%	0.75	3	Use validator to monitor relative drift in channel 1/channel 2 system gain ratio
Pointing
Control/knowledge	$25 / 100 μrad$	0.01	negligible	Heterodyne drives this; N/A for aircraft
On-line/Off-Line Pointing Offset
Control/knowledge of along-track pointing offset			negligible	Use channel 1 and channel 2 heterodyne signal difference frequency as diagnostic

Parameter	Value
${CO}_{2}$ line center frequency	$4875.749 {cm}^{- 1}$
JPL LAS ON frequency JPL LAS OFF frequency	$4875.882 {cm}^{- 1}$ $4875.225 {cm}^{- 1}$
Laser output power	$100 mW$
Transmit/receive telescope apertures	$10 cm$ diameter
Receiver FOV (diffraction-limited)	$60 μrad$
Photomixer type	InGaAs
Receiver heterodyne frequency window	$10 - 20 MHz$
Signal digitization	$14 bits / 50 MHz$

Error Source	Uncertainty/Assumption	Measurement Error (% full column DAOD)	Measurement Error (ppmv ${CO}_{2}$ )	Comments
Atmosphere
Surface pressure	$1 hPa$	0.15	0.6	Eliminate by measuring DAOD in layers bounded by multiple-level overflights
Pressure at aircraft altitudes	$1.5 hPa$	0.22	0.9
Temperature profile	$2 K$	0.11	0.4	Applicable to $R (30)$ line
Water vapor mixing ratio: (1) conversion to dry ${CO}_{2}$ (2) $H_{2} O$ line interference	10% in “tropical” atmosphere (worst case)	0.1 0.1	0.4 0.4	Inferred ${CO}_{2}$ error can be $\sim 3$ times less for LEO than for low flying aircraft measurements, due to water vapor vertical profile
Aerosol and thin cloud spectral scattering/extinction		negligible	negligible	Negligible spectral variability of scattering within $Δ λ \sim 0.3 nm$
Surface
Surface elevation	$3 m$	0.1	0.4	Eliminate by measuring DAOD in layers bounded by multiple-level overflights
Aircraft Position
Aircraft altitude (GPS)	$3 m$	0.1	0.4	Relative error depends on layer thickness
Laser Transmitter
Power transmitted	0.5%	0.75	3	Use validator to monitor drifts
Frequency knowledge/control	$2 MHz$	0.08	0.3	On-line frequency offset from line center
System Gain Ratio Stability	0.5%	0.75	3	Use validator to monitor relative drift in channel 1/channel 2 system gain ratio
Pointing
Control/knowledge	$25 / 100 μrad$	0.01	negligible	Heterodyne drives this; N/A for aircraft
On-line/Off-Line Pointing Offset
Control/knowledge of along-track pointing offset			negligible	Use channel 1 and channel 2 heterodyne signal difference frequency as diagnostic

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Applied Optics