We report progress in assessing the feasibility of a new satellite-based laser-sounding instrument to measure CO2 and other trace gas abundances in the lower troposphere from space

Abshire, James B.

Allan, Graham

Chen, Jeffrey

Kawa, S. Randy

Mayo, Jian-Ping

Riris, Haris

Stephen, Mark

Sun, Xiaoli

Weaver, Clark

Wilson, Emily

NASA Technical Reports Server

Laser Sounder Approach for Global Measurement of Tropospheric C02  
Mixing Ratio from Space 
James B. Abshire, Haris Riris, Graham ~ l l an ' ,  S. Randy Kawa, Jian-Ping illao2, Emily Wilson, Mark 
Stephen, Jeffrey Chen, Xiaoli Sun, Clark Weaver3 
iVASA-Goclrlard Space Flight Center Scierzce and Tecimology Directorates 
Corles 690, 613.3, and 554, Greenbelt MD 20771 USA , James.Abshire@gsfc.nasa.gov 
I 
- Sigrtza Space Im., NASA Goddard Code 694, Greertbelt M D  20771 USA 
- RSIS Inc. 165 1 Old Meadow Road McLean VA 22102 USA 
' - GEST/C'MBC, NASA Goddard Code 613.3, Greenbelt ,MD 20771 USA 
Abstract: We report progress in assessing the feasibility of a new satellite-based laser-sounding instrument to 
measure C02 and other trace gas abundances in the lower troposphere from space. 
1. INTRODUCTION 
C02  measurements from ice cores show that 
atmospheric C02  concentrations are higher now than 
they have been in the past 400,000 years. It is 
becoming increasingly important to understand the 
nature and processes of the C02 sinks, on a global 
scale, in order to make predictions of future 
atmospheric composition. Accurate measurements of 
tropospheric C02 abundance with global-coverage, 
300 km spatial and monthly temporal resolution are 
needed to quantify processes that regulate C02 
storage by the land and oceans [I]. 
The NASA Orbiting Carbon Observatory (OCO) is 
the first space mission focused on atmospheric C02 
for measuring total column C02 and O2 by detecting 
the spectral absorption in reflected sunlight. The 
OCO mission will yield important new information 
about atmospheric C02  distributions. However there 
are unavoidable limitations imposed by its 
measurement approach. These include best accuracy 
only during daytime at moderate to high sun angles, 
interference by cloud and aerosol scattering, and 
limited signal from C02  variability in the lower 
tropospheric C02  column. The recent NRC Decadal 
Survey for Earth Science [2] has recommended 
addressing these un-met needs in a laser-based C02 
measuring mission called ASCENDS. 
We have been in developing a laser technique for the 
remote measurement of the tropospheric COz 
concentrations from orbit [3-61. Our initial goal is to 
demonstrate a lidar technique and instrument 
technology that will permit measurements of the C02 
column abundance in the lower troposphere from 
aircraft at the few ppm level. Our final goal is to 
develop a space instrument and mission approach for 
active C 0 2  measurements. This would allow 
continuous measurements of C02  mixing ratio from 
orbit, both day and night, over land and ocean 
surfaces, and under realistic atmospheric scattering 
conditions. 
2. APPROACH 
Previous and some ongoing efforts to develop laser 
instruments for measuring atmospheric C02 have 
used the 4.88 um [7] and 2 um [8-111 bands. Our 
approach is to use the 1570nm band and a dual 
channel laser absorption spectrometer (ie DIAL lidar 
used an altimeter mode), which continuously 
measures at nadir from a near polar circular orbit 
(Figure 1). 
Measurss: 
- COZ tropospher~ miumn 
- 02  tropospheric mlumn 
-Cloud backscanering profile L-' ' 
SSokm aithude 
- 
sun sync arM 
COLA-15% GOAS 
02 i,nm 
a a U d S s n d n d  i - t m  
Figure 1- Measurement concept for space-based C02 Laser 
Sounder. 
We use several tunable fiber laser transmitters 
allowing simultaneous measurement of the 
absorption from a C02 absorption line in the 1570 nm 
band [12] , O2 extinction in the oxygen A-band, and 
surface height and aerosol backscatter in the same 
https://ntrs.nasa.gov/search.jsp?R=20090001869 2019-08-30T05:55:34+00:00Z
measurement path. The approach directs pulsed co- 
aligned laser beams from the instrument's lasers 
toward nadir, and measures the energy of the laser 
echoes reflected from land and water surfaces. 
During the measurement the lasers are stepped across 
a selected C02  line near 1572 nm and a pair of O2 
lines near 765 nm lines at kHz rates. 
The lasers are a MOPA architecture using tunable 
diode seed lasers and fiber amplifiers, and have 
spectral widths much narrower than the gas 
absorption lines. The receiver uses a -1.5-m 
diameter telescope and photon counting detectors 
[13], and measures the background light and energies 
of the laser echoes from the surface along with 
scattering from any clouds and aerosols in the path. 
The gas extinction and column densities for the C02 
and O2 gases are estimated from the ratio of the on- 
and off-line signals via the differential optical 
absorption technique. Pulsed laser signals and time 
gating are used to isolate the laser echo signals from 
the surface, and to exclude photons scattered from 
clouds and atmospheric aerosols. 
The 1570 nm C02 band 1131 shown in Figure 2 is 
well suited for this measurement. It is largely free 
from interference, has absorption lines with the 
needed temperature insensitivity and strengths [14], 
and is within the spectral range of high power lasers 
and sensitive photon counting detectors. 
Wovelen th (nm) 
1580 1535 1570 
6320 6330 6340 6350 6360 6370 6380 
Wove Number (cm-') 
Figure 2 - 2-way atmospheric absorption from space for 
the 1570 nm C02 band (from HITRAN). 
sensitivity to C 0 2  changes in the boundary layer 
where variations caused by surface sources and sinks 
are largest. Simultaneous measurements of total 
pressure use a pair of lines in the Oxygen A-band 
near 765 nm. Laser alt~metry and atmospheric 
backscatter profiles are also measured 
simultaneously, which permits determining the 
surface height and measurements made to cloud tops 
and through aerosol layers. 
The laser sounder approach has some fundamental 
advantages over measurements with passive sensors 
using reflected sunlight. It measures gas absorption in 
a common nadirlzenith path and the narrow laser 
divergence produces small laser footprints. The laser 
sources allow measurements In sunlight and darkness 
allowing global coverage. It can measure 
continuously over the ocean, to cloud tops and 
through broken clouds. The lasers are pulsed and 
potential measurement errors from scattering from 
clouds and aerosols are greatly reduced by using time 
gating in the receiver. Nonetheless, the optical 
absorption change due to a few ppm change in C02 is 
quite small, <l%, which makes achieving 
measurement sensitivity and stabilities challenging. 
Signal-to-noise ratios and measurement stabilities of 
> 700:l are needed to allow C02 mixing ratio 
estimates at the few ppm level. 
We have calculated several characteristics of the 
technique, and have demonstrated key aspects of the 
laser, detector and receiver approaches in the 
laboratory. We have also measured O2 over a 206 m 
long path and C02  over 206m, 400m, 1.3 and 2.2 km 
long open horizontal paths [6] using a breadboard 
version of the sensor (Figures 3-5). 
S c a n  of Oxygen A-Band Doublet 
10-5-07 
Our technique uses the on-line wavelengths tuned to 
the sides of the gas absorption line. This exploits the 
atmospheric pressure broadening of the gas lines to 
weight the measurement sensitivity to the 
atmospheric column below 5 km. This maximizes 
Wavelength ( m i )  
Figure 3 - Measurements of 02 line pair using breadboard 
02 sensor in van over 220 m hortzontal path compared to 
calculated line shapes from HITRAN. 
Figure 4 - Measured series of CO, absorption from with 
breadboard sensor from lab over 405 m honzontal path 
over 24 hr time span in May 2007 compared to end-point 
C 0 2  measurements made at top of laboratory building. 
Laser Sounder lleasured C02  Absorption 11-15-07 
compared to Weighted C 0 2  Concentration measured at Erie Tower 
Mountain Standard Time (i3:H.M) 
318:1800 319W:W) 
I h I I 4sa < 
" 
.- +- 0 & 14.5 428 a 
8 2 14 410 2 
v € E 13.5 lo0 
2 D 
13 390 ' 5 u3 iii
12s 3w 
I! 
- 
318:18:00 31Y:OO:W 319:@6:00 - 
Universal Time (D:H:M) UTC 
Figure 5 - Preliminary analysis of C02 absorption 
measurements made using breadboard sensor in van over 
1.3 km slant path to side of NOAA C02 tower in Erie CO, 
compared to estimate from in-situ readlngs from tower. 
Measurement date was 11-15-07. 
We will describe more details of the approach and 
our measurements in the talk. 
3. ACI(N0WLEGEMENTS 
This work is supported by the NASA EST0 IIP 
program and by the NASA-Goddard IRAD program. 
REFERENCES 
2. National Research Council, "Earth Sc~ence and 
Applications from Space: National Imperatives for 
the Next Decade and Beyond," Jan. 2007, available 
from http://www.nap.edul. 
3. J.B. Abshire, et al." Laser Sounder Technique for 
Remotely Measuring Atmospheric C02 
Concentrations", Eos Trans. AGU, 82(47), Fall 
Meet. Suppl., Abstract GC32A-022 1 ,  December 
2001. 
4. M.A. Krainak, et al.,"Measurements of 
atmospheric C02  over a horizontal path using a 
tunable-diode-laser and erbium-fiber-amplifier at 
1572 nm," CLEO '03, June 2003. 
5. J.B. Abshire, et al, "Laser Sounder for Global 
Measurement of C02  Concentrations in the Lower 
Troposphere from Space: Progress," Fall AGU 
2004, Paper SF43A-0783, December 2004. 
6. J.B. Abshire et al., "Laser Sounder for Global 
Measurement of C02  Concentrations in the 
Troposphere from Space," 2007 EGU General 
Assembly, Abstract 2007-A-10014, Vienna Austria, 
April 2007. 
7. J.L. Bufton et al., Appl. Optics 22 (17), 2592 
(1983). 
8. N. Sugimoto et al., Appl. Optics 32 (33), 6827 
(1993). 
9. Ismail, S., et al., " A 2-micron DIAL system for 
profiling atmospheric C02", International Laser 
Radar Conference, Proceedings, (2002). 
10. Ismail, S. et al., "Technology development for 
tropospheric profiling of C02 and ground-based 
measurements", International Laser Radar 
Conference, Proceedings, (2004) 
1 I. Koch, G., et al., Coherent differential absorption 
lidar measurements of COY, Appl. Opt., 43(26), 
5092, (2004). 
12. J. Henningsen et al., J.  Mol. Spectrosc. 203 (I), 
16 (2000). 
13. X. Sun et al., CLEO 2002, paper CFF3 (2002). 
14. J. Mao et al., "Sensitivity Studies for a Space- 
based C02  Laser Sounder Development", 2007 
EGU General Assenzbly, Abstract 2007-A-1 1150, 
Vienna Austria, April 2007. 
1. R.J. Engelen et al., J. of Geoplzysical Res.- 
Atnzospheres 106 (D 17): 20055 (2001). 


Laser Sounder Approach for Global Measurement of Tropospheric CO2 Mixing Ratio from Space

Laser Sounder Approach for Global Measurement of Tropospheric CO2 Mixing Ratio from Space

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