NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
1
James B. Abshire, G. James Collatz, Xiaoli Sun ,Haris Riris, Arlyn E. Andrews, Michael Krainak
Supported by:
ESTO ATI program, GSFC IR&D funding
GSFC DDF (initial seed funding)
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
2
CO2 1570nm Absorption Band and Line
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
3
Reference Channel - O2 Band and Lines
Calculated ground-space absorption forcandidate O2 lines.
HITRAN calculation of absorption inthe O2 A-Band near 761 nm for ahorizontal path of 100m at STP.
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
4
Satellite with active sounder can give global coverage at dawndusk sampling times
The CO2 flux and atmospheric concentrationvary significantly in both space and time andthe measurement requirements (precision andstability) are demanding.
Sun-synchronous Orbit – 1 month
600 km
550 km
Altitude (m)
Local Time (hr)
GCM Simulation (CSU)
• Diurnal variations depend on the type ofecosystem, and can be extremely large.
• Most diurnal variations are confined the lowestfew kilometers of the atmosphere.
• Sounder technique permits near dawn and dusksampling of diurnal cycle
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
5
Laser and Co2 spectroscopy Breadboard
Tunable seed laser
Er-doped Fiber amp
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
6
Tunable seed laser scans of CO2 lines(note calculations and measurements are at different pressures)
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
7
PMT cooler unit
8.7”x8.7”x13”
Test setup
PMT
LN2
Multichannel
Scaler (rcvr
Data collection)
HV Power
supply
Fiber
attenuator
CO2 Sounder Breadboard
Receiver Test Setup
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
8
Initial Demonstration of Measurement using Breadboard Receiver
Laser pulse
Noise cts
(dark andbackground~18cts/100us)
Signal + noise cts(~24 cts/100us)
Averagedover 1000 shots
Averagedover 25,000 shots(25 sec)
Cts from a single shot
Processing:
-Integrate all counts over the pulse period;
- Integrate all counts over the periodimmediately before the signal pulsefor background noise estimation
- Take the difference as the net signal.
NASA
ESTO
ATIP
Laser Sounder
for Remotely Measuring Atmospheric CO2 Concentrations
12/12/01
NASA Goddard - Laser Remote Sensing Branch
9
Laser Sounder Technique for Remotely Measuring Atmospheric CO2 Concentrations
Abshire, J B, Collatz, G, Sun, X ,Riris, H , Andrews, A E, Krainak, M
We are developing a lidar technique for the remote measurement of the tropospheric CO2 concentrations. Our goal is todemonstrate a technique and technology that will permit measurements of the CO2 column abundance in the lower troposphere fromaircraft at the few ppm level, with a capability of scaling to permit global CO2 measurements from orbit. Accurate remote sensingmeasurements of CO2 mixing ratio from aircraft and from space are challenging. Potential error sources include possible interferencesfrom other trace gas species, the effects of clouds and aerosols in the path, and variability in dry air density caused by pressure ortopographic changes. Some potential instrumental errors include frequency drifts in the transmitter and sensitivity drifts in the receiver.High signal-to-noise ratios are needed for estimates at the few ppm level.
Our laser sounder approach uses 3 laser transmitters to permit simultaneous measurement of CO2 and O2 extinction,and aerosol backscatter at 1064 nm in the same atmospheric path. It directs the co-aligned laser beams from the lidar toward nadir, andmeasures the energy of the laser backscatter from land and water surfaces. During each measurement period, the two narrow linewidthlasers are rapidly tuned on and off the selected CO2 and O2 absorption lines. The receiver records and averages the energies of thelaser echoes. The column extinction and column densities of both CO2 and O2 are estimated via the differential absorption lidartechnique. For the on-line wavelength, the side of the gas absorption line is used, which weights its measurements to 0-4 km in thetroposphere. Simultaneous measurements of O2 column abundance are made using an identical approach using an O2 line near 770nm. Atmospheric backscatter profiles are measured with the 1064 nm channel, which permits identifying and excluding measurementscontaining clouds.
Our technique has advantages over passive spectrometers in its high (MHz) spectral resolution and stability, the abilityto measure at night and in dim-light, a narrow measurement swath, and the ability to simultaneously detect and exclude measurementswith clouds or aerosols in the path. For space, our concept is a lidar measuring at nadir in sun-synchronous orbit. Using dawn and duskmeasurements make it possible to sample the diurnal variations in CO2 mixing ratios in the lower troposphere. A 1-m telescope is usedas the receiver for all wavelengths. When averaging over 50 seconds, a SNR of ~1500 is achievable for each gas at each on- and off-line measurement. Such a mission can furnish global maps of the lower tropospheric CO2 column abundance at dawn and dusk. Globalcoverage with an accuracy of a few ppm with a spatial resolution of ~ 50,000 sq. km appears achievable each month.
We have demonstrated some key elements of the laser, PMT detector and receiver in the laboratory, including stablemeasurements of CO2 line shapes in an absorption cell using a fiber amplifier seeded by a tunable diode laser. Our plans for the nextyear are to complete our laboratory work and demonstrate the performance of the CO2 channel over a horizontal path. We haveexamined the feasibility of an airborne instrument operating at a 5 km altitude. Using commercially available fiber lasers and a 10 cmreceiver lens appears sufficient for useful measurements with 5 second integration time.