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Tellus (2010) Tellus B C
2010 International Meteorological Institute in Stockholm
No claim to original US government works
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TELLUS
Pulsed airborne lidar measurements of atmospheric
CO2column absorption
By JAMES B. ABSHIRE1∗, HARIS RIRIS1, GRAHAM R. ALLAN2, CLARK J. W EAVER3,
JIANPING MAO3,XIAOLI SUN1, WILLIAM E. HASSELBRACK2, S. RANDOPH KAWA1and
SEBASTIEN BIRAUD4,1NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA; 2Sigma Space
Inc., Lanham, MD 20706, USA; 3Goddard Earth Sciences and Technology Center, University of Maryland Baltimore
County, Baltimore, MD 21228, USA; 4Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
(Manuscript received 29 December 2009; in final form 22 July2010)
ABSTRACT
We report initial measurements of atmospheric CO2column density using a pulsed airborne lidar operating at 1572 nm.
It uses a lidar measurement technique being developed at NASA Goddard Space Flight Center as a candidate for the
CO2measurement in the Active Sensing of CO2Emissions over Nights, Days and Seasons (ASCENDS) space mission.
The pulsed multiple-wavelength lidar approach offers several new capabilities with respect to passive spectrometer and
other lidar techniques for high-precision CO2column density measurements. We developed an airborne lidar using a
fibre laser transmitter and photon counting detector, and conducted initial measurements of the CO2column absorption
during flights over Oklahoma in December 2008. The results show clear CO2line shape and absorption signals. These
follow the expected changes with aircraft altitude from 1.5 to 7.1 km, and are in good agreement with column number
density estimates calculated from nearly coincident airborne in-situ measurements.
1. Introduction
Atmospheric CO2is presently understood as the largest anthro-
pogenic forcing function for climate change, but there is con-
siderable uncertainty about the global CO2budget. Accurate
measurements of tropospheric CO2abundances are needed to
study CO2exchange with the land and oceans. To be useful
in reducing uncertainties about carbon sources and sinks the
atmospheric CO2measurements need to have high resolution,
with ∼0.3% precision (Tans et al., 1990; Fan et al., 1998). The
GOSAT mission (Yokota et al., 2004) is making new global CO2
measurements from space using a passive spectrometer and sur-
face reflected sunlight. However sun angle limitations restrict
its measurements to the daytime primarily over mid-latitudes.
A concern for measurement accuracy with passive instruments
is optical scattering from thin clouds in the measurement path
(Mao and Kawa, 2004; Aben et al., 2007). Optical scattering in
the measurement path modifies the optical path length and thus
the total CO2absorption viewed by the instrument. For mea-
∗Corresponding author.
e-mail: James.B.Abshire@nasa.gov
DOI: 10.1111/j.1600-0889.2010.00502.x
surements using spectrometers with reflected sunlight optical
scattering can cause large retrieval errors even for thin cirrus
clouds (Uchino et al., 2009).
To address these issues, the US National Research Coun-
cil’s 2007 Decadal Survey for Earth Science recommended a
new space-based CO2measuring mission called Active Sens-
ing of CO2over Nights, Days, and Seasons, or ASCENDS (US
NRC, 2007). The goals of the ASCENDS mission are to pro-
duce global atmospheric CO2measurements with much smaller
seasonal, latitudinal, and diurnal biases by using the laser absorp-
tion spectroscopy measurement approach. The mission’s goals
are to quantify global spatial distribution of atmospheric CO2
with 1–2 ppm accuracy, and quantify the global spatial distri-
bution of terrestrial and oceanic sources and sinks of CO2on
1-degree grids with 2–3 week time resolution. The ASCENDS
approach offers continuous measurements over the cloud-free
oceans, at low sun angles and in darkness, which are major
improvements over passive sensors. ASCENDS mission orga-
nizers held a workshop in 2008 to better define the science
and measurement needs and planning for future work (NASA,
2008). ESA has also conducted mission definition studies for
a similar space mission called A-SCOPE (ESA, 2008; Durand
et al., 2009). Although the ASCENDS mission concept requires
Tellus (2010) 1
PUBLISHED BY THE INTERNATIONAL METEOROLOGICAL INSTITUTE IN STOCKHOLM
SERIES B
CHEMICAL
AND PHYSICAL
METEOROLOGY
2 J. B. ABSHIRE ET AL.
a simultaneous dry air column measurement, the A-SCOPE mis-
sion concept does not. The lidar sensitivity and spectroscopic
analyses performed as part of the A-SCOPE definition activi-
ties have been recently published (Ehret et al., 2008; Caron and
Durand, 2009).
2. Candidate lidar approach for ASCENDS
NASA Goddard Space Flight Center has been developing a
pulsed lidar approach for the measurement of atmospheric
CO2concentrations as a candidate for the ASCENDS mission
(Abshire et al., 2001, 2007; Riris et al., 2007). The approach
uses a dual band pulsed laser absorption spectrometer and the
integrated path differential absorption (IPDA) lidar technique
(Measures, 1992). The instrument concept uses two tunable
pulsed laser transmitters allowing simultaneous measurement
of the absorption from a CO2absorption line in the 1570 nm
band, O2absorption in the oxygen A-band, and surface height
and atmospheric backscatter in the same path. A tunable laser
is stepped in wavelength across a single CO2line for the CO2
column measurement, while simultaneously a laser is stepped
across a pair of lines near 765 nm in the Oxygen A-band for an
atmospheric pressure measurement (Stephen et al., 2007, 2008).
Both lasers are pulsed at a ∼8 kHz rate, and the two absorption
line regions are repeatedly sampled at typically ∼1kHz.Both
laser transmitters utilize tunable diode lasers followed by laser
fibre amplifiers. The direct detection receiver measures the time
resolved laser backscatter from the atmosphere and the surface.
After suitable averaging, the gas extinction and column densi-
ties for the CO2and O2gases are estimated from the sampled
wavelengths of the surface reflected line shapes via the IPDA
technique.
This approach measures the CO2lineshape at several spec-
trally resolved points, which provides several capabilities. This
allows calculating atmospheric weighting functions at two to
three heights (Mao et al., 2007). Sampling at multiple wave-
lengths across the absorption line allows for solving for
wavelength offsets via a line fitting process. The distributed
wavelength sampling across the line region also allows the in-
strument’s response to be characterized as a function of wave-
length. These capabilities allow modelling and reducing the im-
pacts of wavelength dependent responses in the lidar. Using
pulsed lasers and a time resolved receiver also allows post de-
tection signal processing to isolate the laser echo signals from the
surface, and to reject laser photons scattered from the atmosphere
which arrive earlier. Hence it allows isolating the full column
measurement from bias errors caused by atmospheric scatter-
ing (Mao and Kawa, 2004; Aben et al., 2007). The time gate
used in the receiver processing also substantially improves the
receiver’s signal-to-noise ratio (SNR) by reducing the amount
of noise included from the detector and solar background. This
paper describes an initial airborne demonstration of the CO2
column measurement using this technique.
3. Previous lidar measurements of CO2
Several groups have measured atmospheric CO2absorption us-
ing DIAL lidar techniques. Researchers have reported lidar mea-
surements using the CO2absorption lines in the 2051–2062 nm
region using coherent detection. Phillips et al. (2004) describe a
compact dual wavelength continuous wave (CW) laser absorp-
tion spectrometer designed for airborne integrated path measure-
ments using a CO2line at 2051 nm and a receiver using coherent
detection. Koch et al. (2004) have developed and demonstrated
a ground-based range-resolved CO2backscatter profiling lidar,
using a CO2line near 2050 nm, a pulsed Ho:Tm:YLF laser and a
coherent receiver. They demonstrated CO2absorption measure-
ments to within a few percent to a range of 3 km. Subsequently
Koch et al. (2008) demonstrated an increase in laser power and
vertical profiling of CO2near a CO2measuring tower. Gibert
et al. (2006) have developed and demonstrated a ground based
range resolved CO2backscatter profiling lidar, operating at a line
near 2062 nm, based on a pulsed Ho:Tm:YLF laser and using
a coherent receiver. They demonstrated CO2absorption mea-
surements over horizontal path lengths of 2 km. Subsequently,
Gibert et al. (2008) demonstrated and analysed numerous hor-
izontal, slant path and vertical profiling measurements, as well
as integrated path measurements to clouds.
Researchers have also reported lidar measurements using sev-
eral different CO2lines in the 1570 nm CO2absorption band
with direct detection receivers. Krainak et al. (2003) reported
integrated path CO2absorption measurements over a 200-m
horizontal path to a reflective target. Their lidar used a tun-
able CW laser, consisting of a wavelength scanned diode laser
followed by erbium-doped fibre amplifier, to repeatedly sweep
across the 1572.33 nm line. The direct detection receiver used
a PIN photodiode detector. Riris et al. (2007), and Allan et al.
(2008) describe the evolution of this lidar, its use for longer du-
ration CO2absorption measurements over 0.4 and 1.6 km long
horizontal paths, and comparison of its measurements with in
situ sensor readings.
Amediek et al. (2008) reported on CO2measurements made
using a lidar operating on the 1572.9 nm line, using a pulsed
Optical Parametric Oscillator (OPO)-based laser transmitter,
pumped by a Nd:YAG laser. Their direct detection receiver used
a PIN photodiode detector. Integrated path CO2absorption mea-
surements were made over a 2-km-long horizontal path to the
sides of a tree stand and compared to an in situ sensor. Sakaizawa
et al. (2009) reported on a ground-based backscatter profiling li-
dar using a Nd:YAG laser pumped OPO transmitter, a CO2line
near 1572 nm, and a direct detection receiver using a photomul-
tiplier (PMT) detector. They report measuring height resolved
CO2absorption profiles to 5 km, and relative errors of 1% at
<7 km height. Kameyama et al. (2009) have developed a dual
wavelength sine-wave modulated CW lidar for integrated path
CO2measurements. It used the 1572.9 nm line, an 11 cm diam-
eter receiver and a PIN photodiode detector. They report CO2
Tellus (2010)
PULSED AIRBORNE LIDAR MEASUREMENTS OF ATMOSPHERIC CO23
absorption measurements over a 1 km long horizontal path with
4 ppm fluctuations.
Several researchers have recently reported on airborne li-
dar relevant to CO2measurements. Amediek et al. (2009) have
made airborne measurements of ground and water reflectance at
1573 nm in a set of flights over western Europe. Their lidar used
a broadband OPO transmitter and a PIN photodiode detector
and made normalized backscattered pulse energy measurements
from 1.7 to 3 km altitudes. Browell et al. (2009) have been mak-
ing measurements with an airborne lidar measuring integrated
path CO2absorption from the aircraft to the surface. Their li-
dar uses a CO2line near 1571 nm, two CW fibre lasers whose
powers are sine-wave modulated at different frequencies, and a
direct detection receiver using lock-in detection for each modu-
lation frequency.They reported good agreement with CO2values
measured with in situ sensors on a number of flights to 7.5 km
altitudes.
4. Airborne lidar measurement approach
We report here on the initial airborne measurements of CO2col-
umn density made with a pulsed lidar using the IPDA technique,
a wavelength tunable laser with a fibre amplifier, and a direct
detection photon counting receiver. The IPDA technique is a
well-established technique for open-path laser absorption spec-
troscopy measurements (Measures, 1992; Weitkamp, 2005). It is
essentially a special case of differential absorption lidar, where
a scattering target (such as the ground, a water surface, trees,
and cloud tops) is used at the end of the path. Typically two laser
wavelengths are used, which have linewidths much narrower
than the gas absorption line. The target is illuminated with the
laser alternatively tuned onto the gas absorption line, and off it,
at a nearby region. The path-integrated gas absorption attenu-
ates the on-line laser energy relative to the off-line wavelength.
By measuring the optical depth of the gas absorption line, and
by knowing the difference in gas absorption cross-sections and
path length, one can solve for the path integrated gas number
density.
Our lidar uses a pulsed narrow linewidth laser, based on a
tunable diode laser and fibre amplifier, which is repeatedly step-
scanned in wavelength across the selected CO2absorption line.
Twenty wavelength steps were used for these flights and the
wavelength step size and other lidar parameters are summarized
in Table 1. The receiver records and accumulates the backscat-
tered photon counting profiles for the scan during the integra-
tion time. This contains the range resolved backscatter from any
clouds or aerosols in the path as well as the surface echo pulses
at each wavelength.
The quality of the lidar measurement depends on its signal and
noise characteristics and the magnitude of bias errors. A detailed
analysis must account for many factors, including variability in
the lidar parameters, atmospheric temperature and pressure, tur-
bulence, laser speckle, changing surface reflectivity and range,
Table 1. 2008 airborne lidar parameters
CO2line center wavelength 1572.33 nm
Laser min & max wavelengths 1572.29 nm, 1572.39 nm
Laser wavelength steps across line 20 (these flights)
Laser wavelength change/step ∼5pm
Laser peak power, pulse width 25 watts, 1 μsec
Laser pulse energy 25 μJ
Laser divergence angle 470 μrad (these flights)
Seed laser diode type DFB: Fitel FOL15DCWD
Laser Pulse Modulator (AOM) NEOS Model: 26035–2-155
Fiber coupled CO2cell 80 cm path, ∼200 Torr pressure
Fiber Laser Amplifier (EDFA) IPG EAR-10K-1571-LP-SF
Laser line scan rate 450 Hz
Laser linewidth per step ∼15 MHz
Receiver Telescope type Cassegrain, f/10 (Vixen)
Telescope diameter 20 cm
Receiver FOV diameter 200 μrad
Receiver optical bandwidth 800 pm FWHM
Receiver optics transmission (incl
laser loss overfilling FOV)
16%
Detector type PMT: Hamamatsu H10330A-75
Detector quantum efficiency 2% (this device)
Detector dark count rate ∼325 kHz
Receiver signal processing Photon counting/histogramming
Histogram time bin width 64 nsec (these flights)
Receiver integration time 1 s per readout
Recording duty cycle 50% (1 s every 2 s)
Instrument rack size & mass: ∼90 cm tall, total: 147 kg
Sensor head size and mass ∼25 ×60 ×60 cm, 41 kg
etc. (see Ehret et al., 2008 and Caron and Durand, 2009). The
following is a simplified treatment for this approach for an open
atmospheric path and target at a fixed range R, which illustrates
some of the important dependencies. The measurement’s signal
and noise are determined from the lidar equations. The average
signal detected at a measurement wavelength for a single laser
pulse is given by
Nsig(λ)=ηdet
Elas(λ)
hc/λ
rsl
π
Arcvr
R2τopt(λ)τ2
atm(λ).(1)
The total detector noise counts within the laser pulse period,
caused by detected reflected sunlight and detector dark noise, is
Nn≈ηdet
Isol
hc/λ λBPF θFOV
22rsb
πArcvrτopt (λ)Tp+˙
NdTp,(2)
where ηdet is the detector photon counting efficiency; Elas(λ)is
the laser pulse energy at a given wavelength; his the Planck’s
constant; cis the speed of light; λis the laser wavelength; rsl
is the target surface’s effective diffuse reflectivity to the laser
signal; rsb is the target surface’s effective diffuse reflectivity to
sunlight in the receiver’s line-of-sight; Arcvr is the collecting area
of receiver telescope; Ris the range from the instrument to the
surface; τopt(λ) is the receiver optical transmission at a given
wavelength; τatm(λ) is the one-way atmosphere transmission at
the laser wavelength; Isol is the solar spectral irradiance; λ
BPF
Tellus (2010)
4 J. B. ABSHIRE ET AL.
is the receiver optical bandwidth; θFOV is the diameter of receiver
field of view; ˙
Ndis the detector dark noise count rate (Hz) and
Tpis the receiver pulse integration time, usually slightly larger
than the laser pulse width.
The two-way atmospheric transmission is a function of the
laser wavelength. It is related to the total column CO2density
by
τ2
atm(λ)=τ2
off exp −2R
0
[σ(λ, r)−σ(λoff ,r)] nCO2(r)dr,
(3)
where τ2
off is the two-way atmosphere transmission, when the
laser is tuned off the absorption line, σ(λ,r)istheCO
2molecular
absorption cross-section at the laser wavelength λand range
r,σ(λoff,r) is the offline CO2absorption cross-section, and
nCO2(r)istheCO
2molecular volume density.
For airborne measurements though a nadir (vertical) path,
the pressure and temperature both change with r.Thevary-
ing pressure and temperature change the line shape and causes
σ(λ,r) to vary with range, and hence cause an altitude depen-
dence (or weighting) in absorption. However, for the simplest
case, when the path’s temperature and pressure conditions are
approximately uniform, the line shape and cross-sections about
constant along the path, so for it
τ2
atm(λ)=τ2
off exp −2[σ(λ)−σ(λoff)] R
0
nCO2(r)dr.(4)
For this simpler case, and for using a single wavelength for the
online measurement, the total column CO2abundance can be
computed from the ratio of the numbers of the detected photons
on- and off-line from the CO2absorption wavelength, as
Ron-off ≡Nsig(λon )
Nsig(λoff )
Elas(λoff )τopt (λoff)
Elas(λon )τopt(λon)
=exp −2[σ(λon)−σ(λoff )] R
0
nCO2(r)dr(5)
or
NCO2≡R
0
nCO2(r)dr=−1
2[σ(λon)−σ(λoff )] ln (Ron-off ).
(6)
The optical depth of the line absorption is defined as - ln (Ron−off).
Here we have defined Ron-off as the ratio of the received signal
photons measured for the on- and off-line wavelengths multi-
plied by a fraction, which depends on the lidar’s wavelength
response versus wavelength. Equation (5) assumes that the sur-
face reflectivity rsl is equal at both wavelengths. This is an
approximation for a wavelength-stepped lidar measuring from
a moving aircraft, particularly if the surface area (and reflectiv-
ity) viewed by the lidar changes at an appreciable fraction of
the lidar’s wavelength step rate (Amediek et al., 2009). How-
ever, as discussed in Section 8, it was a good approximation for
these airborne measurements over nearly uniformly reflecting
terrain, where there were usually several wavelength scans per
illuminated measurement spot.
For measuring a uniform path with two wavelengths, the frac-
tional error in the average column CO2measurement error can
be approximated as
εN≡δ(NCO2)
NCO2≈1
ln (Ron-off)
δ(Ron-off)
Ron-off
,(7)
where δ(x) denotes the error in the measurements of x.
The random errors are due to the statistical uncertainties
(finite signal-to-noise ratio) of the received signal. The frac-
tional random error in the ratio of the net on- and off-line signal
can be approximated as
δ(Ron-off)≈∂Nsig (λon)
Nsig(λoff )
=Ron-off −∂[Nsig(λoff )]
Nsig(λoff )+∂[Nsig (λon)]
Nsig(λon ).(8)
Hence, for this case, the standard deviation of the fraction error
of the total column CO2number density due to random errors
can be written as
σεN =−1
ln (Ron-off)1
SNRoff +1
SNRon .(9)
In general, for a direct detection lidar, the detected signal fluc-
tuates from both speckle and shot noise. The speckle noise con-
tribution can be estimated from λ, the laser beam divergence,
and Arcvr (Tsai and Gardner, 1985). For these experiments the
number of speckle correlation cells captured by the receiver tele-
scope per laser firing, Kswas ∼6100, which was much larger
than the detected number of signal photons per firing. Hence
the speckle noise effects were negligible and the random errors
were caused by shot noise in the signal and background. The
signal-to-noise ratio at each wavelength, can be computed for
each laser shot from
SNRi=Ntot −Nn
√Ntot +Nn=Nsig
Nsig +2Nn
,(10)
where Ntot,Nnand Nsig are the total detected photons, the de-
tected background and dark counts accumulated over the laser
pulse width, and the detected signal photons, respectively, for
that wavelength. Note that the receiver has to estimate the
noise photon count separately, which can be done by integrat-
ing the detector output after the occurrence of the ground echo
pulse.
After accumulating photon counts for an integration time Tint,
the averaged signal-to-noise ratios in (9) at each wavelength are
given by
SNR(λ)=f(λ)lasTint SNRi,(11)
where f(λ)las is the laser pulse rate at wavelength λand
f(λ)lasTint is the total number of pulse measurements averaged.
The total column CO2measurement error in abundance can be
Tellus (2010)
PULSED AIRBORNE LIDAR MEASUREMENTS OF ATMOSPHERIC CO25
obtained by multiplying σεNin (9) by the nominal CO2abun-
dance.
Bias errors occur when there are errors in the mean values of
the lidar and experiment parameters. For laser absorption spec-
trometers, including lidar based on the IPDA technique, there
are many potential sources of bias errors. These include errors in
line strengths from spectroscopy, errors in estimating the laser
powers and wavelengths, errors in estimating R, non-linearities
in detector response with power, etc. For these experiments the
accuracy of the range (altimetry) measurement was estimated
to be ∼5 m, and the relative error from path length errors was
small. The linearity of the photon counting receiver used for
these experiments is being evaluated.
A common error is from small changes (a few per cent or less)
in the instrument ‘baseline’, that is, in the product Elas(λ)τopt (λ),
versus wavelength in eq. (5). Ideally this product is a constant,
but typically it varies with wavelength, and also changes with
time and temperature. Variability in the baseline response with
wavelength is usually the limiting error source in laser absorp-
tion spectrometers (Werle et al. 1993, 2004). However sampling
the absorption line region at multiple wavelengths around the
line allows the lidar’s wavelength variability to be modelled,
and the modelled values may be used in eq. (5). This approach
can significantly reduce the error in NCO2. More wavelength
samples usually allows for more accurate modelling. However,
with a fixed time delay between laser pulses, using more wave-
length samples also slows the line scan rate. As the line scan
rate is reduced, an airborne lidar becomes more sensitive to
variability from surface reflectance changes, which introduce
measurement errors. For these initial flights the dominant er-
ror source was the ∼8% variability in τopt(λ) caused by etalon
fringes (see for example, Hecht, 2000) from the aircraft’s plane-
parallel nadir window. For these flights we adjusted the lidar to
sample the CO2line region at 20 wavelength samples, in or-
der to allow about four wavelength samples per window etalon
fringe period. This resulted in a 450 Hz line scan rate, and al-
lowed modelling of window etalon fringe transmission from the
measurements as part of the retrieval approach, which simulta-
neously solved for the line absorption depth and for the baseline
response.
5. CO2spectroscopy and line choice
The near infrared vibration-rotation bands of CO2at 1.57, 1.6
and 2.1 μm have been recommended for remote sensing ( Kuang
et al., 2002; O’Brien and Rayner, 2002; Dufour and Breon, 2003;
Mao and Kawa, 2004; Caron and Durand, 2009). We used a
line in the 1570 nm band (Fig. 1) for the CO2measurement
(Mao and Kawa, 2004). This vibration-rotation band of CO2
has an appropriate range of absorption that provides good sen-
sitivity to the surface echo signal and to variation in CO2in
the lower troposheric column. This band has minimal interfer-
ences from other atmospheric species like H2O, and has several
Fig. 1 . Calculated CO2band absorption (top panel) and the 1572.335
nm line shape (bottom panel) calculated from HITRAN 2004 for
two-way path from a 5 km aircraft altitude to ground, based on the US
standard atmosphere. The line shape is plotted both in transmission
(solid) and in optical depth (dashed).
different lines, which are sufficiently insensitive to changes in
atmospheric temperature.
The shorter wavelength lines in the R-branch are a better
match to available laser and detector technologies. The centre-
line of R-branch at 1572.335 nm, shown in Fig. 1, has been
analysed and recommended as an attractive line for CO2mea-
surements (Mao et al., 2007). It has the minimum temperature
sensitivity, particularly to the lower atmospheric temperature
changes. It also provides the maximum CO2absorptioninthe
R-branch. Absorption measurements on this line at a several dif-
ferent wavelengths yield the line shape and CO2vertical column
densities with absorption weighting functions peaking at several
different altitudes.
6. Airborne lidar description
We first developed a ground-based lidar to demonstrate CO2
absorption measurements over horizontal paths to cooperative
Tellus (2010)
6 J. B. ABSHIRE ET AL.
Fig. 2 . Top panel: NASA Glenn Lear-25 aircraft. The nadir window
assembly is just below the NASA logo. Bottom panels: photograph of
the lidar installed on the aircraft showing the sensor head assembly
(left-hand panel) and the dual aircraft racks (right-hand panel).
targets. This lidar used a continuous-wave distributed feedback
(DFB) diode laser, operating at a selected CO2line near 1572
nm, followed by an erbium doped fibre amplifier (EDFA). The
laser wavelength was swept across the CO2line at KHz rates by
tuning the current to the diode laser and the output was gated
by a mechanical chopper. The receiver was a 20 cm diameter
telescope and a PIN photodiode detector, followed by an ana-
logue to digital converter. The ground based lidar was used to
make long-term laboratory measurements of absorption from
CO2in a cell, and in over open paths using cooperative targets.
We also made field measurements of integrated path CO2ab-
sorptions over 0.2–1.6-km-long horizontal paths at two different
sites (Riris et al., 2007; Allan et al., 2008).
We subsequently modified the ground-based instrument for
use on the NASA Glenn Lear-25 aircraft shown in Fig. 2. A
block diagram of the flight instrument (Abshire et al., 2009a) is
shown in Fig. 3. Modifications to the ground based lidar included
converting the laser transmitter to pulsed operation by adding
Fig. 3 . Block diagram of the airborne lidar.
an acousto-optic modulator (AOM) between the diode laser and
the fibre amplifier, removing the chopper wheel, and improving
the receiver sensitivity by using a PMT detector, followed by
a discriminator and multichannel scaler (MCS). The airborne
lidar specifications are listed in Table 1.
For the airborne instrument, the laser signal source is DFB
laser diode, which is stabilized near 1572.33 nm by controlling
its temperature and current. A voltage ramp from a signal gener-
ator was used to sweep the current to the diode laser, and hence
its output wavelength. The diode’s CW output is then gated into
pulses using an acousto-optic modulator (AOM). The laser pulse
timing is synchronized to the tuning of the laser wavelength so
that the CO2absorption peak occurs in the middle of the scan.
A small percentage of the CW seed laser output is split off and
directed through a fibre-coupled CO2absorption cell and to a
PIN detector. The CO2cell serves as a monitor for centre wave-
length of the sweep. An initial calibration procedure was used
on the ground to test and determine the wavelength of each of
the transmitted laser pulses. This used a commercial wavemetre
with 0.1 pm resolution to measure the wavelength of the diode
laser as it was stepped through the nominal voltages of the ramp.
Subsequent testing showed some curvature in the actual dynamic
ramp signal, so a more accurate model of the laser wavelength
versus pulse position was a quadratic function, which was used
in data analysis.
The output of the transmitter is a sequence of 1 μswidelaser
pulses every 100 μs (e.g. a 10 KHz pulse rate) as is shown in
Fig. 4. The peak power was approximately 25 W. Each laser
pulse contains about 25 μJ and over 90% of the pulse energy.
A sample of the laser diode sweep through the internal cell
containing CO2is shown in Fig. 5, along with a sample of the
pulsed transmitter wavelength sweep. The optical power versus
time waveform of a single pulse from the transmitter is shown
in Fig. 6.
The collimated transmitted laser signal exits through the nadir
aircraft window. The laser backscatter is collected by the re-
ceiver’s 20 cm diameter Cassegrain telescope, which views nadir
through the same window in a bistatic configuration. A multi-
mode optical fibre is used to couple the optical signal from
the telescope focal plane to the receiver optics. After passing
Fig. 4 . Timing diagram of the laser pulse and wavelength sweep used
to generate pulsed wavelength scans of CO2line. A wavelength scan
with 20 pulses was used for the December 2008 flights.
Tellus (2010)
PULSED AIRBORNE LIDAR MEASUREMENTS OF ATMOSPHERIC CO27
Fig. 5 . Example of laser wavelength scan. (Red trace and left hand
axis)—Sample cw wavelength scan of the diode laser (before the
modulator) through the instrument’s internal low pressure CO2cell,
showing CO2absorption and diode laser power variability versus
wavelength. (Black trace and right hand axis)—Detected laser power
versus time from the laser transmitter’s power monitor.
Fig. 6 . Typical single laser pulse from the airborne laser transmitter.
The pulse shape shows decay as the fibre amplifier gain is depleted. The
1μs wide part of the laser pulse contains over 90% of the pulse energy.
through an optical bandpass filter, the signal is focused onto a
PMT detector. The PMT has a single photon detection efficiency
of ∼2%. The electrical pulse output from the PMT was amplified
and passed through a threshold detector.
The pulses from the discriminator are binned and accumulated
by the MCS. One MCS sweep records all detected PMT pulses
for the sequence of 20 laser pulses. The start time of the MCS
sweep is synchronized with the first laser pulse trigger and hence
start of the pulsed wavelength sweep. Each MCS sweep contains
a histogram of PMT pulse counts versus time for the wavelength
sweeps (i.e. the laser backscatter profiles for all 20 pulses). At
the end of 1 s, each MCS bin contains the total receiver counts, at
its respective time delay, for the 450 laser sweeps. The receiver
histogram record is then read and stored. Due to the time required
for the readout, data was stored every other second. The laser
trigger and data acquisition is synchronized to timing markers
from the GPS receiver. The computer also digitizes other signals,
including those from eight thermocouples distributed across the
sensor head and electronic rack, the inertial guidance system
output from the aircraft and GPS position and time. A nadir
viewing video camera also captures the visible image though
the nadir window during flight.
7. Airborne campaigns
The NASA Glenn Lear-25 aircraft (NASA-Glenn, 2010) was
selected for these flights based on maximum altitude capability.
For work related to space missions, it is important to provide a
high altitude path, which includes expected effects such as scat-
tering from cirrus clouds. The airborne CO2lidar was integrated
onto the Lear-25 in early October 2008 for two engineering
flights. The airborne lidar was configured into two half-racks
and a ‘sensor head’, which contained the receiver telescope and
the transmitter optics. A photograph of the sensor when inte-
grated on the aircraft is shown in Fig. 2.
The sensor head was mounted above the aircraft’s nadir view-
ing window. The original design called for antireflection (AR)
coated, wedged optical windows to be used. However, due to
window delivery delays, these first flights were performed with
the aircraft’s standard quartz nadir camera window.
The experiment team flew six flights over Ohio and Okla-
homa during October and December 2008. Each flight lasted
just over 2 h, which was limited by the aircraft’s fuel capacity.
These flights allowed testing and recording performance under
different measurement conditions. These included measuring to
the ground through broken and thin clouds. An example of these
measurements is shown in Fig. 7. It shows the time resolved
double-echo pulses measured when viewing the ground over the
DOE ARM site at 7.2 km altitude through thin clouds ∼1km
below the aircraft. The first pulse in each pair is the reflection
from the cloud, while the second is reflection from the ground.
Without range gating, the echo pulse signals and measurements
from the two different path lengths are mixed. Using the pulsed
measurement approach allows using range gating in the data
processing to isolate the signal from the surface and eliminates
optical path length errors from cloud scattering.
The earlier flights also illustrated the impact of the wavelength
variability introduced by the etalon fringes from the aircraft’s
uncoated plane-parallel nadir window. The raw CO2absorption
line shape measurements were distorted by the ±4% transmis-
sion variability caused by etalon fringes from the window. These
were approximately sinusoidal with ∼4 cycles across the sweep.
These changed with time and temperature and caused τ(λ)opt to
vary in flight. On these flights this variability limited our capa-
bility to estimate the CO2line shape and absorbance. For sub-
sequent flights (Abshire et al., 2009b) these effects were greatly
reduced by replacing the aircraft’s standard nadir window with
two wedged and AR coated windows.
Tellus (2010)
8 J. B. ABSHIRE ET AL.
Fig. 7 . Left panel: Example of a time resolved airborne measurement through a thin cloud deck, made from 7.2 km altitude with 20 wavelength
steps across the CO2line. The integration time was 1 s and the measurement time resolution was 64 ns bin−1(9.6 m bin−1), with 16 bins pulse−1.
For clarity the plot used an eight bin running average. For each group the first pulse is from cloud and second pulse is from the surface. The cloud to
ground distance is ∼6.2 km. The received energy of the surface reflected pulses show the attenuation from CO2line absorption, with a transmission
minimum at approximately bin 12500. This measurement is not corrected for wavelength variability (etalon fringes) caused by the aircraft’s nadir
window. Right panel: Expanded view of the second pulse pair.
8. Airborne CO2measurements
and calculations
Airborne CO2column measurements were made from flights on
December 7, 2008 above Department of Energy (DOE) South-
ern Great Plains ARM (Atmospheric Radiation Measurements)
site near Lamont Oklahoma. There were two 2-h long flights,
one in the early afternoon and one in the evening. Lidar mea-
surements were made at stepped flight altitudes from 1.5 to 7
km. The flight patterns are shown in Fig. 8 where the length
of the straight-line segments were ∼32 km. The patterns were
flown with three segments at constant altitude, and the altitude
was stepped, upward or downward, during the eastern-most seg-
ment. The lidar functioned well during the flights and a plot of a
detected single off-line wavelength signal count versus altitude
measured over several flights is shown in Fig. 9. The received
signal levels followed the R−2dependence predicted by eq. (1),
with about 1500 detected counts s−1for an off-line wavelength
at 8 km altitude. The daytime detected solar background count
rate was about 550 KHz. For 1 s averaging time the noise counts
per laser pulse were about 250 counts, which is consistent with
values predicted by eq. (2). At 8 km altitude the SNR for an
off-line wavelength was ∼33 for a 1 s averaging time, and the
relative error in the received energy estimate was ∼3%.
For the experiments above the DOE ARM site, the land sur-
face was wintertime prairie and was fairly uniform in reflectivity.
However if the surface reflectivity viewed by the moving lidar
varies rapidly (i.e. at a significant fraction of the 450 Hz line scan
rate), the changing reflectivity may introduce some residual vari-
ability in the average detected signal energy and in the line shape
measurement. Hence the spatial variability in reflectivity along
the flight track, the wavelength scan rate and degree of foot-
print overlap on the ground can be important (Amediek et al.,
2009). Some calculations for this experiment are summarized in
Table 2. They show the Lear-25 speed increases modestly with
altitude, and with a fixed laser divergence, the laser spot diameter
on the ground increases linearly with altitude. For these flights
the aircraft travelled ∼32 cm per line scan. The laser footprint
areas for the middle half of the scans were ∼64% overlapped at
2.44 km altitude, and the fraction increased to 84% at 7.1 km.
Hence this experiment was most sensitive to any rapid (m-scale)
surface reflectance changes at lower altitudes. With the fairly
uniform surface reflectivity for these experiments, the signal
variability due to any reflectance changes was small, and was
almost always less than the single pulse SNR from the signal
shot noise. Due to their increasing diameters on the surface, the
laser spots are more overlapped at higher altitudes, such as those
made in 2009, and so their sensitivity to reflectivity changes is
smaller.
In order to estimate the actual CO2column density, mea-
surements of atmospheric temperature, moisture and pressure
vertical profiles were used from the DOE radiosonde balloons,
which were launched from Lamont, OK every 6 h. Their param-
eters were used in a 40 layer atmospheric model to compute dry
Tellus (2010)
PULSED AIRBORNE LIDAR MEASUREMENTS OF ATMOSPHERIC CO29
Fig. 8 . The airborne flight pattern for the two flights above the DOE
ARM site near Lamont Oklahoma on December 7, 2008. Upper panel:
a map of the flight patterns, which were flown in a counterclockwise
direction. The radiosonde site at Lamont, OK (97.48◦W, 36.62◦N) is in
the middle of the pattern (source: Google Earth). Lower panel: the
altitude profiles versus time for both flights. For each flight rotation
three segments were flown at constant altitude and the eastern most leg
was used to change altitude.
air column density versus height to 8 km altitude. The December
7th airborne flights were also coordinated with DOE investiga-
tors. They used a small single engine Cessna aircraft carrying an
in situ quick-response infrared absorption gas analyser to mea-
sure CO2concentrations. It sampled air and CO2concentrations
every second from takeoff to ∼6 km (its altitude limit) and back
to the ground. Two Cessna flights were made on December 7,
2008, each lasting about 2 h, and their measurements provided
vertical profiles of CO2mixing ratios.
Figure 10 shows the vertical profiles of CO2mixing ratio mea-
sured by the in situ analyser on both Cessna flights. The upward
leg (black dots) was a direct ascent, but the descent measure-
ments (blue dots) are flown in downward stepped pattern with 10
and 5 min long legs flown horizontally at every 305 m (1000 ft)
Fig. 9 . Symbols: the measured lidar detected signal photon counts for
a single off-line wavelength and 1 s integration times at different flight
altitudes for three flights during December 2008. Solid line: predicted
signal levels from calculations using eq. (1).
Table 2. Airborne Surface Sampling vs Altitude and Aircraft Speed
Altitude (km) 2.44 5.3 7.15
Nominal Lear speed (m/sec) 133 162 175
Receiver FOV diameter on ground (m) 0.3 1.06 1.43
Lear forward motion during 1 scan (m) 0.27 0.32 0.35
Receiver FOV fraction moved per CO2
line scan (%)
89 30 24
Spot overlap fraction for middle 50%
of scan (%)
64 81 84
altitude. The aircraft speed was about 50 m s−1andthelegdi-
rections were approximately perpendicular to the wind speed.
The spread of values in the descent measurements indicate the
spatial variability of the CO2concentrations sampled at the alti-
tude steps. This is especially noticeable at lower altitudes within
the boundary layer, and for the second flight. Given this spatial
variability, for this computation the simplified straightline ap-
proximations, shown in red, were used for the column density
calculation. Figure 11 shows the two-way (aircraft-to-ground
and back) optical depth and transmittance computed based on
HITRAN 2004 and the Cessna and radiosonde measured con-
ditions versus altitude for two adjacent 1572 nm lines, for the
early afternoon flight on December 7, 2008. As expected the
line transmittance decreased with increasing flight altitude and
column length. Figure 12 shows the averaged two-way optical
depth for the 1572.335 nm line and the CO2column number den-
sity plotted versus flight altitude. Both increase smoothly with
height. These calculations provided a reference comparison for
the airborne lidar measurements and showed how the CO2line
shapes and depths should respond to flight altitude.
Tellus (2010)
10 J. B. ABSHIRE ET AL.
Fig. 10. In situ CO2concentrations versus altitude measured by the DOE Cessna aircraft with an infrared gas analyser. Left-hand profile is from a
noon flight and the right-hand profile from a late afternoon flight on December 7, 2008. The black dots are measurements made during the aircraft’s
direct ascent, and the blue dots are those measured during the altitude-stepped decent, where the aircraft sampled CO2while flying horizontally for
5–10 min per altitude. The spread in blue dots at the altitude bands is caused by the spatial variability of CO2concentrations at the altitude. The red
lines are the functions used to approximate the concentration profiles.
Fig. 11. The two-way optical transmittance of the 1572.335 nm CO2
line as a function of flight altitude for December 7, 2008 computed
from the radiosonde and Cessna readings.
9. CO2measurement processing
and line retrievals
For the flights above the ARM site, the lidar recorded the
time- and wavelength-resolved laser backscatter with the photon
counting timing system with 1 s integration time. In subsequent
analysis, the measurements at each flight altitude step where
averaged, using between 15 and 100 s of lidar measurements
per altitude. We used a CO2line retrieval approach based on
the Gauss–Newton method (Rodgers, 2000) to analyse the air-
borne line shape measurements. This approach has sufficient free
parameters to model and correct for instrument effects, to fit
Fig. 12. The computed mean two-way optical depth (top scale) and
number density (bottom scale) for the 1572.335 nm CO2line measured
in the flights as a function of flight altitudes for the flights of December
7, 2008.
the resulting CO2line shapes, and to estimate the correspond-
ing CO2column densities (and mixing ratios) at each altitude.
The CO2retrieval algorithm yields an estimate of the mean
CO2concentration over the laser path length based on line ab-
sorption strength. The input observations were the ratio of the
photon counts in the surface echo signals at each wavelength
after they were normalized by an estimate of transmitted pulse
energy. The error covariance matrix for the observed signals
was diagonal and equally weighted for all but the last received
wavelength.
Tellus (2010)
PULSED AIRBORNE LIDAR MEASUREMENTS OF ATMOSPHERIC CO211
The algorithm required several other fixed quantities as in-
puts. First is a vertical profile of temperature, pressure and water
vapour content with altitude. This was estimated from gridded
meteorological fields from the Goddard Modelling and Assimi-
lation Office for Lamont OK on December 7, 2008 at the time
of the flights. It uses the wavelength resolved absorption spectra
of atmospheric CO2based upon HITRAN 2004. Finally, it used
the path length from the aircraft to the surface, which was cal-
culated from the laser pulse’s time of flight to the surface. Based
on both ground-based testing and subsequent airborne measure-
ments, the range resolution for these flights was estimated to be
∼5 m. Since this was 0.3–0.1% of the column height, for these
flights its contribution to the overall CO2measurement error was
negligible.
The algorithm fits the sampled CO2line shape using a model
with several sets of variables. The first is the reduction of the
photon count ratio near the 1572.335 nm line due to CO2ab-
sorption. Since the photon counts for the line shape samples
are measured as a function of pulse number, they are converted
to wavelength before comparing the observed spectra with the
HITRAN data. For these experiments the lidar’s wavelength
(i.e. wavelength per laser pulse number) was modelled as a
quadratic function, and the three wavelength coefficients were
solved for, using the ground calibration as a prior constraint.
The final set of variables modelled the changes in the lidar’s
baseline response, Elas(λ)τopt (λ), with wavelength. This prod-
uct varied during flight due to wavelength dependent laser power
and etalon fringe pattern in the aircraft’s window transmission.
For these experiments the dominant source of variability was
a sinusoid in transmission caused the aircraft’s nadir window.
There were about 4 cycles per wavelength scan, which changed
more considerably more rapidly with wavelength than did the
CO2absorption. The phase of the etalon fringe pattern changed
with time and window temperature. We modelled and normal-
ized for an estimate of the baseline variability, Elas (λ)τopt(λ), by
using a quadratic energy dependence with a simple sinusoidal
etalon transmission model. For these flights 10 adjustable pa-
rameters were used to solve for the baseline variability and the
three wavelength coefficients of the sweep function. To solve for
the best-fitting line shape, the CO2concentration, the linear and
polynomial coefficients were varied simultaneously to minimize
the difference between the computed (modelled) photon count
ratios and the measured ones. Since each lidar measurement
provided the received energy counts for the 20 wavelengths,
the algorithm’s requirement for 10 inputs was considerably over
determined. This permitted flexibility in line fitting and error
determination. In subsequent flights (Abshire et al., 2009b) we
used custom aircraft windows, which are both wedged and AR
coated, and the etalon fringes and baseline variability were re-
duced by an order of magnitude.
An example of calculated and the observed (retrieved) line
shapes for the flights over the ARM site are shown in Fig. 13. The
retrieved line shapes have similar shape and altitude dependence
Fig. 13. Some CO2transmission line shapes versus altitude for flights,
with altitudes colour coded in same way as indicated in insets. Solid
lines: calculated shapes based on airborne in situ readings. Dots: lidar
measured line shape after processing with CO2retrieval algorithm with
estimated 1σerror bars.
as the calculated ones. However their shape is not as smooth.This
indicates that for these experiments the lidar had some residual
wavelength variability, which was not accurately modelled by
the retrieval algorithm. The measurements at the lowestaltitudes,
with their smaller illuminated spot sizes, are more likely to be
impacted by any variability in surface reflectivity, while the
higher altitude measurements were made with fewer detected
photons. The shape agreement was considered good, given the
impact of the aircraft window’s etalon effects and that these were
the initial lidar flights.
The line shape fits were then used to compute the CO2col-
umn number density and, given the dry air column density es-
timate from the radiosonde, the mixing ratio. The results are
shown in Fig. 14, along with the calculated values from the in
situ measurements. The centre dots represent the mean of all
the retrieved values (typically 20) for the altitude, and the in-
dividual retrievals were based on an average of 15 s of lidar
measurements. The error bars are ±1 standard deviation. The
amount of CO2predicted from the lidar readings increased as
expected with the laser path length (flight altitude) and were
generally consistent with the in situ measurements. The aver-
age difference between the computed column densities and the
4 measurements between 4.3 and 7.1 km altitude was 2.5%,
or equivalently 9.8 ppm. These are larger than the shot noise
limit. The primary causes of variability in these initial measure-
ments were the significant variability in the measured wave-
length response caused by etalon fringing from the aircraft’s
window and incomplete calibration of the lidar’s wavelength
and receiver responses. These sources of error have since been
greatly reduced. Some of the average difference between cal-
culated number densities and the measurements may be from
Tellus (2010)
12 J. B. ABSHIRE ET AL.
Fig. 14. Lidar estimated CO2column number density to surface with 1
standard deviation error bars (in black) made at the altitude steps
versus the calculated CO2column density to surface (in orange) from
the in situ sensor as a function of flight altitude.
errors in the HITRAN 2004 database for the 1572.335 nm CO2
line.
10. Summary
We have demonstrated initial airborne measurements of CO2
absorption and column abundance using a new pulsed direct
detection lidar based on the IPDA technique. The lidar operates
by scanning its laser wavelength across a CO2line near 1572 nm
at a 450 Hz rate with 20 wavelength samples across the line. It
measures the time resolved backscatter and absorption line shape
in the column to the surface. Unlike previous airborne CO2lidar,
it uses low energy (25 μJ) laser pulses and a sensitive photon
counting PMT detector. A pulsed and time gated measurement
approach is used to allow CO2column measurements through
thin clouds.
Initial airborne lidar measurements were demonstrated dur-
ing December 2008 and the lidar functioned well during flights
between 3 and 11 km altitudes. Measurements were made us-
ing the 1572.335 nm CO2line in flights over the DOE ARM
site including some through cirrus clouds. They showed clear
absorption line shapes, which increased in optical depth with in-
creasing aircraft altitude. CO2concentrations and dry air profiles
made from radiosondes were used to estimate the column CO2
number density, and expected line shapes were calculated using
HITRAN 2004. The instrument’s line shapes were estimated via
aCO
2line shape retrieval algorithm, which permitted solving
and correcting for known instrument factors, including baseline
offset, transmission variability, and the wavelength sweep pa-
rameters. The post-processed line shapes agreed well with those
calculated from in situ measurements and radiosondes. The col-
umn number density versus altitude showed similar agreement
with calculations, and the estimated CO2number density agreed
within an average 2.5% for the higher altitudes. The limiting
error sources for these initial flights were a significant vari-
ability in optical transmission versus wavelength caused by the
aircraft’s nadir window, instrument temperature changes and in-
complete instrument calibration. These errors have since been
reduced.
These initial airborne measurements have demonstrated a
candidate lidar technique for CO2needed for the ASCENDS
mission. In order to meet the ASCENDS measurement require-
ments, a lidar approach to measure the dry air column is needed,
as well as improvements in calibration, precision, stability, read-
out rate and power scaling. In subsequent work, we have made
several improvements to this airborne lidar. We have greatly re-
duced the variability in the instrument’s wavelength response by
replacing the aircraft window with ones both wedged and AR
coated. We have improved the instrument’s optical transmis-
sion, calibrations and receiver SNR. We have made additional
flights during August 2009, to 13 km altitude, over various sites,
and with additional measurements from in situ sensors (Abshire
et al., 2009b). Those measurement results are being analysed
and will be reported in the future.
11. Acknowledgments
We acknowledge the support of the NASA Earth Science Tech-
nology Office’s Advanced Instrument Technology and Instru-
ment Incubator Programs, the NASA Carbon Cycle Science
Program, and the Goddard IRAD program. We appreciate the
collaboration with the NASA Glenn Aircraft Operation office,
and with Marc Fischer of Lawrence Berkeley Laboratory on the
airborne in situ CO2measurements. We also appreciate the valu-
able work of other members of the Goddard CO2Sounder team,
and the many suggestions from the reviewers.
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