Aerosol extinction profiles are derived from backscatter data by constraining the retrieval with column aerosol optical thickness (AOT), for example from coincident MODIS observations and without reliance on a priori assumptions about aerosol type or optical properties. The backscatter data were acquired with the NASA Langley High Spectral Resolution Lidar (HSRL). The HSRL also simultaneously measures extinction independently, thereby providing an ideal data set for evaluating the constrained retrieval of extinction from backscatter. We will show constrained extinction retrievals using various sources of column AOT, and examine comparisons with the HSRL extinction measurements and with a similar retrieval using data from the CALIOP lidar on the CALIPSO satellite

Burton, S. P.

Cook, A. L.

Ferrare, R. A.

Hair, J. W.

Haper, D. B.

Hostetler, C. A.

Kittaka, C.

Obland, M. D.

Rogers, R. R.

NASA Technical Reports Server

USING AIRBORNE HIGH SPECTRAL RESOLUTION LIDAR DATA TO EVALUATE 
COMBINED ACTIVE PLUS PASSIVE RETRIEVALS 
OF AEROSOL EXTINCTION PROFILES 
S. P. Burton1,  R. A. Ferrare2, C. Kittaka1, C. A. Hostetler2, J. W. Hair2, M. D. Obland2, R. R. Rogers1, 
A. L. Cook2, D. B. Harper2 
 
1Science Systems and Applications, Inc., Hampton VA, 23669, USA, Sharon.P.Burton@NASA.gov, 
Chieko.Kittaka-1@NASA.gov, Raymond.R.Rogers@NASA.gov 
2NASA Langley Research Center, MS 401A, Hampton, VA, 23681, USA, Richard.A.Ferrare@NASA.gov, 
Chris.A.Hostetler@NASA.gov, John.W.Hair@NASA.gov, Michael.D.Obland@NASA.gov, 
Anthony.L.Cook@NASA.gov, David.B.Harper@NASA.gov 
 
 
ABSTRACT 
Aerosol extinction profiles are derived from backscat-
ter data by constraining the retrieval with column aero-
sol optical thickness (AOT), for example from 
coincident MODIS observations and without reliance 
on a priori assumptions about aerosol type or optical 
properties.  The backscatter data were acquired with 
the NASA Langley High Spectral Resolution Lidar 
(HSRL).  The HSRL also simultaneously measures 
extinction independently, thereby providing an ideal 
data set for evaluating the constrained retrieval of ex-
tinction from backscatter.   We will show constrained 
extinction retrievals using various sources of column 
AOT, and examine comparisons with the HSRL extinc-
tion measurements and with a similar retrieval using 
data from the CALIOP lidar on the CALIPSO satellite. 
INTRODUCTION 
Accurate aerosol measurements are required for under-
standing shortwave radiative transfer since aerosols 
affect climate by scattering and absorbing solar radia-
tion and also by altering the lifetime and development 
of clouds.  Aerosol radiative forcing depends strongly 
on the vertical distribution of aerosols [1].  Scattering 
particles exhibit a greater forcing when most of the 
aerosol mass is located in the lower troposphere be-
cause of the increase in aerosol size with relative hu-
midity.  In contrast, absorbing aerosols produce a 
greater radiative forcing when the aerosol mass is 
above cloudy layers or when the underlying surface 
albedo is high [1].  
Lidar remote sensing is a valuable means of measuring 
the vertical distribution of aerosol properties.  The lidar 
equation relates the total backscatter and extinction to 
the lidar signal as a function of range from the instru-
ment. 
( ) ( ) ( )[ ] ( ) ( )[ ] ⎪⎭
⎪⎬⎫⎪⎩
⎪⎨⎧ ′′+′−+= ∫r
0
2 2exp rdrrrrr
CrP amam σσββ  
where r is the range, P(r) is the measured signal, C is a 
calibration constant, βm and βa are the molecular and 
aerosol backscatter coefficients, respectively, and σm 
and σa are the molecular and aerosol extinction coeffi-
cients, respectively.  For a backscatter lidar such as the 
Cloud-Aerosol Lidar with Orthogonal Polarization 
(CALIOP) sensor, both the aerosol backscatter and 
extinction at a given range are unknown variables to be 
retrieved, so the equation is underdetermined. The two 
unknowns can be related to each other by the aerosol 
extinction to backscatter ratio or “lidar ratio,” Sa. The 
lidar ratio is frequently either assumed or inferred from 
additional measurements and used as a means of solv-
ing the lidar equation. 
Recently, work has been done on retrieving both aero-
sol extinction and backscatter profiles from lidar data 
by constraining the solution with column aerosol in-
formation from coincident satellite measurements, the-
reby avoiding the need to assume a value for Sa [2], [3].  
A successful implementation of such a technique 
would potentially improve extinction retrievals from 
the CALIOP sensor on board the Cloud-Aerosol Lidar 
and Infrared Pathfinder Satellite Observation 
(CALIPSO) satellite. The CALIOP extinction retriev-
als rely on accurately specifying Sa, and uncertainties 
in the lidar ratio are a major source of uncertainty in 
the CALIOP extinction retrievals.  
One active+passive method of retrieving extinction 
was performed by Ferrare et al. [4] from the NASA 
Langley UV-DIAL lidar using aerosol optical thickness 
(AOT) measurements from the Moderate Resolution 
Imaging Spectroradiometer (MODIS) and the Model of 
Atmospheric Transport and Chemistry (MATCH).  A 
time series of column AOT values along the airborne 
https://ntrs.nasa.gov/search.jsp?R=20080024125 2019-08-30T04:39:44+00:00Z
lidar flight track is derived.  For each point along the 
track, the retrieval of extinction from backscatter re-
turns is constrained by requiring that the resulting AOT 
matches the measured AOT. 
Here we discuss a similar calculation, using satellite 
aerosol optical thickness from MODIS and from 
PARASOL (Polarization and Anisotropy of Reflec-
tances for Atmospheric Sciences coupled with Obser-
vations from a Lidar) to constrain the retrieval of 
extinction from total (i.e., aerosol plus molecular) atte-
nuated backscatter profiles obtained by the NASA 
Langley High Spectral Resolution Lidar (HSRL) [5].  
The HSRL provides a unique opportunity to validate 
and evaluate this technique, because it also provides a 
direct extinction measurement, which can be used as 
ground truth in determining the strengths and limita-
tions of the method.  Retrieving extinction from HSRL 
total attenuated backscatter measurements of course is 
not done for the sake of the derived extinction profiles 
themselves, but because it can provide valuable insight 
into the AOT-constraint technique, a technique that 
may help reduce uncertainties in aerosol extinction 
profiles derived from CALIOP measurements.  
The HSRL instrument measures backscattering and 
depolarization at two wavelengths, 532 nm and 
1064 nm.  It also measures extinction at 532 nm, by 
using the Doppler broadening of the molecular back-
scattering signal to separate the molecular and aerosol 
components of backscattering [6].  Since the molecular 
backscattering is also known from the atmospheric 
density profile (as in standard lidar techniques) the 
attenuation of the molecular backscatter signal by at-
mospheric extinction can be obtained. 
The HSRL instrument has flown aboard the NASA 
Langley King Air B200 on over 120 flights since 
March 2006 during a variety of missions including the 
Gulf of Mexico Atmospheric Composition and Climate 
Study (GOMACCS) and the Cumulus Humilis Aerosol 
Processing Study (CHAPS).  Approximately 40 of 
these flights contain segments coordinated with 
CALIPSO overpasses, providing an opportunity to 
validate extinction retrievals from the CALIOP sensor.  
EXTINCTION RETRIEVAL  
To retrieve aerosol extinction from HSRL attenuated 
backscatter signals, we implement a Fernald near-field 
solution [7] using a given Sa value.  Rather than assume 
a value for Sa, however, we search for the value of Sa 
that satisfies the requirement that the column integral 
of the extinction values must correspond to a column 
AOT value provided by a coincident MODIS or 
PARASOL measurement or another source such as an 
aerosol transport model.  The solution is obtained in a 
straightforward way by forming the Fernald equation 
and the numerical integration into root-finding form 
(i.e., an equation with one side equal to zero) and solv-
ing for Sa with an off-the-shelf non-linear root finder.  
By these means, the ratio of aerosol extinction to back-
scattering is free to vary with time and location along 
the flight track, although the method still contains the 
underlying assumption that Sa is constant with altitude. 
There are several possible sources of AOT data to con-
strain the extinction retrieval.  The simplest to start 
with is the AOT integrated from the extinction meas-
ured by the HSRL instrument itself.  The spatial coin-
cidence is of course perfect so we expect to get very 
good agreement.  Figure 1 shows the results of the re-
trieval for a three hour HSRL flight over the Mid-
Atlantic and off the coast of Virginia on August 4, 
2007.  Laser shots with cloud attenuation have been 
excluded from the retrieval.  The agreement between 
the measured extinction and the retrieved extinction is 
very good.  The percent difference is shown for each 
bin where the extinction is greater than 0.01, which is 
each bin below 4.5 km.  For these altitudes, the percent 
difference is between -5% to 12%.  These results indi-
cate that, for this case, the assumption of a constant 
lidar ratio with altitude is valid.  
 
Figure 1. Median profiles [left] and percent difference profile 
[right] in ½ km bins of the extinction retrieved from HSRL 
attenuated backscatter data constrained using the HSRL AOT 
(gray), compared to the measured HSRL extinction (black).  
These data are from a flight on August 4, 2007 off the coast 
of Virginia.  Cloudy shots have been excluded from the re-
trieval.  The error bars show the 25th and 75th percentiles of 
the extinction values and of the difference.  
0.00 0.05 0.10 0.15 0.20 0.25 0.30
Extinction (km-1)
0
1
2
3
4
5
A
lti
tu
de
 (k
m)
-40 -20 0 20 40
Percent dif {calc-meas}/meas
0
1
2
3
4
5
It is desirable to obtain the aerosol optical thickness 
from a coincident passive sensor such as MODIS or 
PARASOL, to evaluate the active+passive extinction 
retrieval for use with lidar measurements that do not 
have associated AOT measurements.  Figure 2 demon-
strates the extinction retrieval using HSRL attenuated 
backscatter data with AOT from MODIS on Aqua.  
Only the shots over land are shown.  The AOT that is 
used in the extinction retrieval is “Corrected_Optical_
Depth_Land” at 550 nm from coincident MODIS Aqua 
measurements from an overpass at 18:25 UT (during 
the HSRL flight), from which all the pixels within 25 
km of the HSRL track are averaged for each point on 
the track.  The 550-nm AOT is also converted to the 
HSRL wavelength of 532 nm and a further adjustment 
is made to reduce the MODIS AOT for the fact that 
HSRL backscatter measurements, and thereby the re-
sulting derived extinction, are available only in a par-
tial column (i.e. only below the aircraft).  The 
agreement between the retrieved extinction and the 
measured extinction in this case is about as good as 
before, with percent differences from -6% to 14%. 
Figure 2.  Like Figure 1, but the AOT constraint to the ex-
tinction retrieval was from MODIS Aqua, as described in the 
text. Only the data over land are included. 
The results for the portion of the same flight over water 
are less successful, as shown in Figure 3.  In this case, 
the retrieval underestimates the extinction at all alti-
tudes, and the percent difference varies from -5% to 
-44%.  Direct comparison of the MODIS Aqua AOT 
used in this experiment with the AOT derived from the 
HSRL extinction measurements reveals a large differ-
ence as well, in Figure 4. With such a difference in 
AOT between MODIS and HSRL over much of the 
flight track, it is no surprise that the extinction retrieval 
produces much smaller extinctions than the HSRL 
measurements for this case. 
 
Figure 3.  Like Figure 2, but for the water portion of the Au-
gust 4, 2007 flight. 
The MODIS Terra swaths for two overpasses that oc-
curred a few hours earlier, at 15:05 and 16:45 UT, also 
include the HSRL flight track.  Using the same spatial 
coincidence criteria and applying the same adjustments 
for wavelength and for the partial column produces 
along-track AOT for MODIS Terra that is also shown 
in Figure 4. There is a large bias between the MODIS 
Aqua and MODIS Terra AOT.  Even given the differ-
ence in time between the Aqua and Terra overpasses 
this large difference is surprising.  The reason for the 
difference is currently under investigation, but we have 
seen similar bias in occasional other cases over water.  
Because of the Terra vs. Aqua difference for the water 
portion of this flight, we examine another source of 
AOT measurements.  PARASOL is a wide-field imag-
ing radiometer and polarimeter and another member of 
the “A-train” satellite constellation, trailing Aqua by 
about 3 minutes.  Since PARASOL measurements over 
water are made at 670 nm and 865 nm, extrapolation of 
aerosol optical thickness measurements to the HSRL 
wavelength of 550 nm is required.  For this experiment, 
the Ångström coefficient is derived by a multiple linear 
regression on the 670 nm and 865 nm data within a 
rectangular box surrounding the entire flight track.  The 
extrapolated AOT is then treated as before to combine 
with the HSRL measurements.  The adjusted along-
track AOT for PARASOL is also shown in Figure 4.  
While there are still significant differences with the 
measured HSRL AOT, there is better agreement than 
0.05 0.10 0.15 0.20 0.25 0.30
Extinction (km-1)
0
1
2
3
4
5
A
lti
tu
de
 (k
m)
-40 -20 0 20 40
Percent dif {calc-meas}/meas
0
1
2
3
4
5
0.0 0.1 0.2 0.3 0.4
Extinction (km-1)
0
1
2
3
4
A
lti
tu
de
 (k
m)
-40 -20 0 20 40
Percent dif {calc-meas}/meas
0
1
2
3
4
with the MODIS Aqua AOT.  These values have been 
used in the active+passive retrieval with HSRL back-
scatter measurements to produce the results shown in 
Figure 5.  Here the percent difference varies from -11% 
to 6%.  
  
Figure 4. The partial AOT value from HSRL extinction mea-
surements (black) and derived from MODIS Aqua (dark 
gray), MODIS Terra (medium gray) and PARASOL (light 
gray) along the Aug. 4, 2007 HSRL flight track over water.  
The MODIS and PARASOL AOT have been averaged and 
converted to partial column AOT at 532 nm as described in 
the text. 
SUMMARY  
Aerosol HSRL measurements are used to evaluate 
aerosol extinction profiles derived by combining lidar 
backscatter profiles and satellite retrievals of aerosol 
optical thickness. The HSRL data indicate the vertical 
variability of the lidar ratio can be small enough in 
some cases to assume a single altitude-independent 
lidar ratio.    
In these cases, using AOT derived from MODIS or 
PARASOL provides a useful constraint to derive aero-
sol extinction profiles that are in generally good 
agreement with extinction profiles derived from air-
borne measurements using the HSRL technique.   For 
measurements over water, we have the option of using 
the AOT from the MODIS ocean algorithm, or, for 
some flights, aerosol data from the PARASOL sensor.  
We hope to be able to better characterize when the ac-
tive plus passive technique described here works well, 
and also when and why it does not always work.  We 
will also show comparisons with extinction from a sim-
ilar retrieval using CALIOP backscatter returns. 
Figure 5.  Like Figure 3, but the AOT constraint used in the 
retrieval was from PARASOL as described in the text.  Once 
again, only shots over water are shown. 
REFERENCES 
[1] Haywood, J. M., et al., A limited-area-model case 
study of the effects of sub-grid scale variations in rela-
tive humidity and cloud upon the direct radiative forc-
ing of sulfate aerosol, Geophys. Res. Letters, 24, 143-
146, 1997. 
[2] Kaufman, Y. J., J. M. Haywood, P. V. Hobbs, W. 
Hart, R. Kleidman, B. Schmid, Remote  Sensing of 
vertical distributions of smoke aerosol off the coast of 
Africa during SAFARI 2000, Geophys. Res. Lett. 30, 
1831, 10.1029/2003GL017068, 2003. 
[3] Léon, J.-F., et al., Profiling of a Saharan dust out-
break based on a synergy between active and passive 
remote sensing, J. Geophys. Res., 108, doi:10.1029/
2002JD002774,  2003. 
[4] Ferrare, R. A., et al., The Vertical Distribution of 
Aerosols: Lidar Measurements vs. Model Simulations, 
23rd International Laser Radar Conference, Nara, Ja-
pan, 2006.  
[5] Hair, J. W., L. M. Caldwell, D. A. Krueger, and 
C.-Y. She, High-spectral-resolution lidar with iodine-
vapor filters: measurements of atmospheric-state and 
aerosol profiles, Applied Optics, 40, 5280-5294. 
[6] Grund, C. J. and E. W. Eloranta, The University of 
Wisconsin High Spectral Resolution Lidar, Optical 
Engineering, 30, 6-12, 1991. 
[7] Fernald, F. G., Analysis of atmospheric lidar obser-
vations: some comments. Applied Optics 23, 652-653, 
1984. 
 
0.05 0.10 0.15 0.20 0.25 0.30
Extinction (km-1)
0
1
2
3
4
5
A
lti
tu
de
 (k
m
)
-40 -20 0 20 40
Percent dif {calc-meas}/meas
0
1
2
3
4
5
18.5 19.0 19.5
Time UTC (hr)
0.0
0.2
0.4
0.6
0.8
1.0
Ae
ro
so
l O
pti
ca
l T
hic
kn
es
s
HSRL partial AOT
MODIS Terra Ocean
MODIS Aqua Ocean
PARASOL Ocean


Using Airborne High Spectral Resolution Lidar Data to Evaluate Combined Active Plus Passive Retrievals of Aerosol Extinction Profiles

Using Airborne High Spectral Resolution Lidar Data to Evaluate Combined Active Plus Passive Retrievals of Aerosol Extinction Profiles

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server