Optical measurements of atmospheric particles from airborne platforms: in situ and remote sensing instruments for balloons and aircrafts

Abstract

Multiwavelength laser backscattersondes (MAS) have been widely used from a variety of airborne platforms for in situ measurements of optical properties of clouds and atmospheric particulate as well as their phase and composition. Recently, a new miniaturized LIDAR (MULID) has been developed using state-of-art technology for balloon borne profiling of the same quantities. A description of the two instruments, a survey of preliminary results obtained during test flights and indications for future use are given.

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ANNALS OF GEOPHYSICS, VOL. 49, N. 1, February 2006
Key words remote sensing – LIDAR – backscatter-
sonde – aerosol
1. Introduction
In response to the changing needs of atmos-
pheric aerosol and cloud research, two new
lightweight optical instruments have been de-
veloped: a microlidar (MULID) and a Multi-
wavelength Aerosol Scattersonde (MAS) (Adri-
ani et al., 1999; Buontempo et al., 2003). Pri-
mary goal of MAS was its use on board of M55
stratospheric airplane Geophysica (Stefanutti
et al., 1999) in the context of polar campaigns.
Although Polar Stratospheric Clouds (PSCs)
and stratospheric aerosol are known to play an
important role in ozone depletion (Crutzen and
Arnold, 1986), our understanding of PSCs is
still in progress and many questions remain
open. It is crucial to understand, in terms of par-
ticles concentration, sizes and optical parame-
ters, (Rosen et al., 1989, 1990) the microphysi-
cal processes underlying PSC structure (Adri-
ani et al., 1992).
For these reasons MAS has been extensively
used in several polar campaigns in the context of
the European Airborne Polar Experiment (APE)
(Stefanutti et al., 1999). The incredible amount
of valuable information obtained by MAS during
these campaigns led us to extend the use of this
instrument to other cloud research fields. We fo-
cused our interest in tropical region mainly be-
cause of the increasing importance of the equa-
torial stratosphere in climate modeling. The use
of a scattersonde in the Tropical Tropopause
Layer (TTL) could help our understanding of
formation, growth and decay of ultrathin tropical
Optical measurements of atmospheric
particles from airborne platforms:
in situ and remote sensing instruments
for balloons and aircrafts
Carlo Buontempo (1), Francesco Cairo (1), Guido Di Donfrancesco (2), Roberto Morbidini (1),
Maurizio Viterbini (1) and Alberto Adriani (3)
(1) Istituto di Scienze dell’Atmosfera e del Clima (ISAC), CNR, Roma, Italy
(2) ENEA C.R. Casaccia, Divisione Ambiente Globale e Mediterraneo,
Sezione Clima, Santa Maria di Galeria (RM), Italy
(3) Istituto di Fisica dello Spazio Interplanetario (IFSI), CNR, Roma, Italy
Abstract
Multiwavelength laser backscattersondes (MAS) have been widely used from a variety of airborne platforms for
in situ measurements of optical properties of clouds and atmospheric particulate as well as their phase and com-
position. Recently, a new miniaturized LIDAR (MULID) has been developed using state-of-art technology for
balloon borne profiling of the same quantities. A description of the two instruments, a survey of preliminary re-
sults obtained during test flights and indications for future use are given.
Mailing address: Dr. Francesco Cairo, Istituto di
Scienze dell’Atmosfera e del Clima (ISAC), CNR, Via del
Fosso del Cavaliere 100, 00133 Roma, Italy; e-mail: f.cai-
ro@isac.cnr.it

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Carlo Buontempo, Francesco Cairo, Guido Di Donfrancesco, Roberto Morbidini, Maurizio Viterbini and Alberto Adriani
cirrus. Such faint clouds are supposed to play an
important role in radiation balance of the tropical
atmosphere because of their extension and ubiq-
uity. Moreover the Sub Visual Cirrus (SVCs) ap-
peared to be directly linked to dehydration of the
stratosphere with tropical tropopause playing the
role of cold trap for ascending air (Holton et al.,
1995). Such thin cirrus layers are composed of
ice particles and can exist in regions where there
is balance between ascent rate, mixing ratio and
gravity evaporation. Moreover, as in the polar
stratosphere, the presence of a solid substrate
could play an important role in the heteroge-
neous reaction of ozone depletion (Molina,
1991). The experimental device has been com-
pletely rebuilt to better fulfill its tropical task.
Although in situ measurements are extreme-
ly important to study the life cycle of cirrus
cloud, knowledge of the atmospheric environ-
ment below the cloud level is important as well.
For this reason we have developed a remote
sensing instrument. The lightweight microlidar
(MULID) designed to be used onboard of
stratospheric balloon is, as far as we know, the
first balloon borne microlidar ever realized, al-
though in the last two decades many airborne li-
dars have been developed (Spinhirne, 1982) .
Both the instruments have the capability to
measure the depolarized signal backscattered
by cloud particles. Such measurements are eas-
ily linked to phase and dimension of the scatter-
ing particles (Spinhirne et al., 1983).
2. Optical and mechanical layout
MAS and MULID use a similar layout, but
they mainly differ in the acquisition mechanism
and software. For both of them light source is a
solid state laser while an optical device is used
to collect the backscattered light and to divide it
according to its wavelength. Table I shows the
optical and electronic characteristics of the two
systems.
2.1. MAS
The system was designed to sample air
parcels located few meters away from the plane,
and consequently adopts a 50 mm common cam-
era lens capable to detect the backscattering sig-
nal. It uses three different wavelength (532,1064,
and 1550 nm) to better constrain microphysical
and optical characteristics of the sampled parti-
cles. The SNR of the system is directly related
the ratio between the Field of View (FoV) and the
laser beam divergence. A large FoV induces an
increase in the background light while a FoV re-
duction permits the laser beam to complete over-
lap the FoV only at a very far distance from the
instruments, so reducing the «near field» power
signal. A 50 µ wide pin hole (FoV 1 mrad) has
been tested initially but after some test flights it
clearly appeared that the drawbacks were not bal-
anced by the reduction of the background noise.
It was finally set at 3 mrad.
A photocell device was used to control a met-
al shutter at the focal point of the system, to
avoid photodetectors damages induced by direct
sun light exposure. The incoming beam is colli-
mated and then divided according to its wave-
length thanks to a beam splitter. Polarizing cubes
Table I. Optical and electronic characteristics of
MULID and MAS instruments.
MULID
Filter bandwidth
Wavelength
Telescope focal length
Telescope diameter
Laser peak energy
Pulse repetition rate
Total weight
Photo-detector QE @532 nm
5 nm
532 nm
2000 mm
210 mm
1.5 µJ
1.1 kHz
10 kg
12%
MAS
Filter bandwidth 532 nm
Wavelength
Telescope focal length
Telescope diameter
Laser peak energy
Pulse repetition rate
Total weight
Photo-detector QE @522 nm
5 nm
532 nm
50 mm
48 mm
1.5 µJ
1 kHz
50 kg
12%

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Optical measurements of atmospheric particles from airborne platforms
will split it again according to its polarization.
We adopted photodiode devices for IR channels
and photo multipliers for the visible ones.
The system uses interferential filter to select
the requested wavelength. A narrow filter im-
proves the SNR but needs a more accurate ther-
malization to prevent the transmission peak
drifting too far away from the required wave-
length. MAS flies on board of Geophysica M55
a stratospheric research airplane thus it is sub-
jected to abrupt changes in temperature. Such
thermal shocks alter the laser cavity dimensions
and consequently the output power. The system
adopts an amplified photodiode to monitor laser
beam intensity. We realized that not taking into
consideration such sharp variation (50-70%) of
laser power, leads to a misleading result. The
instrument is divided into two parts mounted
one above the other inside a fibreglass sand-
wich shield. The optical box is located on top,
below the acquisition hardware (fig. 1). MAS
has four different channels of acquisition: one
Fig. 1. MAS at the end of mechanical buildup. From above note the optical box with the supports for all the
optics, the telescope with the two lasers on its side and the periscope.

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Carlo Buontempo, Francesco Cairo, Guido Di Donfrancesco, Roberto Morbidini, Maurizio Viterbini and Alberto Adriani
per wavelength and one for the housekeeping
data. Various stages of data elaboration can be
identified by following data stream from the
photo detector toward its solid state storage on
the PC drive as shown in fig. 2:
– A pulse shaper used to maximise the elec-
tronic dynamic.
– An A/D converter that digitalize the profile.
– A First In First Out device (FIFO) used to
accumulate data during the pre-trigger phase.
– A Digital Signal Processor (DSP) used to
integrate the profile over a preset repetition in-
terval.
– The acquisition PC.
At the end of the flight, after landing, record-
ed data are retrieved using PC serial ports.
2.2. MULID
The system is based on a 1.5 µJ laser that
operates at a repetition rate of 1 kHz. A picture
describing the instrument is shown in fig. 3.
Laser operates at 532 nm, the well known neo-
dinium-YAG source. Sensors are Hamamatsu
5783p photo-multipliers modules used in pho-
ton counting mode. Optical and electronic char-
acteristics of the system are shown in table I.
The whole system has been embedded in an in-
sulating polyurethane shield to reduce heat ex-
change with external environment and externally
covered by a reflecting Mylar microfilm. Signal
acquisition is based on Texas Instrument DSP,
characterized by a very low power consumption.
The DSP has been piloted by a MS-DOS based
PC104.
A pack of commercial alkaline batteries was
mounted as power supply during short duration
flights while Lithium batteries were preferred for
long duration flights. To operate on a balloon
platform, MULID needs to be lightweight, com-
pact and very robust. The results were well above
our best expectations. After a shock landing, in
one of our test flights, major damage was report-
ed only in the telescope while impressively opti-
cal and electronic devices were still operating
thanks to a shock absorbing carbon fibre struc-
ture. The optical layout of MULID is similar to
Fig. 2. MAS, electronic data stream: schematic lay-
out.
Fig. 3. MULID during the assembling phase. The
squared structure on the top is necessary to sustain
the carbon fiber and mylar reflecting shield. The op-
tical box is mounted near the focal point of the tele-
scope.

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Optical measurements of atmospheric particles from airborne platforms
the MAS one but it operates only at one wave-
length instead of the three used by MAS. The
system operates in photon counting mode to in-
crease SNR, for this reason data stream is slight-
ly different from the MAS one. The signal com-
ing from photon-multipliers is sent to an elec-
tronic device that eliminates low amplitude
events caused by thermal background, and re-
shapes the other pulses to a TTL level (discrimi-
nator). The DSP receives the signal and uses its
memory as a shift register for the profile. In or-
der to avoid multiple counts of a single event, a
synchronization between photon pulse and DSP
trigger is performed at this stage. DSP sums the
whole profile bin by bin over an interval set by
the user (normally 30 s). After the integration pe-
riod the summed profile is sent to the PC where
is stored on a solid state memory (2 MB). The
use of a 40 MHz DSP allows to average profiles
at the laser firing frequency (1.1 kHz).
Several problems are related to the use of
such an accurate optical system in an environ-
ment as hard as the stratosphere. The photon-
multipliers are not designed for such low pres-
sure use: when external pressure falls below 50
mb, dielectric breakdown occurs in the air be-
tween dinodes. Such electric discharges dam-
age PMTs permanently. To fix the problem the
photon multiplier windows were treated with a
varnish to create a multilayer external coating
that avoids air loss during ascent.
3. Data inversion
For both the instruments it is necessary to in-
vert and calibrate the signal to obtain atmospher-
ic data. Most of the algorithms used to invert LI-
DAR data assume the signal to be pure molecu-
lar at a certain height. For MULID the procedure
is straightforward although the molecular point
is located in the close range. Hereafter we
analyse the way we invert MULID data and then
we will describe how to extend this technique to
an in situ instrument that does not have a pure
molecular point per profile. For a ground based
LIDAR the signal equation is the following:
(3.1)
( )( )
r
( )
r
( )
r drS r Ar
$
1
ma
r
2
0
$$
=+
bbv
^h #
where A is a constant related to overall efficien-
cy of the system, r is the distance from the ob-
server; S is the signal; the two terms in brackets
represent the molecular and the aerosolic con-
tribution to the scattering respectively, the inte-
gral term represents the signal extinction. In a
ground based LIDAR the inversion is per-
formed assuming as pure molecular the signal
coming from the upper stratosphere and then
using an iterative method to calculate the inte-
gral term in the eq. (3.1), taking for granted the
relation between molecular backscattering and
extinction. In our case the inversion algorithm
is calibrated in the short range.
During balloon ascent, gondola housekeep-
ing data allow us to calculate the density profile.
We use such profile to calculate molecular con-
tribution at every height. To calculate aerosol
scattering ratio (R) we postulate that a few kilo-
metres above the tropopause the signal has to be
almost pure molecular so that
^
(3.2)
This is a reasonable assumption since it is well
known that the aerosol profile exhibits a mini-
mum above the tropopause (Jager et al., 1995).
The range corrected signal is calibrated us-
ing this assumption with an iterative method
calculating the whole profile. At this stage the
cloud extinction in the signal profile is being
considered. A problem exists for those profiles
that do not include any stratospheric point in
the profile as for instance those obtained during
tropospheric ascent. In these cases we have cal-
ibrated the profile considering valid the calibra-
tion of the first stratospheric profile. Because of
the insulator shield, radiative cooling inside the
microlidar is not too fast, as good approxima-
tion we assume that only minor laser tempera-
ture variation happens during the balloon fast
ascent. It is possible to check such approxima-
tion looking at the surface reflectivity, that is
supposed to be almost constant.
For an in situ probe the inversion technique
is even more complicated. MAS does not ac-
quire a profile as MULID does, it uses the
backscattered value integrated on the whole
profile. For the increase of the SNR of the in-
. 1 1
.
=
( )
r
( )
r
b
( )
r
R ZZ
tropopause
m
ma
+
+
d
bb
.
^h
h

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Carlo Buontempo, Francesco Cairo, Guido Di Donfrancesco, Roberto Morbidini, Maurizio Viterbini and Alberto Adriani
strument, the software subtracts the signal av-
eraged over a region where the beam is out of
the telescope FoV, from the backscattered inte-
grated peak value. For MAS, lasers are mount-
ed on the telescope fig. 1, where outside air is
very likely to rapidly cool the instrument. Tak-
ing into account laser power variation has a first
order importance for MAS. We use laser moni-
tor to correct such variations and performed
several lab tests to calibrate laser meter output
to actual laser beam power and we are absolute-
ly confident in the accuracy of our calibration
technique.
When spurious laser effects are eliminated
from the signal, airplane data stream is used to
calculate the molecular contribution to the scat-
tering and then a scattering ratio R value is cal-
culated. To calibrate the R value we assume the
value of the scattering ratio is known at some
particular height.
We normally use 19000 m as a reference for
R=1.1 scattering ratio. For MAS it is sufficient
to have at least a couple of points in the strato-
sphere to calibrate the whole flight because of
the real-time laser power calibration.
Particular attention was paid to the comput-
er synchronization for both instruments. The
clock of the acquisition PC has a different
speed from the geographical data stream one.
We have observed CPU clock drift as big as 700
s in a 4000 s flight span. This is surely due to
the hazardous condition encountered by the
system during the flight. Not to synchronize the
clocks leads to erroneous results.
4. Results
In this paper we present some of the first re-
sults of both systems. Data have to be consid-
ered as preliminary. Both of them refer to two
technical flights. For MULID we present (fig.
4) data from a tropical flight performed from
Bauru (Brazil) on February 23rd 2003. Color
represents the scattering ratio that is supposed
to be 1 in absence of cloud. Three distinct fea-
ture are visible on the plot. At the bottom it is
possible to notice a shallow layer where R is
clearly above 1. This layer represents the noc-
turnal residual layer. The increase in scattering
Fig. 4. MULID profile of backscattering ratio obtained in Brazil during a night time short duration flight. PBL
is detected as a shallow area near the ground where scattering is above 1. A cloud is detected at the end of ac-
quisition period.

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Optical measurements of atmospheric particles from airborne platforms
ratio is mainly due to high concentration dust
and cloud droplet often found below this mix-
ing layer. At the bottom, on the right, there is a
single cloud layer similar to a low level stra-
tocumulus. Finally, it is possible to notice the
ground reflectance as a sharp increase in scat-
tering ratio.
We have not included in the plot the data
coming from places where the laser beam is not
completely inside the telescope FoV. It has to
be mentioned that to increase SNR we volun-
tary reduced spatial and temporal resolution by
the use of a running average, to 150 m and 5
min respectively.
For MAS we present data from a flight per-
formed on February 6th 2003 from Kiruna
(Sweden) during the EUPLEX campaign. In the
plot (fig. 5) it is possible to follow the complete
sequence of calibration operated on the raw da-
ta. From the top you can notice the peak signal
and the background. The difference between
the two leads to the fourth line (second of the
second plot box) that represents the signal not
yet normalized by laser power variations. Those
laser variations are indicated by the third line
(first of the second plot box). Taking into ac-
count the laser effect we have the corrected sig-
nal plot. If we calibrate the signal above tropo-
pause as mentioned in the previous section we
obtain R signal.
5. Conclusions
We have presented two innovative light-
weight optical instruments. MULID is the first
balloon borne microlidar ever built. As our pre-
liminary results demonstrates, it will be very
useful in the tropics. At low latitude accurate
observation of microphysical properties of cir-
rus clouds are very important: both for cirrus
modeling and for the comprehension of the
stratosphere to troposphere exchange process
(Danielsen, 1993).
MULID will provide an interesting dataset
regarding faint tropical cirrus clouds. Also ac-
Fig. 5. MAS data stream. From above signal of the backscattered peak and background signal, laser monitor
line, actual signal (peak-background) not yet corrected for the laser power variations, corrected signal and final-
ly calibrated scattering ratio. In this case the absence of clouds is evidenced by the R constancy.

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Carlo Buontempo, Francesco Cairo, Guido Di Donfrancesco, Roberto Morbidini, Maurizio Viterbini and Alberto Adriani
curate in situ measurements are basic for the
understanding of cloud microphysics.
MAS represents the state of the art back-
scatter sonde. It works, both at night and day, at
three different wavelengths in the two polariza-
tions. Here we have presented only the data of
the visible channel.
The backscattering at three wavelengths and
the color ratio will help us to better understand
the formation of PSCs and cirrus cloud.
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