EPJ Web Conferences 237, 02009 (2020)
ILRC 29
https://doi.org/10.1051/epjconf/202023702009
CHARACTERIZATION OF AEROSOL SIZE AND MICROPHYSICAL
PROPERTIES FROM MULTI-WAVELENGTH RAMAN LIDAR
MEASUREMENTS: INTER-COMPARISON WITH IN SITU SENSORS
ONBOARD THE ATR 42 IN THE FRAMEWORK OF HYMEX-SOP1
Benedetto De Rosa1 , Paolo Di Girolamo1 , Donato Summa1 Dario Stellitano1
1
Scuola di Ingegneria, Università degli Studi della Basilicata, Potenza, 85100, Italy
*Email: bundit@hotmail.it
ABSTRACT
This extended abstract reports measurements that
were carried out by the Raman lidar system
BASIL in the frame of the Hydrological Cycle in
the Mediterranean Experiment – Special
Observation Period 1 (HyMeX-SOP1). A specific
case study was selected revealing the presence of
variable aerosol properties at different altitudes.
Specifically, Raman lidar measurements on 02
October 2012 reveal the presence of two distinct
aerosol layers, a lower one extending up to ~3 km
and an upper one extending from 3.5 km to 4.7
km. Aerosol and size microphysical properties are
determined from multi-wavelength measurements
of particle backscattering and extinction profiles
based on the application of a retrieval scheme
which employs Tikhonov’s inversion with
regularization. Inversion results suggest a size
distribution with the presence, in both the lower
and upper aerosol layer, of two particle modes (a
fine mode, with a radius of ~0.2 m, and a coarse
mode, with radii in the range 2-4 m), volume
concentration values of 2-4 mm3cm-3 and effective
radii in the range 0.2-0.6 m.
This effort benefited from the dedicated flights of
the French research aircraft ATR42, equipped
with a variety of in situ sensors for measuring
aerosol/cloud size and microphysical properties.
Aerosol size and microphysical properties
retrieved from multi-wavelength Raman lidar
measurements were compared with simultaneous
and co-located in-situ measurements.
1. INTRODUCTION
Tropospheric aerosols
are a fundamental
atmospheric component of the Earth’s radiation
budget. Increased aerosol concentrations intensify
atmospheric scattering and absorption processes
for both solar and planetary radiation.
Additionally, increased tropospheric aerosol
concentrations may lead to enhanced nucleation
and intensified cloud formation processes The
above mentioned direct and indirect aerosol effect
have a strong impact on the Earth’s radiation
budget. A quantitative assessment of these effects
is difficult to obtain and estimates are typically
affected by large uncertainties.
Despite the recognized importance of having
vertically resolved measurements of aerosol size
and microphysical properties, a limited number of
techniques can provide this information..
Airborne in situ sensors may be effective.
However, their exploitation is complex and costly,
and consequently their use is limited. Remote
sensing techniques have strong potential, but their
applicability and the reliability of results from
need to be verified against independent
measurements. Among others, multi-wavelength
Raman lidar measurements appear to be very
promising. The dataset reported and discussed in
the present research effort represents a unique
opportunity to verify the quality of the results in
terms of size and microphysical properties
obtained through the application of obtained the
retrieval scheme based on Tikhonov’s inversion
with regularization.
.
Reported measurements were carried out in the
frame of HyMeX-SOP1, which took place in the
period September–November 2012 over the
North-Western Mediterranean Sea and its
surrounding coastal regions in France, Italy and
Spain. In the frame of HyMeX-SOP1, the
University of BASILicata ground-based Raman
Lidar system (BASIL) was deployed in the
Cévennes-Vivarais site (Candillargues, Southern
France, Lat: 43°37' N, Long: 4° 4' E, Elev: 1 m).
© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0
(http://creativecommons.org/licenses/by/4.0/).
EPJ Web Conferences 237, 02009 (2020)
ILRC 29
https://doi.org/10.1051/epjconf/202023702009
not exceed 5 %, while the uncertainty affecting
particle extinction measurements must not exceed
10 % [10-11].
As part of HyMeX-SOP1, HyMeX-SOP1 the
French research aircraft ATR42, equipped with a
variety of in situ sensors for turbulence and
aerosol/cloud
microphysical
measurements,
performed more than 60 flight hours, 8 of which
as part of the EUFAR project “WaLiTemp. A
specific flight pattern was defined for this project,
with the aircraft making spirals up and down
around a central location, originally aimed to be
the atmospheric supersite in Candillargues, but,
because of air-traffic restrictions, 20 km eastward
of the supersite.
3. RESULTS
3.1 Case of study 2 October 2012
We have focused our attention on the case study
on 2 October 2012. The color map in figure 1
illustrates the time evolution of particle
backscattering coefficient at 355 nm covering a
period of approx. 4 h from 19:20 to 23:40 UTC.
On this day the ATR 42 flew from 19:20 to 22:20
UTC.
For the purpose of retrieving particle size
distribution parameters as a function of altitude,
we focused on a time interval when aerosol
concentration was higher. In order to get high
enough signals-to-noise ratios, and consequently
low statistical uncertainty, for the retrieval scheme
to be applicable, we considered a 2-h averaging
for both particle backscattering and extinction
coefficient, i.e. the time interval from 20:30 to
22:30 UTC.
2. METHODOLOGY
2.1 The lidar system
The lidar transmitter is developed around a
Nd:YAG laser source, including both second and
third harmonic generation crystals.. Laser pulses
at 1064, 532 and 355 nm are simultaneously
transmitted in the atmosphere along the zenith.
BASIL is capable to perform high-resolution and
accurate measurements of atmospheric water
vapour and temperature, both in daytime and
night-time, based on the application of the
vibrational and rotational Raman lidar techniques,
respectively, in the UV [1, 2, 3, 4, 5]. Besides
water vapour and temperature, BASIL, in its
HyMeX
configuration,
also
performed
measurements of the particle backscattering
coefficient at 355, 532 nm and 1064 of the particle
extinction coefficient at 355 nm and 532, and of
particle depolarization at 355 nm. In addition to
HyMeX-SOP1, BASIL has been deployed in
several international field campaigns [6-9].
A retrieval algorithm, exploiting Tikhonov’s
inversion with regularization, is applied to multiwavelength particle backscattering and extinction
measurements in order to retrieve particle size and
microphysical parameters. The retrieval scheme
makes use of kernel functions for spherical
particles. Veselovskii et al. [10-11] demonstrated
that an input data set consisting of particle
backscattering coefficient profiles at 355, 532 and
1064 nm and particle extinction coefficient at 355
and 532 is sufficient to allow retrieving specific
aerosol size and microphysical parameters, such
as number, surface, volume density, effective
radius and complex refractive index. In order form
this to be possible the statistical uncertainty
affecting particle backscatter measurements must
Fig.1 Time evolution of the backscattering coefficient
at 355 nm in the time interval 19:20-23:40 UTC on 2
October 2012.
Figure 2 shows the 5-days back-trajectory analysis
as determined with NOAA Lagrangian backtrajectory model HYSPLIT. The analysis reveals
that air masses reaching the measurement site at
an altitude of 600 m were originated over the
Northern Atlantic Ocean, South of Iceland, and
passed at low altitudes (500-600 m) over
continental areas with high anthropic impact
(Ireland, England and Northern France).
The chemical sensors onboard the ATR 42 show
high concentration values for NO3 SO4 NH4,
which are typical of a polluted aerosol (figure 3).
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EPJ Web Conferences 237, 02009 (2020)
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Extinction
The back-trajectory analysis also reveals that air
masses at 4000 m were originated over the
Northern Atlantic Ocean, off-shore the Canadian
coast, and overpassed the Northern coast of Spain
before reaching the measurement site. This is a
marine aerosol, with a high concentration of
sulfates.
Both particle backscattering and extinction
coefficient profiles, shown in figure 4, reveal the
presence of the two different aerosol layers. The
lower polluted layer extends up to ~3 km, while
the upper marine layer extends from 3.5 to 4.7 km.
To reduce the statistical noise the backscattering
coefficient was determined with a vertical
resolution of 300 m, while the extinction
coefficient with a vertical resolution of 600 m.
The backscatter coefficient at 355 and 532 was
determined based on the application of the Raman
technique, while at 1064 the Klett-modified
method was applied [12-13]. The extinction at
both 355 and 532 nm was calculated using the
approach proposed by Ansmann [14].
0,000000
5000
0,000015
0,000030
0,000045
b355
b532
b1064
a355
a532
4500
4000
Height, m
0,000060
3500
3000
2500
2000
1500
0,0000000
0,0000001
0,0000002
0,0000003
0,0000004
backscattering
Figure 4: Vertical profile of the backscattering
coefficient at 355, 532 and 1064 nm and the extinction
coefficient at 355 and 532nm.
Aerosol size and microphysical properties
obtained from the multi-wavelength Raman lidar
data based on the application of the above
mentioned inversion algorithm are compared with
those simultaneously measured
from in-situ
sensors on-board the ATR42. Figure 5 illustrates
the comparison expressed in terms of volume
concentration measurements, with the two
instruments showing similar values both in the
lower (2-4 mm3cm-3 for the Raman lidar and 2-5
mm3cm-3 for the in-situ sensors) and upper layer
(2-3 mm3cm-3 for the Raman lidar and 1-3.5
mm3cm-3 for the in-situ sensors).
5000
Basil
SMPS and OPC Grimm
4500
Heigth [m]
4000
3500
3000
2500
Figure 2 Back-trajectory analysis as determined with
NOAA HYSPLIT model. The three illustrated
trajectories are those ending over the lidar site at 600 m,
4000 m and 6000 m (in red, blue and green,
respectively) at 20:00 UTC on 2 October 2012.
2000
1500
0
1
2
3
4
3
V, mm /cm
5
6
7
3
Figure 5: Comparison in terms of volume
concentration measurements between BASIL and the
on-board in situ sensors.
Figure 6 shows the variability with altitude of the
effective radius, with values in the range 0.2-0.6
m. The agreement between BASIL and the insitu sensors is good at all altitudes, with values
measured by the in-situ sensors always within the
error bar of the corresponding measurements by
the Raman lidar, with the only exception of the
values at ~4 km. As expected, values in the upper
Figure 3: Measurements carried out by the chemical
sensors on board the ATR 42.
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EPJ Web Conferences 237, 02009 (2020)
ILRC 29
https://doi.org/10.1051/epjconf/202023702009
layer are slightly larger than those in the lower
layer, as in fact marine aerosols are typically
larger than urban continental aerosols.
3,0
2,0
3
dV/dlnr, m /cm
3
2,5
Basil, 3975 m
Basil, 4275 m
In situ sensors, 3700-4300 m
Basil
In situ sensors
4000
1,5
1,0
3500
0,5
Heigth [m]
3000
0,0
2500
0,1
1
10
Radius, m
2000
Figure 9: Comparison in terms of size distribution at
3975 m measurements between BASIL and the onboard in situ sensors.
1500
1000
0,2
0,4
0,6
0,8
1,0
1,2
1,4
1,6
At 2 and 2.7 km the size distribution is dominated
by the continental aerosols, while at 4 km most
aerosols have a marine origin and have crossed
the Atlantic before reaching the site.
Effective radius, m
Figure 6: Comparison in terms of effective radius
measurements between BASIL and the on-board in situ
sensors.
Figures 7, 8 and 9 show the aerosol size
distribution at 2, 2.7 and 4 km. The agreement
between Raman lidar and in-situ sensors is good
at all altitudes. It is to be specified that the in-situ
sensors are not sensitivity to particle with
diameters in excess of 2 m.
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Basil, 2175 m
In situ sensors, 1700-2300 m
2,5
3
dV/dlnr, m /cm
3
2,0
1,5
1,0
0,5
0,0
0,1
1
10
Radius, m
Figure 7: Comparison in terms of size distribution at
2175 m measurements between BASIL and the onboard in situ sensors.
Basil, 2775 m
In situ sensors, 2400-2900 m
1,6
1,4
3
0,8
dV/dlnr, m /cm
1,0
3
1,2
0,6
0,4
0,2
0,0
-0,2
0,01
0,1
Radius, m
1
Figure 8: Comparison in terms of size distribution at
2775 m measurements between BASIL and the onboard in situ sensors.
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