Measurement of Mount Etna plume by CO<sub>2</sub>-laser-based lidar

doi:10.1364/OL.34.000800

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Measurement of Mount Etna plume by CO₂-laser-based lidar

Luca Fiorani, Francesco Colao, and Antonio Palucci »View Author Affiliations

Optics Letters, Vol. 34, Issue 6, pp. 800-802 (2009)
http://dx.doi.org/10.1364/OL.34.000800

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Abstract

The $C O_{2}$ laser-based agile tuner lidar for atmospheric sensing has been used to profile the volcanic plume of Mount Etna during its most recent eruption. Owing to the transmitted wavelength, this system is practically insensitive to air molecules while it detects aerosol loads, and thus the path attenuation of the laser beam is strongly affected by volcanic particulate. Vertical profiles of extinction coefficient were retrieved up to an altitude above ground level of $5000 m$ . The observed extinction coefficient ranges from $10^{- 5} to 5 \times 10^{- 4} m^{- 1}$ . The lidar was able to accurately track the spatiotemporal evolution of the volcanic plume thanks to a spatial resolution of $15 m$ and a temporal resolution of $1 \min$ .

Volcanic particulate can strongly affect stratospheric and tropospheric temperatures, thus contributing to climate change. Although lidar [1

R. M. Measures, Laser Remote Sensing (Krieger, 1992).

] soundings of volcanic particulate have been carried out in the lower stratosphere after major eruptions [2

H. Jaeger, Geophys. Res. Lett. 19, 191 (1992). [CrossRef]

], lidar measurements of volcanic plumes [3

P. Weibring, H. Edner, S. Svanberg, G. Cecchi, L. Pantani, R. Ferrara, and T. Caltabiano, Appl. Phys. B 67, 419 (1998). [CrossRef]

] are quite unusual, even if they offer the attractive capability of retrieving flux rates [4

J. N. Porter, K. A. Horton, P. J. Mouginis-Mark, B. Lienert, S. K. Sharma, E. Lau, A. J. Sutton, and T. Elias, Geophys. Res. Lett. 29, 1783 (2002). [CrossRef]

]. Moreover, to our knowledge, it is the first time that a

C O_{2}

-laser-based lidar is used to profile a volcanic plume. According to Mie theory, a

C O_{2}

-laser-based lidar, because of its wavelength of operation around

10 μ m

, has the advantage of efficiently detecting the relatively large aerosols that form volcanic plumes and, at the same time, being practically insensitive to molecules and small aerosols of a different origin.

Recently, the agile tuner lidar for atmospheric sensing (ATLAS) [5

L. Fiorani, M. Aglione, I. Okladnikov, and A. Palucci, in Proceedings of the First ‘Atmospheric Science Conference’ (ESA, 2006).

] has been developed and mounted on the ENVIronmental LABoratory (ENVILAB), hosted in a small truck. ATLAS (Table 1 ) can be decomposed into four subsystems: transmitter, receiver, detector, and analog-to-digital converter (ADC). The main parts of the transmitter are a tunable transverse excited atmospheric

C O_{2}

laser (WH-20, Scientific Development & Integration, South Africa) and an off-axis reflective beam expander consisting of two oxygen-free high-conductivity copper mirrors manufactured in our laboratory. The laser is tunable thanks to the agile tuner consisting of a diffraction grating and a scanning mirror actuated by a computer-controlled galvo motor. The receiver is based on a Newton telescope. A liquid-nitrogen-cooled mercury-cadmium-telluride photodiode (PV-12-1, Fermionics, United States), coupled with a preamplifier designed to compliment it (PVA-500-10, Fermionics, United States), has been chosen as the detector.

The most recent eruption of Mount Etna started on May 13, 2008. Lidar measurements were carried out on July 14, 2008, while the volcano was still active. ATLAS was located in Santuario Magazzeni (latitude, 37.75986; longitude, 15.10459; altitude,

1025 m

), i.e., about

10 km

east from the main craters (altitude,

3329 m

), and was pointed vertically. A strong wind came from the west, blowing the plume just over the system so that it remained around the altitude level of Mount Etna and thus above the top of the planetary boundary layer (PBL). An approximate position and altitude of the plume were visually checked from the system site and from locations some kilometers away from Santuario Magazzeni before, during, and after the data acquisition.

Within the

C O_{2}

laser spectrum

(9 - 11 μ m)

, the beam is weakly attenuated by the atmosphere, i.e., the extinction coefficient is of the order of

10^{- 4} m^{- 1}

for visibility larger than

5 km

A. Ben-David, Appl. Opt. 38, 2616 (1999). [CrossRef]

], unless the emission wavelength is tuned at the absorption line of an air molecule as in differential absorption lidar [1

R. M. Measures, Laser Remote Sensing (Krieger, 1992).

] experiments. The spectral stability is not critical in elastic backscatter lidar [1

R. M. Measures, Laser Remote Sensing (Krieger, 1992).

] experiments, because in this case the phenomena dominating the extinction and the backscattering coefficients depend weakly on the wavelength [6

A. Ben-David, Appl. Opt. 38, 2616 (1999). [CrossRef]

]. Nevertheless, it was checked, and there was no difference observed between the wavelength computer controlled by the galvo motor and the wavelength measured with a spectrum analyzer.

An example of the lidar signal is given in Fig. 1 . The repetition rate was

5 Hz

, and ATLAS was operated in the elastic backscatter lidar mode, transmitting a wavelength of

10.591 μ m

corresponding to the 10P20 emission line of the

C O_{2}

laser. The experiment started at 10:45 a.m. (local civil time, i.e., universal time

coordinated + 2

) and 3000 lidar returns were averaged corresponding to a temporal resolution of

10 \min

. The spatial resolution depends on the sampling rate and is

15 m

The experimental points are compared with a calculation [1

R. M. Measures, Laser Remote Sensing (Krieger, 1992).

] assuming a constant extinction coefficient of

0.25 {km}^{- 1}

(this value has been retrieved from the signal slope between 500 and

1000 m

) and with the level corresponding to the specified noise equivalent power (NEP) of the detector. The assumption of a constant extinction coefficient is very crude, as demonstrated by the not negligible discrepancy between experimental points and the calculated intensity of the signal, and was used only to estimate the maximum theoretical altitude of measurement as the altitude where the calculated intensity of the signal is equal to the specified NEP of the detector (slightly beyond

5000 m

The maximum of the signal around

150 m

is well explained by the overlap function. The altitude of complete overlap is about

300 m

, in good agreement with an analysis of the receiver response [7

R. Velotta, B. Bartoli, R. Capobianco, L. Fiorani, and N. Spinelli, Appl. Opt. 37, 6999 (1998). [CrossRef]

]. Between 1800 and

3300 m

of altitude, the laser beam intercepts the volcanic plume and the number of backscattered photons rises accordingly. Beyond

4000 m

signal and noise are hardly distinguishable, in reasonable agreement with the maximum theoretical altitude of measurement.

Starting from

300 m

, the extinction coefficient was retrieved from the lidar signal (Fig. 2 ) with an inversion method [8

L. Fiorani, in Lasers and Electro-Optics at the Cutting Edge (Nova, 2007).

], derived from the algorithms published by Klett [9

J. D. Klett, Appl. Opt. 20, 211 (1981). [CrossRef] [PubMed]

] and Fernald [10

F. G. Fernald, Appl. Opt. 23, 652 (1984). [CrossRef] [PubMed]

], assuming a constant backscatter-to-extinction ratio of the order of

10^{- 3} - 10^{- 1} {sr}^{- 1}

, in good agreement with aerosol measurements and models [6

A. Ben-David, Appl. Opt. 38, 2616 (1999). [CrossRef]

, 11

A. Deepak, G. S. Kent, and G. K. Yue, Atmospheric Backscatter Model Development for $C O_{2}$ Wavelengths (NASA, 1982). [PubMed]

, 12

M. J. Post, F. F. Hall, R. A. Richter, and T. R. Lawrence, Appl. Opt. 21, 2442 (1982). [CrossRef] [PubMed]

, 13

R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, Appl. Opt. 23, 2510 (1984). [CrossRef] [PubMed]

]. The reference range was

300 m

, and the reference extinction was evaluated with the slope method using the 14 signal points from

300 to 495 m

(owing to the incomplete overlap, it was impossible to extend the evaluation below

300 m

Except the double peak from

1800 to 3300 m

, the extinction coefficient shows a behavior in agreement with the roughly exponential decrease already observed in similar studies [11

A. Deepak, G. S. Kent, and G. K. Yue, Atmospheric Backscatter Model Development for $C O_{2}$ Wavelengths (NASA, 1982). [PubMed]

, 12

M. J. Post, F. F. Hall, R. A. Richter, and T. R. Lawrence, Appl. Opt. 21, 2442 (1982). [CrossRef] [PubMed]

, 13

R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, Appl. Opt. 23, 2510 (1984). [CrossRef] [PubMed]

]. After a more careful inspection, two nearly linear decays can be distinguished; the first one is stronger and extends between 300 and

1000 m

, and the second one is weaker and extends between 1000 and

4300 m

. The most probable explanation of this behavior is that in the intervals

300 - 1000 m

and

1000 - 4300 m

the lidar probes PBL and free troposphere (FT), respectively, while the double peak from

1800 to 3300 m

is due to the volcanic plume. The difference in the slope of the extinction coefficient in the intervals

300 - 1000 m

and

1000 - 4300 m

implies that the optical properties and the aerosol loads of PBL and FT are dissimilar. As far as we know, all published profiles measured by

C O_{2}

lasers start at about

2000 m

[11

A. Deepak, G. S. Kent, and G. K. Yue, Atmospheric Backscatter Model Development for $C O_{2}$ Wavelengths (NASA, 1982). [PubMed]

, 12

M. J. Post, F. F. Hall, R. A. Richter, and T. R. Lawrence, Appl. Opt. 21, 2442 (1982). [CrossRef] [PubMed]

, 13

R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, Appl. Opt. 23, 2510 (1984). [CrossRef] [PubMed]

], and they cannot be compared with this study below that altitude. Nevertheless, the different optical properties of PBL and FT are routinely exploited to observe the temporal evolution of the PBL height by ultraviolet, visible, and near-IR lidars [8

L. Fiorani, in Lasers and Electro-Optics at the Cutting Edge (Nova, 2007).

After the extinction profile of 10:45 a.m., three more measurements were carried out in the same experimental conditions at 12:38 p.m., 13:19 p.m., and 14:00 p.m. (Fig. 3 ). Note that at 12:38 p.m. the profile extends up to

5000 m

, thus approaching the maximum theoretical altitude of measurement. The fact that different signals do not have the same maximum altitude of measurements is not surprising, because the signal-to-noise ratio can vary somewhat according to the change in atmospheric conditions. While the general behavior remains more or less the same, the extinction coefficient between 300 and

1000 m

increases with time and the volcanic plume spreads and stratifies after 10:45 a.m. The increase in extinction coefficient between 300 and

1000 m

in the warmest hours of the day can be explained by the convective activity that mixes the PBL. As soon as the Sun’s radiation heats the Earth’s surface, the aerosols that are below

300 m

in the early morning start to be raised above that altitude by thermal columns [8

L. Fiorani, in Lasers and Electro-Optics at the Cutting Edge (Nova, 2007).

], thus falling in the measurement range of ATLAS.

A moving aerosol plume may spontaneously undergo different microphysical processes that include condensation, evaporation, coagulation, sedimentation, and diffusion [13

R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, Appl. Opt. 23, 2510 (1984). [CrossRef] [PubMed]

]. During this study, owing to the strong wind blowing the plume, it is likely that the change in extinction coefficient beyond

1000 m

is related to transport phenomena, and thus the lidar observation of the spatiotemporal evolution of the volcanic plume gives information on the air mass dynamics of the FT.

Although the extinction profiles of Fig. 3 were acquired with a spatial resolution of

15 m

and a temporal resolution of

10 \min

, ATLAS can follow the spatiotemporal evolution of the volcanic plume with a higher temporal resolution. To demonstrate this, the sample of 3000 lidar returns acquired at 10:45 a.m. was divided into ten consecutive subsamples of 300 lidar returns; then each set of 300 lidar returns was averaged, corresponding to a temporal resolution of

1 \min

, and the extinction coefficient was retrieved per each subsample. Eventually, the extinction profiles were combined in a contour plot of the extinction coefficient as a function of time and altitude (Fig. 4 ) showing in detail the spatiotemporal dynamics of the volcanic plume. It is clear that the optical thickness of the plume increases with time and the second peak develops at the end of the observation period.

In conclusion, the

C O_{2}

-laser-based lidar ATLAS has been used to profile the volcanic plume of Mount Etna. According to Mie theory, ATLAS is particularly sensitive to volcanic particulate. Extinction coefficient profiles have been retrieved up to an altitude above ground level of

5000 m

with a spatial resolution of

15 m

and a temporal resolution of

1 \min

. These performances are adequate to follow the spatiotemporal dynamics of the volcanic plume.

Acknowledgments

The authors are deeply grateful to R. Fantoni for her constant encouragement and A. Aiuppa, G. Giudice, and C. Spinetti for fruitful discussions. We extend a special thank you to D. Del Bugaro for his important involvement, N. du Preez and F. Lamattina for laser assistance, and R. Giovagnoli for mechanical parts. We gratefully acknowledge the valuable support at Mount Etna by G. Giuffrida, R. Guida, and M. Liuzzo. We also deeply appreciated the helpful suggestions of the referees.

References and links

1.	R. M. Measures, Laser Remote Sensing (Krieger, 1992).
2.	H. Jaeger, Geophys. Res. Lett. 19, 191 (1992). [CrossRef]
3.	P. Weibring, H. Edner, S. Svanberg, G. Cecchi, L. Pantani, R. Ferrara, and T. Caltabiano, Appl. Phys. B 67, 419 (1998). [CrossRef]
4.	J. N. Porter, K. A. Horton, P. J. Mouginis-Mark, B. Lienert, S. K. Sharma, E. Lau, A. J. Sutton, and T. Elias, Geophys. Res. Lett. 29, 1783 (2002). [CrossRef]
5.	L. Fiorani, M. Aglione, I. Okladnikov, and A. Palucci, in Proceedings of the First ‘Atmospheric Science Conference’ (ESA, 2006).
6.	A. Ben-David, Appl. Opt. 38, 2616 (1999). [CrossRef]
7.	R. Velotta, B. Bartoli, R. Capobianco, L. Fiorani, and N. Spinelli, Appl. Opt. 37, 6999 (1998). [CrossRef]
8.	L. Fiorani, in Lasers and Electro-Optics at the Cutting Edge (Nova, 2007).
9.	J. D. Klett, Appl. Opt. 20, 211 (1981). [CrossRef] [PubMed]
10.	F. G. Fernald, Appl. Opt. 23, 652 (1984). [CrossRef] [PubMed]
11.	A. Deepak, G. S. Kent, and G. K. Yue, Atmospheric Backscatter Model Development for $C O_{2}$ Wavelengths (NASA, 1982). [PubMed]
12.	M. J. Post, F. F. Hall, R. A. Richter, and T. R. Lawrence, Appl. Opt. 21, 2442 (1982). [CrossRef] [PubMed]
13.	R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, Appl. Opt. 23, 2510 (1984). [CrossRef] [PubMed]

Table 1 Main Specifications of ATLAS

	Pulse Energy	$750 mJ$ (at the 10P20 emission line)
Transmitter	Pulse duration	$60 ns$ (FWHM)
	Repetition rate	$2 \div 20 Hz$
	Transmitted wavelength	$9.2 \div 10.8 μ m$
	Beam divergence	$0.7 mrad$
Receiver	Mirror coating	Au
	Diameter	$310 mm$
	Focal length	$1.2 m$
Detector	Diameter	$1 mm$
	Specific detectivity	$4 \times 10^{10} cm {Hz}^{1 ∕ 2} W^{- 1}$
	Gain	200
	Linear dynamic range	$0.1 \div 1000 mV$
	Bandwidth	$0 \div 10 MHz$
Analog-to-digital converter	Dynamic range	$8 bit$
Analog-to-digital converter	Sampling rate	$10 Ms s^{- 1}$

Fig. 1 Lidar signal compared with a theoretical model and the detector noise. The altitude in all figures is above ground level.

Fig. 2 Extinction coefficient retrieved from the lidar signal of Fig. 1. The double peak from

1800 to 3300 m

appears to superimpose on a nearly linear decay and has been ascribed to the volcanic plume (the dashed curve has been added for the convenience of the reader).

Fig. 3 Extinction coefficient retrieved at 10:45 a.m., 12:38 p.m., 13:19 p.m., and 14:00 p.m. in the same experimental conditions of Fig. 1. The volcanic plume appears to spread and stratify with time.

Fig. 4 Contour plot of the extinction coefficient as a function of time and altitude.

OCIS Codes
(010.1100) Atmospheric and oceanic optics : Aerosol detection
(010.1300) Atmospheric and oceanic optics : Atmospheric propagation
(280.3640) Remote sensing and sensors : Lidar
(280.1350) Remote sensing and sensors : Backscattering

ToC Category:
Atmospheric and Oceanic Optics

History
Original Manuscript: November 20, 2008
Revised Manuscript: January 19, 2009
Manuscript Accepted: January 27, 2009
Published: March 11, 2009

Citation
Luca Fiorani, Francesco Colao, and Antonio Palucci, "Measurement of Mount Etna plume by CO₂-laser-based lidar," Opt. Lett. 34, 800-802 (2009)
http://www.opticsinfobase.org/ol/abstract.cfm?URI=ol-34-6-800

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References

R. M. Measures, Laser Remote Sensing (Krieger, 1992).
H. Jaeger, Geophys. Res. Lett. 19, 191 (1992). [CrossRef]
P. Weibring, H. Edner, S. Svanberg, G. Cecchi, L. Pantani, R. Ferrara, and T. Caltabiano, Appl. Phys. B 67, 419 (1998). [CrossRef]
J. N. Porter, K. A. Horton, P. J. Mouginis-Mark, B. Lienert, S. K. Sharma, E. Lau, A. J. Sutton, and T. Elias, Geophys. Res. Lett. 29, 1783 (2002). [CrossRef]
L. Fiorani, M. Aglione, I. Okladnikov, and A. Palucci, in Proceedings of the First 'Atmospheric Science Conference' (ESA, 2006).
A. Ben-David, Appl. Opt. 38, 2616 (1999). [CrossRef]
R. Velotta, B. Bartoli, R. Capobianco, L. Fiorani, and N. Spinelli, Appl. Opt. 37, 6999 (1998). [CrossRef]
L. Fiorani, in Lasers and Electro-Optics at the Cutting Edge (Nova, 2007).
J. D. Klett, Appl. Opt. 20, 211 (1981). [CrossRef] [PubMed]
F. G. Fernald, Appl. Opt. 23, 652 (1984). [CrossRef] [PubMed]
A. Deepak, G. S. Kent, and G. K. Yue, Atmospheric Backscatter Model Development for CO2 Wavelengths (NASA, 1982). [PubMed]
M. J. Post, F. F. Hall, R. A. Richter, and T. R. Lawrence, Appl. Opt. 21, 2442 (1982). [CrossRef] [PubMed]
R. T. Menzies, M. J. Kavaya, P. H. Flamant, and D. A. Haner, Appl. Opt. 23, 2510 (1984). [CrossRef] [PubMed]

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