www.osa-opn.org40 | OPN October 2007

The ozone hole is an area over the Antarctic strato-sphere in which ozone levels have been depleted to more than 60 percent of what they were before 1975. The hole occurs in the period known as the austral spring—from September through early

December—when strong, westerly winds blow around the con-tintent and isolate polar air masses from mid-latitudes.

As this “polar vortex” passes over the region, it causes an increase in solar radiation that reaches the Earth’s surface. Thus, the ozone hole is thought to play an important role in global climate change. It also places the inhabitants of affected areas at increased risk for contracting skin cancer and certain ocular diseases.

Although there is a lot of science documenting the thinning of the ozone layer and its effects on human health, there are few high-latitude locations from which researchers can make long-term measurements to monitor the problem. Antarctica is, of course, ideal in terms of its proximity to the hole, but its remote location makes it a difficult choice.

Another option is Southern Argentina. Much of this region is located beyond parallel 51, which reaches the edge of the ozone hole during the austral spring. Our research group, from the Lidar Division of the Laser and Applications Research Center at CEILAP in Buenos Aires (348 339S, 588 309W, 20 m above sea level), conducts our measurements from the town of

Using Lidar to Measure the Ozone LayerEduardo Quel, Elian Wolfram, Lidia Otero, Jacobo Salvador, Juan Pallotta, Raúl D´Elía and Marcelo Raponi

South American scientists are

using lidar technology to measure

stratospheric ozone, water vapor

and aerosols in Argentine Patagonia.

Their work is helping to monitor global

climate change, and has led to the

development of a method for alerting

local populations to the presence of

the ozone hole.

OPN October 2007 | 41

Río Gallegos (518 369S, 698 199W, 15 m ASL), the capital of the Santa Cruz province of Argentina.

In addition to its location at one of the best possible lati-tudes, Río Gallegos offers the advantage of clear night skies—which facilitates atmospheric studies that use a wide range of remote sensing techniques.

Quantitative observations of the atmosphereThere are many possible methods for making quantitative observations of the atmosphere. One of the major distinctions is between in situ and remote-sensing measurements. The former are those that are done in the vicinity of the

measurement apparatus, such as the routine meteorological observations made by weather stations, while the latter assess atmospheric properties from a distance. Remote sensing can be done from instruments orbiting in space, from airplanes or balloons, or from ground-based instruments, which measure the electromagnetic radiation emitted, dispersed or transmitted through the atmosphere.

The remote sensing techniques that are used to measure atmospheric parameters can also be divided into two broad groups, according to the source of radiation used: passive and active. In passive remote sensing, the radiation measured is of natural origin—for example, the thermal radiation emitted by

Perito Moreno Glacier, Patagonia, Argentina

Luca Galuzzi

www.osa-opn.org42 | OPN October 2007

the atmosphere, or the solar radiation emitted or dispersed by it.

In active remote sensing, one instrument that is widely used is a lidar (which stands for “light detection and rang-ing”) system. Lidar is based on an emission laser and operates on similar principles to radar (radio detection and ranging) or sonar (sound navigation and ranging). In the case of a lidar, the transmitter sends pulses of radiation into the atmosphere, where they are dispersed in all directions by molecules, aerosols or inhomogeneities in the atmospheric structure.

A small amount of this radiation is directed backwards toward the lidar receiver, and some of it is registered by a detector. Th e principal components of a lidar include a pulsed laser source; a light collector (often a parabolic mirror); a photoreceptor, which transforms the backscattered photons into electrical signals; and a recording and calculation system, which digitalizes the electrical signal as a function of time (or as a function of distance from the light source) in addition to controlling other basic functions of the system.

The project’s beginningsResearchers’ interest in making lidar measurements from the southern tip of the southern hemisphere dates back to 1995. At that time, researchers from CEILAP, along with the late Prof. Gerard Mégie, who was then head of the Service d´Aeronomie in France, were considering conducting a campaign to measure ozone profi les using a system known as DIAL (diff erential abso-prtion lidar), in Patagonia, Argentina. Th e design and construc-tion of the lidar system then began as a collaboration between the two institutions.

For the DIAL technique, scientists use two laser wave-lengths to measure ozone in the atmosphere. One wavelength is strongly absorbed by ozone, while the other is less so. After the wavelengths travel through the atmosphere and scatter back to two receivers, researchers make a ratio of the measurements—which allows direct determination of the ozone concentration as a function of range.

Th e system became operational in 1997 in Villa Martelli, in the province of Buenos Aires, where the headquarters of CEILAP is located. Th e initial version had only one telescope,

which was 50 cm in diameter. It operated successfully until 2002. Later, the number of telescopes was increased to four and a spectrometer was added. Th e apparatus was fi ne-tuned at the Villa Martelli headquarters.

Th e Service d´Aéronomie provided the electronic equipment to the project and a container that had already been used in the Arctic for polar ozone campaigns. However, fi nancing was still an issue. Fortunately, since 1995, CEILAP has maintained a relationship with the Tohoku Institute of Technology in Sendai, Japan. As a result, the Japan International Cooperation Agency decided to fi nance the entire measurement campaign in the south. It made a further contribution with the acquisition of a new Nd:YAG laser, which is essential to the DIAL system. Th e campaign was named SOLAR (stratospheric ozone lidar of Argentina) and initiated in June 2005.

As we installed the system in a very southerly continental region in Argentina, we conducted a feasibility study for the campaign, taking into consideration the nocturnal cloud cover over four towns in Argentine Patagonia. We compared the data with those corresponding to days when the Antarctic polar vortex crosses over these towns.

Th en we gave consideration to diff erent tracers, such as the total ozone column values measured by total ozone mapping spectrometry, the equivalent latitude method and the potential vorticity maps calculated for the mid-stratosphere, accord-ing to studies carried out in collaboration with the Service d´Aeronomie in France and the National Institute for Environ-mental Studies in Japan.

Th e town that met the necessary conditions for these mea-surements was Río Gallegos. It is situated 2,630 km from the city of Buenos Aires, on the banks of the estuary of the River Gallegos, and has a population of 140,000. Like other towns in southern Argentina and Chile, Río Gallegos is reached by the edge of the ozone hole during the austral spring. However, compared with its counterparts, it has a greater number of clear nights or nights with less than one-eighth cloud cover—which meant there would be more opportunities for making measure-ments with the ozone DIAL.

Río Gallegos is also home to the National University of Southern Patagonia, whose staff could take part in the cam-

UV narrow band radiometer GUV-541 Biospherical Inst. Inc. 305, 313, 320, 340 and 380 nm

Sun photometer CIMEL

Pyranometer CM-11, Kipp & Zonen Holland 305–2,800 nm

UV-B radiometer YES UVB -1 280–320 nm

UV-A radiometer YES UVA -1 315–400 nm

1,200, 940, 870, 670, 500, 440, 380, 340 nm

Instrument Model Spectral range

CEILAP’s passive remote sensing instruments in Río Gallegos

OPN October 2007 | 43

paign, and is close to the town of Punta Arenas, Chile, where a research group has used a Brewer instrument to make ozone measurements for years in collaboration with Brazilian scien-tists. (Punta Arenas is about 200 km from Río Gallegos, on the banks of the Magallanes Strait.)

On June 10, 2005, we set off overland for Río Gallegos in two trucks that traveled 2,612 km from Buenos Aires to the Military Air Base in Río Gallegos, where we set up a mobile laboratory. The base is located 18 km from the center of the town.

Measuring stratospheric ozoneThe DIAL system is designed to measure stratospheric ozone. However, it can also detect tropospheric water vapor. It contains two lasers—an excimer laser, which emits radiation that is absorbed by ozone (308 nm) and a frequency-tripled Nd:YAG laser, which emits radiation that is scarcely absorbed by ozone (355 nm). The two laser pulses are emitted vertically. Using optical fibers at each focus, researchers can collect the backscat-tered pulses from both in the four telescopes, which are 50 cm in diameter with a focal length of 1 m. The fibers carry the radiation to a specially designed spectrometer.

The latter enables us to separate Rayleigh lines and thus determine the profile of the stratospheric ozone layer from Earth. This is accomplished by appropriately manipulating the lidar equation. It is also possible to detect the Raman lines that the pulses generate in nitrogen (332 and 387 nm, respec-tively) in order to correct for the possible presence of aerosols in the stratosphere. The design of the optical receiving system is similar to that of the ozone lidar system at Haute-Provence Observatory in France (e.g., Godin-Beekmann et al. J. Environ. Monitoring 5, 57-67).

In addition, the spectrometer allows us to separate the Ra-man line (347 nm) of water vapor generated by the 308 nm laser; thus, we can measure tropospheric water vapor profiles separately from stratospheric ozone.

To overcome the dynamic range of the Rayleigh signals, we measured the ozone profile at low and high altitudes. Interest-ingly, these systems are auto-calibrated, which means that prior

knowledge of the ozone content at a given altitude is not neces-sary. The content is simply obtained directly with each measure-ment—on average, hundreds of thousands of laser pulses.

Each point of the profile comes from thicknesses on the order of 150 m or more. For a two-year period, from 2003 to 2005, we operated the instrument on a trial basis at the CEILAP site at Villa Martelli, near the city of Buenos Aires. After that, we transported it south for the SOLAR campaign. The instrument always operates after midnight in Río Gallegos and when there are clear skies.

The SOLAR campaign has two measurement protocols: an intensive period between August and November each year (which extends from late winter through springtime in the Southern Hemisphere) and a routine measurement period for the rest of the year. During the intensive period, which is coincident with the ozone hole development, the stratospheric ozone layer is monitored for an average of four hours on each available clear night.

Since June 2005, when we deployed the lidar instruments in Río Gallegos, we have performed several experiments. For example, when monitoring the ozone hole during the austral spring, we obtained the ozone profiles shown in the top figure on p. 44. Two of them showed a reduction in ozone content

SOLAR team member align-ing mirrors of the lidar’s optical

bench, which send the laser beam into the atmosphere.

Ozone differential absorption lidar (DIAL)

Fiber optics

Acquisition Spectrometer

Telescopes

Lasers

Laser beam

Lidar (which stands for “light detection and ranging”) is based on an emission laser and operates on similar principles to radar (radio detection and ranging) or sonar (sound navigation and ranging).

www.osa-opn.org44 | OPN October 2007

as a result of the passage of the polar vortex that encloses the ozone hole over the town of Río Gallegos (Wolfram, 2006).

The aerosol studies

Another important measure our team has made is that of particles, or aerosols, in the atmosphere of Río Gallegos. These aerosols, which can be of natural origin or anthropogenic (man-made), are being studied increasingly throughout the world, largely due to their role in trapping the solar radiation that reaches the Earth’s surface.

There are several international networks that measure aero-sols. Perhaps the most important is the AERONET network, which belongs to NASA. The network includes 500 identi-cal instruments, called sun photometers, that are distributed throughout the world. NASA is responsible for keeping them running and calibrating them periodically. However, the Ad-ministration works with the appropriate local governments and organizations to help install them.

Four of the sun photometers are located in Argentina: There is one in Córdoba, a second at CITEFA in Villa Martelli, a third in Trelew, and a fourth in Río Gallegos.

For our aerosol study, we designed another lidar system that allows us to determine the optical properties of aerosols sus-pended in the air and their presence according to altitude. This system, known as a backscatter lidar, detects the three elastic lines generated by an Nd:YAG laser—that is, 355, 532 and 1064 nm—backscattered by aerosols, which we will call 3E. We have such a lidar operating in Río Gallegos.

We also have a similar lidar at the CEILAP headquarters in Villa Martelli. In addition to the three elastic lines previously mentioned, this lidar can also detect the two Raman lines (387 and 607 nm) generated by the 355 and 532 nm lines in nitro-gen, and the Raman line (407 nm) of water vapor generated by the 355 nm line, a system we call 3E+3I. A similar system will soon be installed in Río Gallegos.

Using the 3E+3I lidar system in Villa Martelli, we have been able to determine aerosol contaminations over the atmosphere of Buenos Aires for the past year. For example, we have ob-served aerosol layers originating from biomass burning that has came from as far away as Mendoza, Tucumán or even Brazil.

The middle figure to the left shows the evolution of the atmospheric boundary layer and aerosol layers transported between 1,500 and 6,000 m for September 20, 2004. In the bottom figure, we can see a profile of the aerosol backscatter coefficient for 1,064 nm and 532 nm at 14.00 h local time. The study of the three-day forward trajectories of the air masses, beginning September 17, 2004 at 00 UTC (Coordinated Uni-versal Time), was calculated by the NOAA HYSPLIT model; it indicates that the origin of the air masses that arrived in Buenos Aires were from Paraguay. Also, these air masses were detected by the lidar system at CEILAP in Villa Martelli. In a satel-lite image from September 18, plumes of smoke can be seen advancing over the Argentine territory.

[ Lidar ozone profiles ]

Profiles measured on October 3 (dashed blue line) and October 4 (dotted red line) in 2006. The black line corresponds to the mean lidar ozone profile for October outside the ozone hole.

[ Evolution of atmospheric boundary and aerosol layers ]

[ Aerosol backscatter coefficient ]

Normalized ratio of the aerosol backscatter coefficient measured at 1,064 nm to that measured at 532 nm for September 20, 2004.

Aerosol backscatter coefficient [10-7m-1 sr-1] at 14.00 h local time

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OPN October 2007 | 45

Applications and future directions

The presence of the mobile laboratory in Río Gallegos has enabled the municipality of Río Gallegos, in collaboration with researchers, to develop an ozone warning system for the town’s inhabitants. Using colored lights, the system indicates when the ozone hole is present so that people can take the necessary pre-cautions to minimize their UV exposure. The lights are installed on the banks of the River Gallegos. Data are provided every 10 minutes by the UV sensors installed in the container, and they are compared with those obtained by an in situ UV sensor.

In the coming winter and spring, the ozone lidar system will participate in the Oracle – O3 project, which brings together ozone scientists for the International Polar Year (www.awi-pots-dam.de/www-pot/atmo/ORACLE-O3). It will contribute to the evaluation of polar ozone loss and its influence on southern mid-latitude regions.

Nagoya University is building a millimeter-wave radiometer that will be installed at the National University of Southern Patagonia in Río Gallegos towards the middle of 2009, as part of International Polar Year. In addition, we will install a DOAS (Differential Optical Absorption Spectroscopy) system in collaboration with CIOp (CIC-CONICET) of La Plata, Argentina, for measuring stratospheric NO2. We are also exploring the possibility of installing a SAOZ (Système pour Analyse d’ Observations Zénithales) instrument to measure total O3 and NO2. (For more information, see www.division-lidar.com.ar.) t

The authors wish to thank the following institutions for collaborat-ing in the SOLAR campaign: the Japan International Coopera-tion Agency, the National Institute for Environmental Sciences, Nagoya University and the Sendai Institute of Technology from Japan; Service d´Aeronomie, Ecole Polytechnique and Université Pierre et Marie Curie from France; NASA and AERONET from the United States; Universidad de Magallanes from Chile, and Universidad Tecnológica Nacional, Universidad Nacional de San Martín, Universidad Nacional de la Patagonia Austral, Escuela Superior Técnica, Fuerza Aérea Argentina, Comisión Nacional de Actividades Espaciales, Servicio Meteorológico Nacional, Centro de

Investigaciones Ópticas, Instituto de Física Rosario and TetraPak Company from Argentina. We would also like to acknowledge the contributions of the following colleagues: Hideaki Nakane, Ka-suhiro Asai, Masaji Ono, Akira Mizuno, Tomoo Nagahama, Chan Bong Park, Sophie Godin-Beekmann, Pierre Flamant, Jacques Porteneuve, Brent Holben, Wayne Newcomb, Pablo Ristori, Andrea Pazmiño, Claudio Cassicia, N. Paus Neme, Jorge Tocho and the others members of CEILAP.

[ Eduardo Quel (quel@citefa.gov.ar), E. Wolfram, L. Otero, J. Salvador, J. Pallotta and M. Raponi are with CEILAP (CITEFA-

CONICET) in Buenos Aires, Argentina. ]

[ References and Resources ]

>> Argentina’s UV solar radiation monitoring network: www.dna.uba.ar/

>> AERONET-NASA (Aerosol Robotic Network): http://aeronet.gsfc. nasa.gov

>> Lidar division of the Laser and Applications Research Center, CEILAP: www.division-lidar.com.ar

>> Godin-Beekman S. et al. “Systematic DIAL ozone measurements at Observatoire de Haute-Provence,” J. Env. Monitoring 5, 57-67, 2003.

>> Wolfram E. et al. “SOLAR campaign: stratospheric ozone lidar of Argentina,” Proc. SPIE, 5887, Lidar Remote Sensing for Environmental Monitoring VI. U.N. Singh, ed., 588713, Sep. 12, 2005.

>> Otero L. et al. “Measurement of an aerosol episode in Buenos Aires, Argentina using sunphotometer and lidar,” Proc. SPIE, 5887, 257-64, Lidar Remote Sensing for Environmental Monitoring VI; Upendra N. Singh, ed., 2005.

>> Otero L. et al. “Aerosol optical properties by means of a sunphotom-eter and lidar system in Buenos Aires, Argentina”. Opt. Pura Apl., Especial Third Workshop, “Lidar Measurements in Latin America,” 39(1) 43-7, 2006.

>> Otero L. et al. “Lidar and AERONET measurements in Río Gallegos, Patagonia, Argentina,” reviewed and revised papers presented at the 23rd International Laser Radar Conference, C. Nagasawa and N. Sugimoto, eds., ISBN 4-9902916-0-3. Part II, 747-50, 2006.

>> Wolfram E. et al. “SOLAR Campaign: First Results of Ozone Profile Measurements at Rio Gallegos, Argentina,” 23rd International Laser Radar Conference (ILRC23), Nara, Japan, July 24-28, 2006.

>> Quel E. et al. “SOLAR Project: stratospheric ozone monitoring at Argentina subpolar region.” Proceedings of International Symposium Asian Collaboration in International Polar Year 2007-2008, Tokyo, Japan, published by the National Institute of Polar Research. 85-88, 2007.

Using colored lights, the ozone warning system indicates when the ozone hole is present so that people can take the necessary precautions to minimize their UV exposure.

Members of SOLAR team at the Río Gallegos site. Left to right: Elian Wolfram, Juan Pallotta, Raúl D´Elía and Jacobo Salvador.

Member