**4.2 Differential absorption lidar measurements in Argentina**

The behavior of trace constituents in the Earth's upper atmosphere, dictated by diverse physical processes, is of particular interest for the balance of stratosphere and mesosphere. Expressly, ozone has a principal function by absorbing the shortwavelength UV radiation (which might damage life) and keep the radiative budget stable [68]. For those reasons, ozone has been at the focus of the middle atmosphere research effort [69, 70].

Researchers' interest in performing lidar measurements from the southern region of the southern hemisphere dates back to 1995. Researchers from CEILAP, together with Prof. Gérard Mégie (who was then head of the *Service d'Aéronomie* in France), considered conducting a campaign to measure ozone profiles using a DIAL (differential absorption lidar) system, in Patagonia, Argentina [71]. The configuration and installation of the lidar system began as a collaboration linking the two institutions. For the DIAL technique, two laser wavelengths are used to measure atmospheric ozone. One wavelength is well absorbed by ozone, while the other not. After the wavelengths travel into the atmosphere and are backscattered

#### **Figure 7.**

*Quick-look of the RCS at 532 nm measured at SPU Lidar Station on 27 April 2015. The SPU Lidar Station is installed at the Center for Lasers and Applications of the nuclear and energy research institute (CELAP/ IPEN) in São Paulo. The signal between 18 km and 20 km shows aerosols originating from the Calbuco volcano eruption on 22 April 2015, in Chile.*

to two receivers, it is possible to make a ratio of the measurements, allowing direct determination of the ozone concentration as a function of range.

The instrument became operational in 1997 in Villa Martelli, Buenos Aires, where the headquarters of CEILAP is located. The 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. The apparatus was fine-tuned at the Villa Martelli headquarters.

The *Service d'Aéronomie* loaned the equipment's electronic project and a container, which had already been used in the Arctic. However, financing was still an issue. Fortunately, since 1999, CEILAP has cooperated with the Tohoku Institute of Technology in Sendai, Japan. The Japan International Cooperation Agency (JICA) supported the south's entire measurement campaign. It further contributed to acquiring a new Nd:YAG laser, which is imperative to the DIAL instrument. In this way, the SOLAR (stratospheric ozone lidar of Argentina) campaign started in June 2005 [72].

The campaign's feasibility study was conducted, considering the nocturnal cloud cover over four towns in Argentine Patagonia. The data were compared with those corresponding to days when the Antarctic polar vortex crosses over these towns.

Different tracers were also considered, 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, according to studies carried out in collaboration with the Service d'Aeronomie in France and the National Institute for Environmental Studies in Japan.

The city of Río Gallegos region met the necessary conditions for the measurements. It is located at 2612 km from Buenos Aires, on the River Gallegos estuary banks, and has 140,000 inhabitants. Like other cities in southern Argentina and Chile, Río Gallegos is reached by the ozone hole's edge during the austral spring. However, compared with its counterparts, it has a more significant number of clear nights or nights with less than one-eighth cloud cover, which means more opportunities for making measurements with the ozone DIAL. Río Gallegos also hosts the National University of Southern Patagonia, whose staff could participate in the campaign, and is near to Punta Arenas, Chile, where another research group has used a Brewer instrument to make ozone measurements, in cooperation with Brazilian researchers. On 10 June 2005, the team set off overland for Río Gallegos in two trucks that traveled 2612 km from Buenos Aires to the Military Air Base in Río Gallegos, where a mobile laboratory was set up. The base is located 18 km from the center of the town [72, 73].

A Xe:Cl excimer laser emission at 308 nm is employed for the absorbed wavelength in the DIAL technique, and an Nd:YAG laser at 355 nm third harmonic line is employed as the reference wavelength. Six channels are used for signal acquisition [72]. Four of them detect the emitted wavelengths' elastically backscattered signal (high energy mode for the higher altitude ranges, attenuated energy for the lower ranges), and two correspond to the Raman wavelengths [72]. The CEILAP's DIAL instrument setup is shown in **Figure 8**, and its full description can be found in Ref. [10].

The CEILAP Lidar Division, in cooperation with other national and international institutions, has organized the SOLAR (Stratospheric Ozone Lidar of ARgentina) Campaign as a part of environmental investigations in the Southern Hemisphere [72]. This campaign's objective was to monitor different atmospheric constituents using remote sensing techniques, mainly related to lidar, in Argentina's southern part. The most critical and complex instrument involved in this campaign is a differential absorption lidar capable of producing precise and accurate stratospheric ozone profiles [72, 73].

The most substantial decrease of the ozone column over Río Gallegos through the 2005 spring was observed on 8 October, with a total ozone column of 196 DU *Lidar Observations in South America. Part I - Mesosphere and Stratosphere DOI: http://dx.doi.org/10.5772/intechopen.95038*

**Figure 8.** *Experimental setup of differential absorption lidar (DIAL) developed at CEILAP.*

estimated from integrating an ozone profile based on the lidar measurement and the US Standard 1976. This value expresses a decrease of 45% in the total ozone column concerning the mean total ozone value outside the ozone hole for this month (about 350 DU). **Figure 9** shows the measured lidar profile on this day (dashed line), together with the ozone profile measured on 17 October (dotted line), which corresponds to standard ozone conditions outside the ozone hole (about 357 DU). The figure also shows the climatologic profile (black line) from the SAGE II measurements, which corresponds to the mean of the ozone measurements outside the ozone hole for the 1995–2004 period.

From the full set of lidar measurements, were selected 37 lidar profiles that match the HRLS profiles. The monthly mean lidar profiles were confronted with similar profiles measured by the High-Resolution Dynamics Limb Sounder (HIRDLS) device onboard the NASA-Aura satellite. The collocation criteria for selecting satellite data were set using a distance of up to 500 km from site measurement and a temporal selection of about twelve hours for the measurement time.

#### **Figure 9.**

*Lidar ozone profile inside (dashed line) and outside (gray dotted line) ozone hole in Río Gallegos. Climatologic profile for October from SAGE II data (black line) [74].*

#### **Figure 10.**

*Mean lidar profile (black line - error bar corresponds to ±1 std) and mean HIRDLS (white line) ±1 std. (shadow area) for October.*

The mean stratospheric ozone lidar profile for October in Río Gallegos is shown in **Figure 10**. For comparison, the same quantity from satellite data is included.

In general, good agreement between lidar and satellite data was found (inside the statistical error bar, with a relative difference of around 10%). The maximum disagreement between lidar and satellite data was observed in August mean profiles around 30 km. For October, the agreement was better than 10% above the ozone peak concentration. In general, it was observed that the variability of lidar profile concentrations is higher around the ozone peak, decreasing with height.

Differential Absorption lidar techniques have been demonstrated to be a reliable remote sensing technique to retrieve the stratosphere's ozone profile [73]. Argentina has used DIAL techniques since 1999. In 2005, with French and Japanese researchers' collaboration, the Lidar Division of CEILAP established a new site in Southern Patagonia, the South Patagonia Atmospheric Observatory (OAPA). This device has been part of Network Data for Atmospheric Composition Change (NDACC) since 2008, and the research using its measurements allows the study of ozone hole overpass from South America [75] and the satellite validation in the South Hemisphere. After the SOLAR Campaign, several initiatives were carried out related to stratospheric ozone monitoring in Argentina. For example, the UVO3-Patagonia (2008– 2010) and SAVER-Net projects (2013–2018) were the research activities made in collaboration with JICA, and Japanese and Chilean Researchers went more in-depth the observation of ozone in vertical profiles and total ozone column [76].

### **5. Conclusions**

Part I of this chapter offered the opportunity to give a scientific overview of current and past lidar observation activities conducted in South America, with Cuba's participation. This overview spans over almost 50 years of activities and grants how this part of the world is concerned with laser remote sensing of the atmosphere in almost its whole structure: Mesosphere and Stratosphere. This top-down approach

*Lidar Observations in South America. Part I - Mesosphere and Stratosphere DOI: http://dx.doi.org/10.5772/intechopen.95038*

also followed a chronological delivery of results, with the first results coming from the region in the highest portion of the atmosphere (mesosphere), and going downwards to stratospheric, and finally at the tropospheric studies. If, in the first years, these activities started as individual initiatives at different countries and research groups levels, the creation of a federative lidar network, namely LALINET, helped somehow to have more coordinated measurements. Moreover, the implementation of SAVERNET in Argentina and Chile improved how these joint measurements are conducted. The studies conducted in the mesosphere account for one of the most extended time series of lidar data, being of great importance in the Southern Hemisphere. Also, significant results about Na and K concentrations and their variability over almost three decades are available. The studies of ozone concentration in the stratosphere also provided relevant results, unprecedented for this portion of the globe. Part II of this chapter will be dedicated to tropospheric lidar observations.
