**2. Lidar remote sensing in Latin America: LALINET**

The South American continent, encompassing 42% of the Americas, is a region that shelters the most remarkable ecosystems. Among these, we can cite the Amazon Rainforest, which is the largest tropical forest in the world, the Pantanal (or Chaco), one of the UNESCO World Heritage Sites [12], and the Andes, the most extensive mountain chain in the world, and which hold a plethora of active and inactive volcanoes, extending from Venezuela to Patagonia, crossing all the continent from north to south. Patagonia, the continent's southern region, presents many plants and wildlife, mostly endemic. It also houses another UNESCO World Heritage Site: The National Park Los Glaciares, in Santa Cruz, Argentina [12].

#### **Figure 1.**

*Schematic representation for the location of the LALINET stations in South America. Argentina (AR): 1-) SMN Headquarters (Buenos Aires), 2-) CEILAP Headquarters (Buenos Aires), 3-) Comodoro Rivadavia (Chubut), 4-) Neuquén (Neuquén), 5-) Pilar (Cordoba), 6-) Río Gallegos airport (Santa Cruz), 7-) OAPA Río Gallegos (Santa Cruz), 8-) San Carlos de Bariloche (Río Negro), 9-) San Miguel de Tucumán (Tucumán). Bolivia (BO): 10-) La Paz (La Paz). Brazil (BR): 11-) Manaus (Amazonas), 12-) São Paulo (São Paulo), 13-) Cubatão (São Paulo), 14-) Natal (Rio Grande do Norte). Chile (CH): 15-) Punta Arenas (Magallanes), 16-) Temuco (Cautín). Colombia (CO): 17-) UNAL Medellín (Antioquia), 18-) SIATA Medellín (Antioquia), 19-) Cali (Valle del Cauca). Edited using Google my maps [14].*

Developing a regional ground-based lidar network in Latin and South America is of strategic importance: The knowledge rendered by the high-resolution profiles allows the knowledge of a wide variety of atmospheric phenomena to complement satellite observations and other retrievals by diverse ground-based instruments. Unfortunately, the available infrastructure of lidar stations in Latin America is limited in certain aspects. For example, only a few stations operate regularly (contrasted to Europe and North America), stations have different instrument designs, radiosonde launchings are not occurring nearby all stations, and only a reduced number of sun photometers is distributed across the continent [7, 11]. To get around such limitations and consolidate standard protocols of measurements, data acquisition, quality control, and assurance routines, and data analysis, the Latin America Lidar Network, LALINET, was established in 2001, during the First Workshop on Lidar Measurements in Latin America, held in Camagüey, Cuba, in March 2001 [7, 11, 13]. It was recognized as being part of the GAW (Global Atmospheric Watch) Aerosol Lidar Observation Network (GALION) in 2013 [7, 11, 13]. **Figure 1** shows the location of the LALINET stations [14].

The next sections of this chapter will present information about mesospheric, stratospheric, and tropospheric monitoring by LALINET stations and teams around South America and Cuba, plus some significant results. **Table 1** below shows the operational stations and their characteristics. A detailed description of LALINET origin and its evolution is given in Ref. [7]. The Letter of Agreement between LALINET and GAW can be found in Ref. [15].


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


*Details about the contributing teams, measurement protocols, reports, and equipment can be found on the web page http://www.lalinet.org. Detection of polarized light in the parallel (*∥*) and perpendicular (*⊥*) directions are indicated.*

#### **Table 1.**

*LALINET operational stations and their characteristics [7–11].*

## **3. Mesosphere**

Meteors enter in the upper atmosphere at very high velocities (15–70 km s−1), and the collisions with the atmospheric constituents cause flash heating until the particles melt and their chemical elements vaporize. This ablation process is responsible for the layers of metal atoms as Na, K, Fe, Mg, Ca, Si, among others, which occur globally in the mesosphere and lower thermosphere (MLT). This cosmic dust's primary sources are the sublimation of comets as they approach the Sun on their orbits through the solar system and the collisions between asteroids.

Lidar use for the upper stratosphere, mesosphere, and lower thermosphere investigations started in São José dos Campos, Brazil, in 1969 with a ruby laser operated at 694.3 nm. Clemesha and Rodrigues obtained the first aerosol profile using lidar in South America in 1971 [16]. The height range of measurement was 5 to 35 km due to the use of an 8 x 10″ receiver mirror. Later were obtained profiles up to 90 km in height using a 48″ mirror. In this work, high concentrations of aerosols were observed in the troposphere, a minimum just below the tropopause, around 15 km height, and higher concentrations in the lower stratosphere.

In 1972, when a new "handmade" dye laser became operational (see a Photo of this equipment in **Figure 2**), it was possible to start measurements of the Na layer in the MLT region, using Fabry-Perot interferometers and tuning the laser in the Na D2 line, 5890 Å, with a precision of 0.02 Å [17]. This system enabled the measurement of the mesospheric Na from 75 to 105 km of height [18]. The system continued to be operated regularly for long years obtaining the Na concentration at MLT region with different time and height resolutions, the stratospheric aerosol by Mie Scattering, and the atmospheric density and temperature from 30 to 65 km by Rayleigh scattering. In April 1975, 6 months after the eruption of Volcán de Fuego in Guatemala, a massive increase in aerosol loads was observed in São José dos Campos, which remained in the atmosphere for almost two years [19].

Through Na profiles between 82 and 99 km obtained with the laser beam directed alternately in three positions in the sky, it was possible to estimate the wind's speed in the mesosphere [20, 21]. The velocities vary with height in an oscillatory manner,

#### **Figure 2.**

*The handmade dye laser for Na probing (it operates from 1972 to 1992). See also in the picture Dr. Barclay R. Clemesha (in memoriam), the project's head.*

with the amplitude increasing with height. These wave-like formations vary slowly with time and might be produced by propagating tides in the atmosphere. These structures' common feature is their downward motion with time, consistent with the upward propagation of gravity wave energy. The more extended periods of oscillations are attributed to tides [22, 23]. Lidar measurements of the stratospheric aerosols enabled the observation of the eruption of El Chichón in México, eight months after in São José dos Campos, Brazil [24]. The transport of aerosols of the Pinatubo eruption was much more rapid and could be seen just 45 days after the eruption [25].

Research involving Na has included the first detection of the so-called Sporadic Sodium Layers [26]. The events occurred more frequently through periods of more significant meteor showers, especially in August. It is common to have sporadic E layers coincident with Na enhancement, which suggests that enhanced layers are generated by the wind shear distortion of Na clouds originated from meteor ablation. A significant result was that the long-lived sporadic layers appear to have a different nature from the short-lived ones. The difference is manifested in the more extensive duration and broader thickness and how the events are correlated with sporadic E layers [27].

In 1992, analyses of the vertical distribution of atmospheric Na layer with lidar showed a long-term trend of the centroid height, which decayed by approximately 700 meters between 1972 and 1987 [28]. However, from 1972 to 2001, the trend was 93 meters per decade. This new result appears dramatically diminishes the possibility of long-term cooling of the upper atmosphere [29].

In 1997 a new technique was developed to measure the Doppler temperature of the atmospheric Na layer by using a two-element birefringent filter together with a 0.2 nm free spectral range Fabry-Perot interferometer to produce a linewidth of about 20 pm. It produced a multi-line signal of the laser, with the lines spacing precisely equal to the separation of D2a and D2b transition of Na. With this assembly, it was possible to obtain the mesosphere's temperature with a 5 K precision, a height resolution of 1 km, and a time resolution of 6 minutes [30, 31]. Lately, in 2004 the lidar was equipped with a new laser, which permitted more precise measurements of the mesopause temperature (see the assembly in **Figure 3**) [32, 33]. Gravity wave's effects on the temperature in the mesopause were also studied [34, 35].

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

#### **Figure 3.**

*Photo presenting the continuum narrowband tunable laser for Na concentration and Mesopause temperatures. It operated at São José dos Campos measuring mesopause temperature from 2007 to 2009 and Na concentration from 2006 to 2016. This photo was taken by Barclay R. Clemesha (in memoriam).*

Several mesospheric dynamics studies involving other instruments like photometers, meteor radar, and onboard rocket instruments have been made [23, 36–39]. A mobile lidar has been developed to measure the Na concentration simultaneously with the volume emission profile for the NaD line of airglow in rocket campaigns in the Brazilian equatorial region of Alcântara (2.3728 S, 44.3965 W). An illustrative photo of this system is shown in **Figure 4**. This experiment allowed calculating the branching ratio of the reaction involved in the Na airglow [40].

Along the time, the São José dos Campos lidar underwent many modifications and upgrades. In 1993, the transmitter laser was upgraded with a commercial laser (see its illustration in **Figure 5**). With this upgrade, it was possible to use the Rayleigh signal from the clean atmosphere from 30 to 75 km (below the resonant Na signal) to measure the relative atmospheric density and the absolute temperature. These measurements have been used to study mesospheric temperature general behavior and the effects of atmospheric waves [41]. The long series of measurements have enabled long-term studies of the mesospheric Na, aerosols, and temperatures associated with global change [29, 42, 43]. A dual-beam Na/K lidar was assembled in São José dos Campos, Brazil, to extend the Na layer studies and improve the knowledge about metal layers in the MLT region. This system was installed owing to a cooperative agreement between the National Space Science Center (China) and the National Institute for Space Research (Brazil) in November of 2016.

The lidar uses two laser beams of 589 nm and 770 nm to simultaneously measure Na and K concentrations by the resonant scattering at MLT. The signal-to-noise ratio response allows 3 min time resolution and 96 m of height resolution in the profiles [44]. **Figure 6** shows the Na/K lidar during operation.

It is essential to point out that, up to the present time, this is the unique K lidar system operating in the Southern Hemisphere (SH). For the first time, it was presented the nocturnal and seasonal behavior of K and Na concentrations measured simultaneously at SH [44]. The seasonal variation of these two metals was determined, and it is interesting to note their different behavior even though both are alkali metals and come from meteor ablation. Semiannual variation is observed in both metal concentrations with different maxima: K shows its maxima around the

#### **Figure 4.**

*Photo illustrating the INPE mobile lidar used during rocket campaigns in the Brazilian equatorial region of Alcântara, on 31 may 1992.*

#### **Figure 5.**

*Photos showing the candela laser system assembled at INPE São José dos Campos in 1993. This system operated between 1993 and 2006—Photos taken by B. R. Clemesha (in memoriam).*

solstices more pronounced around June, and Na concentration shows a maximum around May and a broad one centered in September [44]. A plausible interpretation of the different seasonal changes between Na and K concentrations is presented in Ref. [45]. This analysis is based on two points: 1) the neutralization of K+ ions is particularly favored at low temperatures through summer (North Hemisphere), and 2) cycling between K and its primary neutral reservoir KHCO3 is substantially temperature independent [44]. Unfortunately, the first argument is not significant for this latitude, where the mesopause temperature has not a great summer to winter variation [33].

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

#### **Figure 6.**

*Picture showing the dual-beam Na/K lidar located at São José dos Campos, Brazil. The vertical orange beam is at 589 nm for Na scattering and the infrared one at 770 nm for K scattering. This last is not visible in the photo, but the red star indicates the beam position. Liu Zhengkuan took the original photo.*
