**4. Dust transport**

The long-range transport of desert dust has an essential impact on the atmospheric radiative balance, with a global impact on climate, air quality, and human health [52, 53]. It is estimated that between 1 and 2 billion tons of dust are transported from Sahara each year through the atmosphere [54, 55].

Knowledge of the transport mechanisms of dust aerosols from the Sahara Desert to the South American continent can help understand its impact on the balance of nutrients in the Amazon basin [56, 57] and its nutrients deposition in the equatorial Atlantic [58]. Dust transport from the Sahara over the Atlantic Ocean can reach distances greater than 5000 km from its origin and is more intense during December, January and February at 5 N, and during June, July, and August at 20 N latitude. During the boreal winter, desert aerosols are transported across the Atlantic towards the northeast coast of South America, and in the summer, they can reach the Caribbean Sea [59].

The most favorable periods to observe the presence of Saharan dust with a lidar in the atmosphere of the city of Natal (located about 5 S) are in December, January, and February. In this period, Oliveira et al. [60] found the mineral dust

#### *Lidar Observations in South America. Part II - Troposphere DOI: http://dx.doi.org/10.5772/intechopen.95451*

predominance occurred during February, followed by December, January, and March in 2018. Research with lidar on aerosols in the Natal atmosphere focuses on the long-range transport of mineral dust and also smoke from biomass burning from the African continent, occurring under the strong influence of the trade winds, during austral summer, when the Intertropical Convergence Zone (ITCZ) is positioned further to the south [61, 62]. In a case study on 09 February 2018 (Reference [60]), Oliveira et al. show that aerosol plumes were identified below 4000 m altitude.

Landulfo et al. were the first to identify an aerosol plume with a lidar, at 3000 m altitude in Natal's atmosphere, on 01 June 2016 [63]. Direct measurement of the Sun and radiances of the sky with a CIMEL solar photometer may also be used to identify the measured dust aerosol, as in Ref. [60]. Similar aerosol identification studies were carried out [57, 61, 62, 64, 65], showing that African dust's transatlantic transport reaches the Caribbean. Episodes of mineral dust particles transported from Africa to the Amazon between January and April were also observed [66].

The DUSTER Lidar system started operating in 2016 with operational data collection campaigns called MOnitoring aerosol LOng-range Transportation OVer Natal (MOLOTOV) Zero (March 2016 to July 2016.), MOLOTOV I (December 2016 to mid-February 2017), MOLOTOV II (January and February 2018), and APEL (Assessment of Atmospheric Optical Properties During Biomass Burning Events and Long-Range Transport of Desert Dust) (November and December 2017), at the Federal University of Rio Grande do Norte (UFRN), in Natal.

Also, during the campaigns MOLOTOV Zero and MOLOTOV I, eighteen cirrus cloud profiles were analyzed (7 during MOLOTOV Zero and 11 from MOLOTOV I). Part of the research was conducted to calibrate the DUSTER system, applying the Δ90° method [67]. The main physical parameters of cirrus clouds obtained from lidar data were their thickness, base height, top height, and linear particle depolarization (δp) [68, 69]. The determination of parameters such as base height and top height was calculated from the analysis of the depolarization profiles obtained for each data interval. Data from radiosondes launched by the Natal airport (about 7 km from the DUSTER system) were used to obtain the average cirrus temperature. The average depolarization value for cirrus clouds during the MOLOTOV Zero campaign was δp(cirrus) = 0.43 ± 0.15 with an average base height of 14.23 km and an average top height of 15.53 km [70]. The average temperature of cirrus during this campaign was −69.23°C. The average thickness of the cirrus found was 1.30 km. For the MOLOTOV I campaign, the average value was δp(cirrus) = 0.49 ± 0.13 with the average base height equal to 12.96 km and the average top height equal to 14.44 km. The average thickness found was 1.48 km. The average cirrus temperature found for this campaign was −62.06°C [70].
