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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 10 Jan 2019

Research article | 10 Jan 2019

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

A 10-year characterization of the Saharan Air Layer lidar ratio in the subtropical North Atlantic

Alberto Berjón1,2, Africa Barreto3,1,2, Yballa Hernández1, Margarita Yela4, Carlos Toledano2, and Emilio Cuevas1 Alberto Berjón et al.
  • 1Izaña Atmospheric Research Center (IARC), State Meteorological Agency (AEMET), Spain
  • 2Atmospheric Optics Group, University of Valladolid, Spain
  • 3Cimel Electronique, France
  • 4Atmospheric Research and Instrumentation Branch, National Institute for Aerospace Technology (INTA), Spain

Abstract. Particle extinction-to-backscatter ratio (lidar ratio) is a key parameter for a correct interpretation of elastic lidar measurements. Of particular importance is its determination for Saharan Air Layer mineral dust transported into the free troposphere over the North Atlantic region. The location of the two sun photometer stations managed by the Izaña Atmospheric Research Centre (IARC) on the island of Tenerife, and a decade of available micropulse lidar (MPL) data allows us to determine the lidar ratio under almost pure dust conditions. This result can be considered representative of the Saharan dust transported westward over the North Atlantic in the subtropical belt.

Three different methods to calculate the lidar ratio have been used in this work: (1) using the inversion of sky radiance measurements from a sun/sky photometer installed at the Izaña Observatory (2373 m a.s.l.) in free troposphere conditions; (2) the One-Layer method, a joint determination using a micro-pulse lidar sited at Santa Cruz de Tenerife sea-level station and photometric information considering a one layer of aerosol characterized by a single lidar ratio; (3) the Two-Layer method, a joint determination using the micro-pulse lidar and photometric information considering two layers of aerosol with two different lidar ratios. The One-Layer method uses data from a co-located photometer only at Santa Cruz de Tenerife, while the Two-Layer conceptual approach incorporates photometric information at two heights from the observatories of Izaña and Santa Cruz de Tenerife. The almost pure dust lidar ratio retrieval from the sun/sky photometer and from the Two-Layer method give similar results, with lidar ratios at 523 nm of 49 ± 6 sr and 50 ± 11 sr, respectively. These values obtained from a decade of data records are coincident with other studies in the literature reporting campaigns in the subtropical North Atlantic region. This result shows that the Two-Layer method is an improved conceptual approach compared to the single layer approach, that matches well the real lower troposphere structure. The Two-layer method is able to retrieve reliable lidar ratios and therefore aerosol extinction profiles, despite the inherent limitations of the elastic lidar technique.

A lack of correlation between lidar ratio and Ångström exponent (α) indicates that the dust lidar ratio can be considered independent of dust size distribution in this region. This finding suggests that dust is, in most of atmospheric conditions, the predominant aerosol in the North Atlantic free troposphere.

Alberto Berjón et al.
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Alberto Berjón et al.
Alberto Berjón et al.
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Short summary
Lidar ratio is a key parameter for the aerosol characterization using satellite remote sensing platforms as CALIOP. However, there are important differences in the values reported in the bibliography. The geographic characteristics of the IARC observatories location and a 10-year data series allows us to make a unique study of the mineral dust in the Saharan Air Layer. We report lidar ratios at 523 nm of 49 ± 6 sr and 50 ± 11 sr obtained respectively by two different methods.
Lidar ratio is a key parameter for the aerosol characterization using satellite remote sensing...