A 10-year characterization of the Saharan Air Layer lidar ratio in the subtropical North Atlantic
<p>Particle extinction-to-backscatter ratio (lidar ratio) is a key parameter for a correct interpretation of elastic lidar measurements. Of particular importance is the determination of the lidar ratio of the Saharan Air Layer mineral dust transported into the free troposphere over the North A...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-05-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/6331/2019/acp-19-6331-2019.pdf |
| Summary: | <p>Particle extinction-to-backscatter
ratio (lidar ratio) is a key parameter for a correct interpretation of
elastic lidar measurements. Of particular importance is the determination of
the lidar ratio of the 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 allow 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.</p>
<p>Three different methods have been used to calculate the lidar ratio 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.) under
free-troposphere conditions; (2) the one-layer method, a joint determination using
a micropulse lidar sited at the Santa Cruz de Tenerife sea-level station and
photometric information considering one layer of aerosol characterized by a
single lidar ratio; and (3) the two-layer method, a joint determination using the
micropulse lidar and photometric information considering two layers of
aerosol with two different lidar ratios. The one-layer method only uses data from
a co-located photometer 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 <span class="inline-formula">±</span> 6 and
50 <span class="inline-formula">±</span> 11 sr. 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, which matches the real lower-troposphere
structure well. 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.</p>
<p>We found a lack of correlation between lidar ratio and Ångström
exponent (<span class="inline-formula"><i>α</i></span>), which indicates that the dust lidar ratio can be
considered independent of dust size distribution in this region. This finding
suggests that dust is, under most atmospheric conditions, the predominant
aerosol in the North Atlantic free troposphere, which is in agreement with
previous studies conducted at the Izaña Observatory.</p> |
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| ISSN: | 1680-7316 1680-7324 |