Continental pollution in the Western Mediterranean basin: large variability of the aerosol single scattering albedo and influence on the direct shortwave radiative effect
Pollution aerosols strongly influence the composition of the Western Mediterranean basin, but at present little is known on their optical properties. We report in this study in situ observations of the single scattering albedo (<i>ω</i>) of pollution aerosol plumes measured over the...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2016-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/16/10591/2016/acp-16-10591-2016.pdf |
| Summary: | Pollution aerosols strongly influence the composition of the Western
Mediterranean basin, but at present little is known on their optical
properties. We report in this study in situ observations of the single
scattering albedo (<i>ω</i>) of pollution aerosol plumes measured over the
Western Mediterranean basin during the TRAQA (TRansport and Air QuAlity)
airborne campaign in summer 2012. Cases of pollution export from different
source regions around the basin and at different altitudes between
∼ 160 and 3500 m above sea level were sampled during the
flights. Data from this study show a large variability of <i>ω</i>, with
values between 0.84–0.98 at 370 nm and 0.70–0.99 at 950 nm. The single
scattering albedo generally decreases with the wavelength, with some
exception associated to the mixing of pollution with sea spray or dust
particles over the sea surface. The lowest values of <i>ω</i> (0.84–0.70
between 370 and 950 nm) are measured in correspondence of a fresh plume
possibly linked to ship emissions over the basin. The range of variability
of <i>ω</i> observed in this study seems to be independent of the source
region around the basin, as well as of the altitude and aging time of the
plumes. The observed variability of <i>ω</i> reflects in a large
variability for the complex refractive index of pollution aerosols, which is
estimated to span in the large range 1.41–1.77 and 0.002–0.097 for the real
and the imaginary parts, respectively, between 370 and 950 nm.<br><br>
Radiative calculations in clear-sky conditions were performed with the GAME
radiative transfer model to test the sensitivity of the aerosol shortwave
Direct Radiative Effect (DRE) to the variability of <i>ω</i> as observed in
this study. Results from the calculations suggest up to a 50 and 30 %
change of the forcing efficiency (FE), i.e. the DRE per unit of optical
depth, at the surface (−160/−235 W m<sup>−2</sup> <i>τ</i><sup>−1</sup> at 60°
solar zenith angle) and at the Top-Of-Atmosphere
(−137/−92 W m<sup>−2</sup> <i>τ</i><sup>−1</sup>) for <i>ω</i> varying between its
maximum and minimum value. This induces a change of up to an order of
magnitude (+23/+143 W m<sup>−2</sup> <i>τ</i><sup>−1</sup>) for the radiative effect
within the atmosphere. |
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| ISSN: | 1680-7316 1680-7324 |