Continental pollution in the Western Mediterranean basin: large variability of the aerosol single scattering albedo and influence on the direct shortwave radiative effect

Abstract

Abstract. 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 (ω) 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 ω, 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 ω (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 ω 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 ω 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.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 ω 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 −2 τ −1 at 60 • solar zenith angle) and at the Top-Of-Atmosphere (−137/−92 W m −2 τ −1 ) for ω varying between its maximum and minimum value. This induces a change of up to an order of magnitude (+23/+143 W m −2 τ −1 ) for the radiative effect within the atmosphere.

Aethalometer multiple scattering correction <i>C</i><sub>ref</sub> for mineral dust aerosols

“…The SSA decreased with increasing wavelength, as expected for pollution aerosols (e.g. Bergstrom et al, 2007;Di Biagio et al, 2016). The wide range of values indicates the occurrence of particles with very different absorption properties, hence chemical composition (or complex refractive index) and/or different size distribution (e.g.…”

“…For dust aerosols the refractive index was varied in the range 1.47-1.53 (n) and 0.001-0.005i (k) following the literature (see Di Biagio et al, 2017) and D g was set at the mean ± one standard deviation of the values obtained for the different n and k. For kaolinite the OPC diameter conversion was performed by setting the refractive index at 1.56-0.001i. For ambient air the refractive index was set at 1.60-0.01i, a value that represents a medium absorbing urban polluted aerosol (see Di Biagio et al, 2016). The impact of humidity on the refractive index of ambient aerosols and associated changes OPC response were not taken into account.…”

“…Two different approaches were used: for submicrometric ammonium sulfate, the correction proposed by Anderson and Ogren (1998) was applied, while for aerosols with a significant coarse fraction (dust, ambient air and kaolinite), the truncation correction was estimated by optical calculations according to the Mie theory for homogeneous spherical particles using the measured number size distribution as input. In the calculations the real and the imaginary parts of the complex refractive index m (m = n − ik, where n is the real part and k is the imaginary part) were varied in the wide range 1.42-1.56 and 0.001-0.025i for dust , and 1.50-1.72 and 0.001-0.1i for ambient air (Di Biagio et al, 2016), while the value of 1.56-0.001i was assumed for kaolinite (Egan and Hilgeman, 1979;Utry et al, 2015). Then, n and k were set to the values which reproduced the measured β sca at 7-170 • .…”

“…The value most often used in the literature is 2.14 (±0.21), assumed as wavelength-independent (e.g. Sandradewi et al, 2008;Formenti et al, 2011;Di Biagio et al, 2016), which corresponds to the mean of observations at 660 nm for soot aerosols (W2003). Both W2003 and C2010, however, found a dependence of C ref on the aerosol single scattering albedo, with C ref decreasing for increasing SSA.…”

“…The Aethalometer reports equivalent black carbon mass concentration but the spectral absorption by aerosols can be also calculated. Given its large spectral interval, the Aethalometer has been used in the past to investigate the spectral dependence of dust absorption (Fialho et al, 2005;Formenti et al, 2011), as well as the absorption by many aerosol types in different environments (Sandradewi et al, 2008;Segura et al, 2014;Di Biagio et al, 2016;Backman et al, 2016). General reviews on aerosol absorption measurements and their applications are provided by Horvath (1993) and .…”