Size distribution and optical properties of mineral dust aerosols transported in the western Mediterranean

Abstract

Abstract. This study presents in situ aircraft measurements of Saharan mineral dust transported over the western Mediterranean basin in June–July 2013 during the ChArMEx/ADRIMED (the Chemistry-Aerosol Mediterranean Experiment/Aerosol Direct Radiative Impact on the regional climate in the MEDiterranean region) airborne campaign. Dust events differing in terms of source region (Algeria, Tunisia and Morocco), time of transport (1–5 days) and height of transport were sampled. Mineral dust were transported above the marine boundary layer, which conversely was dominated by pollution and marine aerosols. The dust vertical structure was extremely variable and characterized by either a single layer or a more complex and stratified structure with layers originating from different source regions. Mixing of mineral dust with pollution particles was observed depending on the height of transport of the dust layers. Dust layers carried a higher concentration of pollution particles below 3 km above sea level (a.s.l.) than above 3 km a.s.l., resulting in a scattering Ångström exponent up to 2.2 below 3 km a.s.l. However, the optical properties of the dust plumes remained practically unchanged with respect to values previously measured over source regions, regardless of the altitude. Moderate absorption of light by the dust plumes was observed with values of aerosol single scattering albedo at 530 nm ranging from 0.90 to 1.00. Concurrent calculations from the aerosol chemical composition revealed a negligible contribution of pollution particles to the absorption properties of the dust plumes that was due to a low contribution of refractory black carbon in regards to the fraction of dust and sulfate particles. This suggests that, even in the presence of moderate pollution, likely a persistent feature in the Mediterranean, the optical properties of the dust plumes could be assumed similar to those of native dust in radiative transfer simulations, modelling studies and satellite retrievals over the Mediterranean. Measurements also showed that the coarse mode of mineral dust was conserved even after 5 days of transport in the Mediterranean, which contrasts with the gravitational depletion of large particles observed during the transport of dust plumes over the Atlantic. Simulations with the WRF mesoscale meteorological model highlighted a strong vertical turbulence within the dust layers that could prevent deposition of large particles during their atmospheric transport. This has important implications for the dust radiative effects due to surface dimming, atmospheric heating and cloud formation. The results presented here add to the observational data set necessary for evaluating the role of mineral dust on the regional climate and rainfall patterns in the western Mediterranean basin and understanding their atmospheric transport at global scale.

Comparative assessment of GRASP algorithm for a dust event over Granada (Spain) during ChArMEx-ADRIMED 2013 campaign

“…Figure 5 shows the columnar SSA values retrieved by GRASP and AERONET on 16 and 17 June at Granada and Cerro Poyos. Moreover, the SSA value at 530 nm calculated by Denjean et al (2016) for dust layers using airborne measurements during the campaign was 0.95 ± 0.04. SSAs retrieved by GRASP at 532 nm are close to the airborne value.…”

“…Figure 4 illustrates the retrieved columnar RRI and IRI for each day obtained by GRASP and AERONET at Granada and Cerro Poyos. Moreover, RRI and IRI at 530 nm estimated by Denjean et al (2016) using airborne measurements over Granada on 16 and 17 June are included in the plot. AERONET provides RRI and IRI for the whole size distribution, while GRASP is able to provide RRI and IRI for fine and coarse modes separately.…”

“…Several in situ and remote sensing measurements both from surface and on airborne platforms were collected during this campaign using state-of-the-art instrumentation. The measurements were performed at different stations over the western Mediterranean region during summer 2013 to create an updated database of the physical, chemical and optical aerosol properties as well as the vertical distribution of the major "Mediterranean aerosols" Denjean et al, 2016). Data gathered during ChArMEx-ADRIMED campaign give us an excellent opportunity to evaluate the recently developed algorithms for retrieving aerosol microphysical and optical profiles.…”

Black and brown carbon over central Amazonia: Long-term aerosol measurements at the ATTO site

“…It is mobilized from soils and suspended in the atmosphere by windstorms in areas like the Saharan desert in Africa. Dust aerosol particles in the atmosphere efficiently scatter visible radiation and are able to absorb infrared radiation (Andreae, 1996), having a å abs >> 1.0 (Caponi et al, 2017;Denjean et al, 2016). Mineral dust plumes travel over the Atlantic Ocean and are able to reach the American continent.…”

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

“…The D eff,fine varied between 0.24 and 0.62 µm and the D eff,coarse between 2.3 and 6.2 µm for the different cases (Table 2). For mineral dust, D eff,coarse ranged between 2.3 and 3.6 µm, encompassing the value of D eff,coarse ∼ 3 µm reported by Denjean et al (2016b) in their Fig. 11 for Saharan dust, both close to sources and during transport over the Atlantic.…”