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Abstract
Abstract. Mineral dust from arid areas is a major component of global aerosol and has strong interactions with climate and biogeochemistry. As part of the Chemistry-Aerosol Mediterranean Experiment (ChArMEx) to investigate atmospheric chemistry and its impacts in the Mediterranean region, an intensive field campaign was performed from mid-June to early August 2013 in the western basin including in situ balloon-borne aerosol measurements with the light optical aerosol counter (LOAC). LOAC is a counter/sizer that provides the aerosol concentrations in 19 size classes between 0.2 and 100 µm, and an indication of the nature of the particles based on dual-angle scattering measurements. A total of 27 LOAC flights were conducted mainly from Minorca Island (Balearic Islands, Spain) but also from Ile du Levant off Hyères city (SE France) under 17 light dilatable balloons (meteorological sounding balloons) and 10 boundary layer pressurised balloons (quasi-Lagrangian balloons). The purpose was to document the vertical extent of the plume and the time evolution of the concentrations at constant altitude (air density) by in situ observations. LOAC measurements are in agreement with ground-based measurements (lidar, photometer), aircraft measurements (counters), and satellite measurements (CALIOP) in the case of fair spatial and temporal coincidences. LOAC has often detected three modes in the dust particle volume size distributions fitted by lognormal laws at roughly 0.2, 4 and 30 µm in modal diameter. Thanks to the high sensitivity of LOAC, particles larger than 40 µm were observed, with concentrations up to about 10−4 cm−3. Such large particles were lifted several days before and their persistence after transport over long distances is in conflict with calculations of dust sedimentation. We did not observe any significant evolution of the size distribution during the transport from quasi-Lagrangian flights, even for the longest ones (∼ 1 day). Finally, the presence of charged particles is inferred from the LOAC measurements and we speculate that electrical forces might counteract gravitational settling of the coarse particles.
Highlights
The present paper focuses on balloon-borne measurements conducted over the western Mediterranean during desert dust episodes encountered during this summer campaign with the new light optical aerosol counter (LOAC), an optical particle
LOAC observations were compared to the measurements done by other platforms, like the ATR-42 aircraft which embarked upon various aerosol counters, and a backscattering lidar located close to the balloon launching area
Given the limits and uncertainties associated with each measurement system, the agreement was satisfactory, which gave us confidence in the LOAC aerosol distributions
Summary
Mineral dust from arid and semi-arid areas is a major component of the global aerosol and has long been recognised to have strong interactions with climate and biogeochemistry (e.g. Buat-Ménard and Chesselet, 1979; Martin et al, 1991; Swap et al, 1992; Duce, 1995; Alpert et al, 1998; Mahowald et al, 2009, 2011; Maher et al, 2010; Liu et al, 2011; Choobari et al, 2014; Li et al, 2016). We still need observations of dust particle property evolution over an extensive particle size range during their long-range transport; there was a need for a new strategy for multiplying in situ measurements of the dust particle size distributions This was done for the first time in this study during African dust transport events above the western Mediterranean, deploying optical particle counters both below sounding balloons that vertically crossed the dust plume and aboard drifting balloons that remained at constant altitude for quasi-Lagrangian measurements within the atmospheric dust layer. The present paper focuses on balloon-borne measurements conducted over the western Mediterranean during desert dust episodes encountered during this summer campaign with the new light optical aerosol counter (LOAC), an optical particle. We discuss dust particle sedimentation aspects (Sect. 5) and speculations about electrically charged dust particles (Sect. 6), and conclude (Sect. 7)
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