Size distribution, mass concentration, chemical and mineralogical composition and derived optical parameters of the boundary layer aerosol at Tinfou, Morocco, during SAMUM 2006

Abstract

During the SAMUM 2006 field campaign in southern Morocco, physical and chemical properties of desert aerosols were measured. Mass concentrations ranging from 30 μg m−3 for PM2.5 under desert background conditions up to 300 000 μg m−3 for total suspended particles (TSP) during moderate dust storms were measured. TSP dust concentrations are correlated with the local wind speed, whereas PM10 and PM2.5 concentrations are determined by advection from distant sources. Size distributions were measured for particles with diameter between 20 nm and 500 μm (parametrizations are given). Two major regimes of the size spectrum can be distinguished. For particles smaller than 500 nm diameter, the distributions show maxima around 80 nm, widely unaffected of varying meteorological and dust emission conditions. For particles larger than 500 nm, the range of variation may be up to one order of magnitude and up to three orders of magnitude for particles larger than 10 μm. The mineralogical composition of aerosol bulk samples was measured by X‐ray powder diffraction. Major constituents of the aerosol are quartz, potassium feldspar, plagioclase, calcite, hematite and the clay minerals illite, kaolinite and chlorite. A small temporal variability of the bulk mineralogical composition was encountered. The chemical composition of approximately 74 000 particles was determined by electron microscopic single particle analysis. Three size regimes are identified: for smaller than 500 nm in diameter, the aerosol consists of sulphates and mineral dust. For larger than 500 nm up to 50 μm, mineral dust dominates, consisting mainly of silicates, and—to a lesser extent—carbonates and quartz. For diameters larger than 50 μm, approximately half of the particles consist of quartz. Time series of the elemental composition show a moderate temporal variability of the major compounds. Calcium‐dominated particles are enhanced during advection from a prominent dust source in Northern Africa (Chott El Djerid and surroundings). The particle aspect ratio was measured for all analysed particles. Its size dependence reflects that of the chemical composition. For larger than 500 nm particle diameter, a median aspect ratio of 1.6 is measured. Towards smaller particles, it decreases to about 1.3 (parametrizations are given). From the chemical/mineralogical composition, the aerosol complex refractive index was determined for several wavelengths from ultraviolet to near‐infrared. Both real and imaginary parts show lower values for particles smaller than 500 nm in diameter (1.55–2.8 × 10−3i at 530 nm) and slightly higher values for larger particles (1.57–3.7 × 10−3i at 530 nm).

Online differentiation of mineral phase in aerosol particles by ion formation mechanism using a LAAP-TOF single-particle mass spectrometer

“…Field studies show that most dust in the atmosphere is dominated by varying quantities of quartz, K-feldspar, plagioclase, calcite, hematite, kaolinite and the illite-smectite group of clay minerals (Kaufman et al, 2005;Kandler et al, 2007Kandler et al, , 2009Kandler et al, , 2011Formenti et al, 2008;Jeong, 2008;Kaufmann et al, 2016). At a single-particle level, particle size dependence is observed with an abundance of quartz and feldspar grains in the coarse fraction and a fine fraction (< 2 µm) dominated by clay minerals.…”

Seasonal variability of aerosol concentration and size distribution in Cape Verde using a continuous aerosol optical spectrometer

“…Kandler et al (2009) found background PM 2.5 and PM 10 concentrations of 30 and 80 μg.m −3 , respectively, during SAMUM-1 experiment, that took place in Morocco, between May and June 2006. During haze periods, aerosol concentrations increased one order of magnitude and were dependent on wind intensity.…”

Mapping the physico-chemical properties of mineral dust in western Africa: mineralogical composition

“…Another clay family, the smectites, bears the most soluble iron which might become available as a nutrient to marine phytoplankton . The properties of mineral dust are size-dependent, and the dimensional spectrum of mineral dust particles at emission extends over various orders of magnitude from approximately 200-300 nm to 100 µm (Kandler et al, 2009), depending on wind speed at emission. As a consequence, the properties of mineral dust vary as a function of time after emission due to size-dependent deposition, mixing and gaseous condensation.…”