Mixing state of aerosols and direct observation of carbonaceous and marine coatings on African dust by individual particle analysis

Abstract

The mixing state of aerosols collected at M'Bour, Senegal, during the Special Observing Period conducted in January–February 2006 (SOP‐0) of the African Monsoon Multidisciplinary Analysis project (AMMA), was studied by individual particle analysis. The sampling location on the Atlantic coast is particularly adapted for studying the mixing state of tropospheric aerosols since it is (1) located on the path of Saharan dust plumes transported westward over the northern tropical Atlantic, (2) influenced by biomass burning events particularly frequent from December to March, and (3) strongly influenced by anthropogenic emissions from polluted African cities. Particle size, morphology, and chemical composition were determined for 12,672 particles using scanning electron microscopy (automated SEM‐EDX). Complementary analyses were performed using transmission electron microscopy combined with electron energy loss spectrometry (TEM‐EELS) and Raman microspectrometry. Mineral dust and carbonaceous and marine compounds were predominantly found externally mixed, i.e., not present together in the same particles. Binary internally mixed particles, i.e., dust/carbonaceous, carbonaceous/marine, and dust/marine mixtures, accounted for a significant fraction of analyzed particles (from 10.5% to 46.5%). Western Sahara was identified as the main source of mineral dust. Two major types of carbonaceous particles were identified: “tar balls” probably coming from biomass burning emissions and soot from anthropogenic emissions. Regarding binary internally mixed particles, marine and carbonaceous compounds generally formed a coating on mineral dust particles. The carbonaceous coating observed at the particle scale on African dust was evidenced by the combined use of elemental and molecular microanalysis techniques, with the identification of an amorphous rather than crystallized carbon structure.

Chemical characterization and source apportionment of submicron aerosols measured in Senegal during the 2015 SHADOW campaign

“…These interferences may be enhanced during the dry season by internal mixture of BB and DD, as encountered and evidenced during AMMA by Deboudt et al (2010) at the surface and by Hand et al (2010) and Paris et al (2010) at higher altitudes. Consequently, BC absorption coefficients have been recalculated following the method developed by Fialho et al (2005Fialho et al ( , 2006Fialho et al ( , 2014 in order to obtain BC concentrations unbiased by DD influence.…”

“…During one of the AMMA special observing periods (SOPs), in February 2006, a chemical characterization of particles at the ground level was performed near Mbour (Senegal) using filter sampling and individual particle analysis (Deboudt et al, 2010;Flament et al, 2011). The analysis evidenced both internal and external mixing of DD, sea salt (SS) and carbonaceous aerosols (Deboudt et al, 2010) within the surface layer. The compositions of the coarse (2-10 µm) and fine (< 2 µm) fractions were established at the surface; DD dominated the coarse fraction while organic matter was the major constituent of the fine one.…”

“…During the months of JanuaryFebruary (dry season) the transport of desert dust (DD) and biomass burning (BB) aerosols were observed to occur into two layers, one dominated by DD close to the surface (< 1 km) and one containing BB aerosols that is located at a higher altitude, between 2 and 4 km Osborne et al, 2008). During one of the AMMA special observing periods (SOPs), in February 2006, a chemical characterization of particles at the ground level was performed near Mbour (Senegal) using filter sampling and individual particle analysis (Deboudt et al, 2010;Flament et al, 2011). The analysis evidenced both internal and external mixing of DD, sea salt (SS) and carbonaceous aerosols (Deboudt et al, 2010) within the surface layer.…”

Atmospheric tar balls: aged primary droplets from biomass burning?

“…During their long range atmospheric transport tar-ball particles occasionally form aggregates with up to 10 particles, including coagulation with dust particles (Hand et al, 2005;Deboudt et al, 2010).…”

A Computer-Controlled SEM-EDX Routine for Characterizing Respirable Coal Mine Dust

“…It involves manual analysis with a scanning electron microscope equipped with energy dispersive X-ray (SEM-EDX), and has been described in detail elsewhere [25][26][27][28]. SEM-EDX is a very powerful tool for particulate characterization, and is well established for chemical identification of particles (e.g., see [29][30][31][32][33][34][35]) and determination of size and shape parameters (e.g., see [33,36]). The US Environmental Protection Agency (EPA) has produced detailed guidance on the use of SEM-EDX for analysis of particulate samples [37].…”