Estimating chemical composition of atmospheric deposition fluxes from mineral insoluble particles deposition collected in the western Mediterranean region

摘要

In order to measure the mass flux of atmospheric insolubledeposition and to constrain regional models of dust simulation, a network ofautomatic deposition collectors (CARAGA) has been installed throughout thewestern Mediterranean Basin. Weekly samples of the insoluble fraction oftotal atmospheric deposition were collected concurrently on filters at five sites including four on western Mediterranean islands (Frioul and Corsica,France; Mallorca, Spain; and Lampedusa, Italy) and one in the southern FrenchAlps (Le Casset), and a weighing and ignition protocol was applied in orderto quantify their mineral fraction. Atmospheric deposition is both a strongsource of nutrients and metals for marine ecosystems in this area. However,there are few data on trace-metal deposition in the literature, since theirdeposition measurement is difficult to perform. In order to obtain moreinformation from CARAGA atmospheric deposition samples, this study aimed totest their relevance in estimating elemental fluxes in addition to totalmass fluxes. The elemental chemical analysis of ashed CARAGA filter sampleswas based on an acid digestion and an elemental analysis by inductivelycoupled plasma atomic emission spectroscopy (ICP-AES) and mass spectrometry(MS) in a clean room. The sampling and analytical protocols were tested todetermine the elemental composition for mineral dust tracers (Al, Ca, K, Mg and Ti), nutrients (P and Fe) and trace metals (Cd, Co, Cr, Cu, Mn, Ni, Vand Zn) from simulated wet deposition of dust analogues and traffic soot. Therelative mass loss by dissolution in wet deposition was lower than 1 % forAl and Fe, and reached 13 % for P due to its larger solubility in water.For trace metals, this loss represented less than 3 % of the total massconcentration, except for Zn, Cu and Mn for which it could reach 10 %,especially in traffic soot. The chemical contamination during analysis wasnegligible for all the elements except for Cd, which has a very lowconcentration in dust. Tests allowed us to conclude that the CARAGA samplescould be used to estimate the contents of nutrients and trace metals in thelimits of loss by dissolution. Chemical characterization of CARAGA depositionsamples corresponding to the most intense dust deposition events recordedbetween 2011 and 2013 has been performed and showed elemental mass ratiosconsistent with the ones found in the literature for Saharan dust. However,the chemical analysis of CARAGA samples revealed the presence of someanthropogenic signatures, for instance high Zn concentrations in somesamples in Lampedusa, and also pointed out that mineral dust can be mixedwith anthropogenic compounds in the deposition samples collected onFrioul. Results showed that the chemical analysis of CARAGA ashedsamples can be used to trace the origins of elemental deposition. Theelemental atmospheric fluxes estimated from these chemical analyses ofsamples from the CARAGA network of weekly deposition monitoring constitutethe first assessment of mass deposition fluxes of trace metals and P duringintense dust deposition events at the scale of the western Mediterranean Basin. The mass fluxes strongly depend on the distance from dust sources andthe most intense events, while proximity from anthropogenic sources stronglyimpacted the masse fluxes of Zn and Cu at Lampedusa and Frioul.