The effect of metals on the evolved gas analysis of elemental and organic carbon

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Description

Uncertainties in quantification of elemental carbon (EC) and organic carbon (OC) are well known. Agreement between methods has been found repeatedly by inter-comparison studies to be site-specific, presumably because of the site-to-site variation in aerosol chemical composition. In this study, we probe the effect of metals on evolution of organic and elemental carbon in thermo-optical analyses. Lab- generated metal salts were deposited on top of a layer of various mixed elemental and organic carbon samples, including diesel particles, secondary organic aerosols, and biodiesel particles, to investigate their effect on EC and OC quantification with evolved gas analysis (EGA). Investigated metals include alkaline metals (NaCl, KCl, Na2SO4), alkaline-earth metals (MgCl2, CaCl2), and transition metals (CuCl2, FeCl2, FeCl3, CuCl, ZnCl2, MnCl2, CuSO4, Fe2(SO4)3). Metals were observed to reduce the oxidation temperature of EC. The effect of Cu2+, Fe2+, Fe3+, Mn2+, K+, Ca2+ on EC oxidation did not depend on the metal-to-carbon (M/C) ratio; Cu2+ resulted in the largest reduction, followed by Fe2+, Mn2+ > Fe3+, K+ > Ca2+. The effect of Cu+, Na+, and Mg2+ on EC oxidation did depend on M/C ratio, decreasing the oxidation temperature of EC as M/C increased. The amount of charred OC was determined by monitoring darkening of the filter spot. Metals were observed to affect the charring of OC, and this effect was highly dependent on the M/C ratio, and ranged from a -50% decrease to a 100% increase. For Cu2+, K+, Fe3+, Mn2+, Na+, Ca2+, Mg2+, the charring of OC decreased as M/C increased. For Fe2+ and Cu+, the charring of OC increased as M/C increased. Metals were observed to affect the EC/OC ratio; this effect was highly variable and dependent on the M/C ratio. The resulting EC/OC ratio generally was increased by small amounts of metals, with the effect becoming smaller and eventually negative as M/C increased.