The effect of metal salts on quantification of elemental and organic carbon in diesel exhaust particles using thermal-optical evolved gas analysis

摘要

Thermal-optical evolved gas analysis (TOEGA) is a conventional method for classifying carbonaceous aerosols as organic carbon (OC) and elemental carbon (EC). Its main source of uncertainty arises from accounting for pyrolized OC (char), which has similar behavior to the EC originally present on the filter. Sample composition can also cause error, at least partly by complicating the charred carbon correction. In this study, lab generated metal salt particles, including alkali (NaCl, KCl, Na_2SO_4), alkaline-earth (MgCl_2, CaCl_2) and transition metal salts (CuCl _2, FeCl_2, FeCl_3, CuCl, ZnCl_2, MnCl_2, CuSO_4, Fe_2(SO_4)_3), were deposited on a layer of diesel particles to investigate their effect on EC and OC quantification with TOEGA. Measurements show that metals reduce the oxidation temperature of EC and enhance the charring of OC. The split point used to determine classification of EC vs. OC is more dependent on changes in EC oxidation temperature than it is on charring. The resulting EC/OC ratio is reduced by 0-80% in the presence of most of the salts, although some metal salts increase reported EC/OC at low metal to carbon ratios. The results imply that EC/OC ratios of ambient aerosols quantified with TOEGA have variable low biases due to the presence of metals. In general, transition metals are more active than alkali and alkaline-earth metals; copper is the most active. Copper and iron chlorides are more active than sulfates. The melting point of metal salts is strongly correlated with the increase of OC charring, but not with the reduction of EC oxidation temperature. Other chemistry, such as redox reactions, may affect the EC oxidation. A brief discussion of possible catalytic mechanisms for the metals is provided.