Advances in characterization of black carbon particles and their associated coatings using the soot particle aerosol mass spectrometer in Singapore, a complex city environment

Abstract. Atmospheric black carbon can act as a short-lived climate forcer and carrier of toxics. This work aims to utilize aerosol compositions detected by a soot-particle aerosol mass spectrometer to advance our understanding of emission and atmospheric processing of refractory BC (rBC) in Singapore. Positive matrix factorization analysis of rBC and organic aerosols (OA) (PMF_base) identified two local traffic factors with large differences in rBC content and coating thickness, and two secondary OA (SOA) factors impacted by local chemistry and/or regional transport (less-oxidized oxygenated OA (LO-OOA) and more-oxidized OA (MO-OOA)). Including metals in the PMF (PMF_metal) improved the quality of source apportionment significantly. An industrial and shipping influenced OA separated from traffic emissions was strongly associated with heavy metals (e.g., V⁺ and Ni⁺) that might pose higher potential risks to human health. Two biomass burning-influenced OA (BBOA) factors with different degree of oxygenation were also identified. Although the aged-BBOA component was highly oxidized, its strong association with K₃SO₄⁺ differentiated itself from other background SOA. Integration of both metals and inorganic aerosols (IA) into the PMF (PMF_all) was found to provide further insight into the origin of SOA coatings and their chemical processing. PMF_all identified an additional aged-BBOA component that was associated with nighttime IA and organo-nitrate formation. Furthermore, PMF_all revealed concurrent LO-OOA and nitrate formation during daytime, whereas photochemical production of MO-OOA was linked to acidic sulfate formation, indicating the importance of investigating interaction between SOA and IA formation and their mixing state in complex city environments.