Derivation of the correction factor needed for COSMOS observations at high mass concentrations of black carbon

COSMOS is a continuous soot monitoring system for long-term observation of the mass concentrations of black carbon (MBC) in the atmosphere. The system uses a heated inlet to volatilize non-refractory aerosol components, a dual photodetector setup, and filter tape to collect BC particles from the air. In environments with very high MBC, rapid accumulation of BC on the filter tape results in a rapid decrease of filter transmittance (Tr), leading to more frequent filter advances and instrument downtime for changing the filter rolls. Thus, a correction factor is needed to allow accurate determination of MBC values even after large decreases of Tr. Here, by using MBC measurements obtained simultaneously by two COSMOS systems, we characterized Tr and derived a correction factor that permits the derivation of MBC at very low Tr while maintaining reasonable accuracy.