An empirical correction factor for filter-based photo-absorption black carbon measurements

•Transmittance-dependent correction factor for filter-based absorption measurements.•Ambient data set.•Instruments resolved to within 2% after correction.•New type of quartz filter characterised for use with COSMOS.

Filter-based BC measurement techniques such as the Continuous Soot Monitoring System (COSMOS) are particularly well suited to long-term observations of black carbon (BC) due to their relative robustness and reliability. However, caution is required when determining the threshold transmittance, Trthresh (roughly proportional to the time interval between filter changes), in order to ensure that acceptable measurement accuracy is maintained throughout the sampling period. We present a new, empirically derived transmittance-dependent correction factor used to interpret the response characteristics of filter-based aerosol absorption measurements performed by COSMOS. Simultaneous measurements of ambient BC aerosol mass (MBC) were conducted in Tokyo, Japan, using two identical COSMOS instruments operated with different threshold transmittance, Trthresh, values, of 0.95 and 0.6. The derived values for MBC were consistently underestimated by the COSMOS operating at lower Trthresh, as a function of decreasing filter transmittance. The 1-hour averaged values of MBC were underestimated by around 10%, incorporating measurements across the entire range of filter transmittance (1–0.6), with a maximum underestimation at around 17% immediately preceding filter advancement (i.e. Tr=~0.6). An empirical correction factor was derived from these ambient measurements, and was applied to MBC as a function of filter transmittance, resolving the instruments to within 2%.Further to the transmittance-based correction, the operational performance of COSMOS was tested for two types of quartz fibre filter (PALLFLEX and HEPA). Agreement in derived values of MBC for two COSMOS using the same type of filter was around 2%; however, a comparison of the PALLFLEX and HEPA filters demonstrated a systematic overestimation of MBC derived when using HEPA filters, of around 6–8%. A sensitivity study of a radiative transfer model indicated that this enhanced absorption was primarily a result of the increased thickness of the HEPA filter.