An accurate assessment of black carbon (BC) climate and health impacts requires knowledge of its mass absorption cross-section (MACBC) – a parameter linking optical and mass measurements. The mean MACBC for freshly emitted soot typically spans a narrow range of 8 ± 1 m2 g⁻1 at 550 nm1,2 but is modified by subsequent atmospheric aging. Determination of MACBC requires simultaneous measurements of aerosol light-absorption coefficient (βabs) and BC mass. Here, we compile 230 measured MACBC values from 80 atmospheric studies and explore the effects of sampling location, study duration, instrumentation, and measurement wavelength. The compiled data set shows a broad variability in MACBC values (a factor of about 200%). We conclude that this variability is attributable to a combination of the above-mentioned effects with additional instrumental uncertainties (e.g., cross-sensitivities and/or inadequate instrument calibration). The current state of knowledge does not support the use of simplistic generalizations or assumptions about MACBC in the atmosphere, motivating a recommendation to further improve and standardize measurement practices.
