Mass absorption cross-section of ambient black carbon aerosols - a review

Abstract An accurate assessment of black carbon (BC) climate and health impacts requires knowledge of its mass absorption cross-section (MAC BC ) – a parameter linking optical and mass measurements. The mean MAC BC for freshly emitted soot typically spans a narrow range of 8 ± 1 m 2 g⁻ 1 at 550 nm 1,2 but is modified by subsequent atmospheric aging. Determination of MAC BC requires simultaneous measurements of aerosol light-absorption coefficient ( β abs ) and BC mass. Here, we compile 230 measured MAC BC 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 MAC BC 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 MAC BC in the atmosphere, motivating a recommendation to further improve and standardize measurement practices.