New method to determine black carbon mass size distribution

Black carbon (BC) is an important atmospheric component with strong light absorption. Many attempts have been made to measure BC mass size distribution (BCMSD) for its significant impact on climate and public health. Larger-coverage BCMSD, ranging from upper submicron particles sizes to larger than 1 mu m, contributes to a substantial proportion of total BC mass and absorption. However, the current time resolution of larger-coverage BCMSD measurement was limited to 1 d, which was insufficient to characterize variation of larger-coverage BCMSD. In this study, a new method to determine equivalent BCMSD (eBCMSD) was proposed from size-resolved absorption coefficient measured by an aerodynamic aerosol classifier in tandem with an aethalometer. The proposed method could measure larger-coverage eBCMSD with a time resolution as high as 1 h and was validated by comparing the measurement results with refractory BCMSD (rBCMSD) measured by a differential mobility analyzer in tandem with a single-particle soot photometer (DMA-SP2) for particle sizes larger than 300 nm during a field measurement in the Yangtze River Delta. Bulk refractory BC mass concentration (mrBC,bulk) by DMA-SP2 was underestimated by 33 % compared to bulk equivalent BC mass concentration (meBC,bulk) by this method because of the limited size range of measurement for DMA-SP2. Uncertainty analysis of this method was performed with respect to mass absorption cross-section (MAC), transfer function inversion, number fraction of BC-containing particle and instrumental noise. The results indicated that MAC was the main uncertainty source, leading to meBC,bulk values that varied from -20 % to 28 %. With the advantage of a wide size coverage up to 1.5 mu m, high time resolution, easy operation and low cost, this method is expected to have wide applications in field measurement for better estimating the radiative properties and climate effects of BC.