<p>Atmospheric black carbon (BC) is the strongest visible solar radiative absorber in the atmosphere, exerting significant influences on the earth’s radiation budget. The mass absorption cross-section (MAC) is a crucial parameter for converting light absorption coefficient (bab) to mass equivalent BC concentration (mBC). Traditional filter-based instrument, such as AE33, uses a constant MAC of 7.77 m<sup>2</sup>/g to derive mBC, which may lead to uncertainty in mBC. In this paper, a new method of converting light absorption coefficient to BC mass concentration is proposed by incorporating the variations of MAC attributed to the influences of aerosol coating state. Mie simulation showed that MAC varied dramatically with different core-shell structures. We compared our new method with traditional method during a field measurement at a site of North China Plain. The results showed that the MAC was smaller (larger) than 7.77 m<sup>2</sup>/g for particle smaller (larger) than 280 nm, resulting in BC mass size distribution derived from new method was higher (lower) than traditional method for particle smaller (larger) than 280 nm. Size-integrated BC mass concentration derived from new method was 16 % higher than traditional method. Sensitivity analysis indicated that the uncertainty in mBC caused by refractive index (RI) was with in 35 % and the imaginary part of RI had dominant influence on the derived mBC. This study emphasizes the necessity to take variations of MAC into account when deriving mBC from bab and can help constrain the uncertainty in mBC measurements.</p>