Quantifying the effects of the microphysical properties of black carbon on the determination of brown carbon using measurements at multiple wavelengths

Abstract. The absorption Ångström exponent (AAE)-based methods are widely used to estimate brown carbon (BrC) absorption, and the estimated BrC absorption can be significantly different from 0 even for pure black carbon (BC). However, few studies have systematically quantified the effects of BC microphysical properties. Moreover, it is still unclear under which conditions the AAE-based method is applicable. In this work, we used BC models partially coated with non-absorbing materials to calculate the total absorption. Since the total absorption is entirely from BC, the estimated BrC absorption should be 0 if the retrieval methods are accurate. Thus, the estimated BrC absorption (ABS_BrC) should be the absorption from BC that is incorrectly attributed to BrC. The results show that a BC AAE of 1 can generally provide reasonable estimates for freshly emitted BC, since at this time ABS_BrC is generally in the range of -3 % to 4.5 %. However, when BC aerosols are aged, ABS_BrC of about 35 % could be observed. The WDA method does not necessarily improve the estimates, sometimes a negative ABS_BrC of -40 % can be found for partially coated BC. By combining simulations of a global chemical transport model, this work also quantified the effects of BC microphysical properties on BrC global optical absorption aerosol depth (AAOD) estimates. The AAE = 1 method could sometimes lead to a misassigned global mean AAOD of about -0.4 – 0.5 × 10^-3 if BC aerosols have a complex morphology, leading to a global mean direct radiation factor (DRF) of about -0.068 ± 0.0172 to +0.085 ± 0.0215 W/m² from BC, which is incorrectly assigned to BrC. The WDA method does not necessarily improve the estimates. In our cases, the WDA methods based on the spherical models can lead to a range of about -0.9 – 0.05 × 10^-3 of misassigned AAOD, which could lead to a global mean DRF error range of -0.153 ± 0.0387 to +0.0085 ± 0.0022 W/m². At the regional scale, the AAE = 1 method in East Asia sometimes leads to a distributed AAOD of over 3 × 10^-3, resulting in a BC DRF of about +0.51 ± 0.129 W/m², which is incorrectly attributed to BrC. Mie theory-based WDA methods would lead to an estimated AAOD error of more than 6 × 10^-3 in some regions (e.g., East Asia), resulting in an estimated misattributed DRF of +1.0 ± 0.258 W/m².