Assessing direct radiative effect of aging black carbon using an advanced aerosol optics module AI-NAOS and the climate model CAM6

Abstract

Large uncertainties still exist in the estimation of black carbon (BC) radiative forcing due to incomplete representation of BC optical properties. To address this, this study employed the AI-based nonspherical aerosol optical scheme (AI-NAOS), coupled with the Community Atmosphere Model version 6 (CAM6), to comprehensively estimate the optical properties of the aging BC and its direct radiative effect (DRE). The AI-NAOS was obtained from a database of accurate optical properties of encapsulated fractal aggregates computed from the invariant imbedding T-matrix method (IITM). With this scheme, the aging progress of BC in the CAM6 can be explicitly resolved by the volume fraction and the optical properties can be efficiently inferred from the deep neural network (DNN) in real time. Based on decadal-long simulations from 2010 to 2020, the BC DRE of fractal aggregates was estimated to be +0.3 w/m² globally and +1.3 w/m² over East Asia, representing decreases of 40.0% and 38.1%, respectively, compared to spherical assumptions. Additionally, an idealized scenario was considered where BC quantities were increased tenfold. In this scenario, the aging process was minimized due to insufficient hygroscopic aerosols for encapsulating BC aerosols. Compared to the normal scenario, the incremental ratio of radiative effects based on the fractal aggregate model was 11.1 globally and 9.1 over East Asia, whereas it was 7.6 globally and 5.3 over East Asia based on spherical assumptions. These results indicate that, compared to spherical assumptions, stronger enhancement of BC DRE could be produced using more realistic models in scenarios with higher BC emission. Whether the radiative effect is reduced or enhanced using realistic particle models depend on the competing roles of particle nonsphericity and encapsulation (lensing effect) in influencing BC absorption capabilities.