An Aerosol Optical Module With Observation-Constrained Black Carbon Properties for Global Climate Models

Atmospheric black carbon (BC) aerosols have been long-lasting uncertain components in environmental and climate studies. Global climate models (GCMs) potentially overestimate BC absorption efficiency due to a lack of consideration of complex BC microphysical and mixing properties. We extract multiple BC properties from observations and develop an aerosol optical module known as Advanced Black Carbon (ABC) in the framework of the Modal Aerosol Model version 4 (MAM4). The ABC module is implemented in the Community Atmosphere Model version 6 (CAM6) and evaluated by in situ and remote sensing observations. CAM6-ABC addresses the shortcomings of CAM6-MAM4 in terms of BC microphysical and mixing properties, particularly their size, mixing state and optical simulations. Sensitivity simulations show that the global BC absorption aerosol optical depth at 550 nm simulated by CAM6-ABC is reduced by similar to 29% compared with that in CAM6-MAM4. The BC absorption enhancement simulated by CAM6-ABC is reduced from similar to 2.6 of the default MAM4 to similar to 1.4, which is closer to the observed values (mostly less than 1.5). With improved BC absorption estimation, the biases of aerosol single-scattering coalbedo simulations are reduced by 18%-69% compared with global Aerosol Robotic Network observations. Moreover, the globally averaged BC direct radiative effect is reduced from 0.37 to 0.28 W/m2 at the top of the atmosphere. Our new scheme alleviates the overestimation of BC absorption in GCMs by constraining BC microphysical and mixing properties when assessing aerosol radiative and climate effects, and it can be easily implemented in most modal-based aerosol modules of climate models. As a climate warming agent in the atmosphere, black carbon (BC) aerosols remain largely uncertain in their light absorption. In global climate models (GCMs), the BC absorption efficiency tends to be overestimated due to insufficient consideration of the BC microphysical properties and mixing state. Our work quantifies the aerosol representation deficiency in GCMs due to the microphysical and optical properties and compensates for this deficiency in our improved BC and aerosol optical property representation. In this study, we analyze and summarize the key BC properties that highly influence their radiative effects based on multisource observations, for example, size distribution and mixing state, develop an improved aerosol optical module to better account for those observations, and implement this representation in a GCM. We use in situ observations to evaluate our module with improved BC and aerosol optical property representations. The results show that the overestimated BC absorption is much closer to the observations due to a more accurate representation of BC-related microphysical and mixing properties. Correspondingly, our improved BC representation weakens the modeled BC radiative and climate effects by as much as similar to 24%. Observations of black carbon (BC) properties are implemented in the current aerosol optical module of Community Atmosphere Model version 6The modeled BC absorption enhancement and aerosol bulk optical properties are closer to the observationsThe BC top of the atmosphere direct radiative effect decreases from 0.37 to 0.28 W/m2

Automated summary

This study proposes an improved aerosol optical module, Advanced Black Carbon (ABC), to accurately estimate the absorption efficiency and radiative effects of black carbon (BC) aerosols. The module addresses the deficiencies in representing BC microphysical and mixing properties, reducing the discrepancies between model simulations and observations, and mitigating the climate impacts of atmospheric aerosols.