An Integrated Framework Using Observationally Constrained Black Carbon Microphysics, Emissions, and Radiative Forcing: From Regional to Global Perspectives

This work presents a new integrated framework that uses multi-scale observations to constrain the microphysics, emissions, and radiative forcing of black carbon (BC). It demonstrates that BC's climate impact is far more complex, variable, and non-linear than the common assumption of a uniformly absorbing, always-warming aerosol. The framework employs a physically consistent perspective, tracking BC from emission as a particle size distribution, through atmospheric processing and mixing with co-pollutants, to its ultimate radiative interaction and removal.

We first show that realistic emission size distributions and evolving particle mixing states - driven by in-situ and column observations - frequently cause substantial non-linear variations in key optical properties (single scatter albedo and asymmetry parameter). These variations are more complex than Ångström-exponent-based approaches can capture. Second, we use multi-wavelength observations of aerosol optical depth (AOD) and aerosol absorption optical depth (AAOD) to constrain atmospheric column number loadings, total BC mass, and associated scattering aerosol mass. This yields high-resolution minute-scale results in selected areas, daily regional analyses at 5-kilometer scale, and global daily perspectives at 50-kilometer scale, using a suite of remotely sensed products (e.g., AERONET, OMI, TROPOMI, MODIS).

These observationally constrained solutions are used to explore impacts on radiative forcing at the top of the atmosphere (TOA) and within the atmosphere (ATM), effects on greenhouse gas retrievals, and improvements to BC emission source attribution. Analyses span global environments from moderate to extreme pollution, including urban, industrial, fire, and long-range transport scenarios.

Key findings summarize that: (1) BC radiative forcing depends nonlinearly on both per-particle properties and total column loading; (2) while emissions are dominant, particle aging often plays a substantial role over wider areas; (3) BC frequently exerts a net cooling effect at TOA, contradicting most climate models, though it can switch to warming over other areas; (4) BC's atmospheric heating and surface cooling are generally larger than models account for, and can exceed those of CO₂ and CH₄ in heavily industrial areas, implying greater importance for both local and global climate mitigation; (5) significant BC emission sources are missing in space and time from current inventories; (6) global and regional BC atmospheric loadings are often misestimated by models and reanalyses, suggesting a substantial free tropospheric reservoir and highlighting non-linear in-situ processing not captured by current parameterizations; and (7) BC absorption sometimes has a substantial impact on existing satellite retrievals of CH₄ and CO₂, calling into question how existing methods separate these when and where they are co-emitted.