Observation-constrained clear-sky direct radiative forcing of black carbon containing aerosols reveals distinct radiative regimes across urban Asia

Black carbon (BC) aerosols exert a strong yet uncertain influence on the atmospheric energy balance, yet their radiative forcing estimates remain poorly constrained. Current top-down and bottom-up approaches remain limited in resolving how microphysical evolution and column properties jointly govern radiative responses. To address this limitation, we develop an observation-constrained framework that integrates satellite and ground-based remote sensing observations through a dual-constraint approach. Spectral single-scattering albedo observations from TROPOMI and AERONET/SONET, together with their uncertainty bounds, are first used to constrain physically plausible BC size and mixing state across UV-VIS-NIR wavelengths. These particle-scale solutions are then screened using MODIS-MAIAC multi-waveband aerosol optical depth observations and their associated upper uncertainty bounds, which constrain column extinction and loading. This dual-constraint strategy links particle-scale microphysical and column-scale extinction consistency within a unified observation-based framework. The resulting set of BC optical properties is propagated through radiative transfer modeling, supported by ancillary satellite-derived atmospheric parameters. The framework is applied over two contrasting urban environments in Asia - a polycentric urban agglomerate and a monocentric megacity. It produces spatially resolved, observation-constrained estimates of clear-sky direct radiative forcing (DRF) for BC-containing aerosols at 0.05° × 0.05°. The study reveals that, despite comparable atmospheric heating (15.5 ± 1.9 W m −2 , 18.2 ± 1.3 W m −2 , ~0.3 K day −1 ), both regions exhibit distinct top-of-atmosphere responses with persistent cooling (−17.4 ± 2.6 W m −2 , −15.0 ± 1.2 W m −2 ). A sign reversal to weak positive TOA forcing occurs only over Xuzhou, reaching ~ +0.4 W m −2 , whereas Dhaka remains negative throughout the analyzed distribution. Diagnostic analyses further show that BC DRF does not scale with column loading or aerosol optical depth alone, but exhibits a bifurcation into distinct regimes characterized by coupled variation in column loading, particle microphysics and spectral absorption. Coupled variability analysis further reveals a regionally distinct organization of radiative responses. These responses are embedded within coherent multivariate patterns of microphysical optical, and column variability.Overall, the study demonstrates a reproducible pathway for leveraging multi-platform remote sensing observations to resolve regionally distinct BC radiative impacts. These results also provide an observational basis for evaluating how region-specific BC mitigation strategies may alter regional radiative forcing.