Observation-constrained estimates and diagnostic insights into black carbon radiative forcing across contrasting urban environments from multi-platform remote sensing

Black carbon (BC) aerosols are commonly treated as a uniformly warming climate forcer, yet its radiative impact depends sensitively on particle microphysics, column loading, and vertical energy redistribution. Here, we present an observation-constrained assessment of BC radiative forcing over two contrasting Asian urban agglomerations-Dhaka (Bangladesh) and Xuzhou (China), using a multi-platform remote sensing framework that integrates multi-waveband satellite and ground-based observations as constraints. Multi-waveband single-scattering albedo (SSA) from TROPOMI and AERONET/SONET is used to constrain physically admissible BC core-shell size and mixing-state ensembles, which are further filtered using aerosol optical depth (AOD) to enforce column-integrated optical feasibility. The resulting microphysical ensembles and their associated optical properties are coupled with radiative transfer simulations to quantify clear-sky atmospheric (ATM), surface (SFC), and top-of-atmosphere (TOA) forcing at high spatial and temporal resolution.

We find that BC radiative forcing exhibits pronounced regional heterogeneity and a strong vertical redistribution of energy within the atmospheric column. Contrary to the canonical assumption of BC as a strictly warming TOA agent, weighted climatological means reveal substantial net TOA cooling over both regions (-15.0 ± 1.2 Wm^-2 over Dhaka and -17.4 ± 2.6 Wm^-2 over Xuzhou), with occasional episodic warming events. In contrast, atmospheric absorption is markedly stronger (18.2 ± 1.3 Wm^-2 and 15.5 ± 1.9 Wm^-2, respectively), corresponding to localized heating rates approaching ~0.3 K day^-1, while surface cooling frequently exceeds -30 Wm^-2. These results indicate that BC plays a larger role in regulating boundary-layer stability and regional energy balance than implied by TOA forcing alone.

Diagnostic analysis using multivariate decomposition reveals that BC radiative impacts are organized into a limited number of physically coherent pathways. In Dhaka, forcing variability is dominated by emission-driven column loading, producing tightly coupled atmospheric heating and TOA cooling, whereas in Xuzhou, variability is primarily regulated by column-integrated optical efficiency associated with particle aging and mixing state. Local forcing extremes frequently exceed the global mean effective radiative forcing of long-lived greenhouse gases by more than an order of magnitude, underscoring the inadequacy of coarse-scale or globally averaged frameworks for assessing BC-climate interactions. Together, these findings demonstrate that regional climate responses to BC are governed by microphysically mediated energy redistribution, highlighting the need for observation-constrained, high-resolution approaches to inform mitigation strategies in polluted environments.