Divergent Trends of Black and Brown Carbon Driven by Anthropogenic Emissions and Open Biomass Burning Across Asia

Abstract. Black carbon (BC) and brown carbon (BrC) are the dominant light-absorbing carbonaceous aerosols (LACs) and contribute substantially to regional climate warming. Across Asia, heterogeneous changes in anthropogenic and open biomass burning (OBB) emissions driven by clean air policies and climate variability are reshaping LAC composition, yet how these changes alter the relative abundance and radiative roles of BC and BrC remains poorly understood. Here, using the chemical transport model GEOS-Chem combined with machine-learning attribution, we quantify the responses of BC and BrC to concurrent emission changes across East, South, and Southeast Asia during 2012–2019. We identify pronounced spatial heterogeneity in BrC/BC mass ratio trends. East Asia exhibits a significant upward trend, driven by policy-induced suppression of BC-rich anthropogenic sources alongside enhanced BrC-rich OBB from a seasonal shift in crop residue burning under open-fire regulations. In contrast, South and Southeast Asia show declining ratios attributable to residential energy transitions and wildfire variability, respectively. These asymmetric responses propagate into direct radiative forcing (DRF), with the DRF_BrC/DRF_BC ratio increasing from 34.1% to 41.3% in East Asia while declining elsewhere. Notably, in East and Southeast Asia, OBB amounts to up to half of anthropogenically driven radiative forcing changes. Our results demonstrate that emission mitigation can redistribute rather than proportionally reduce LAC warming, highlighting that future climate benefits will critically depend on the concurrent management of open biomass burning.