Strong Shortwave Absorption by Wildfire Brown Carbon from Global Observations and Modeling

Wildfires emit large quantities of brown carbon (BrC), a class of light-absorbing organic aerosols with poorly constrained climate effects. BrC exhibits highly variable absorptivity, from weakly absorbing chromophores in the near-ultraviolet to strongly absorbing "dark BrC" (d-BrC) extending into the visible spectrum, yet the optical properties, global prevalence, and radiative impact of d-BrC remain poorly understood. Here we show that d-BrC is widespread in wildfire plumes globally, based on integrated analyses of aircraft, ground-based, and satellite observations. We found d-BrC mass absorption efficiencies of 0.5–1.5 m²/g at 500 nm, with absorption often comparable to or exceeding that of black carbon (BC). Implementing these observationally constrained optical properties in a global aerosol-climate model, we estimate a direct radiative effect (DRE) of +0.097 W/m² (range: +0.050 to +0.276 W/m²) from wildfire-derived BrC, with the upper bound surpassing BC and extending into mid- and high-latitude regions including the Arctic These findings position d-BrC as a critical but overlooked driver of wildfire radiative forcing, underscoring the need to account for its strong radiative effects on climate.