Enhanced radiative cooling by large aerosol particles from wildfire-driven thunderstorms.

Large wildfires can generate pyrocumulonimbus (pyroCb) clouds, injecting massive quantities of smoke aerosols into the upper troposphere and lower stratosphere (UT/LS), where they persist for months and affect climate. The radiative effects of pyroCb aerosols, however, remain poorly understood because of limited direct measurements. Here, we present in situ aircraft measurements of 5-day-old pyroCb smoke, addressing a critical observational gap in aerosol evolution from freshly emitted to weeks-to-months-aged states. The sampled smoke primarily contained unusually large aerosol particles (500 to 600 nanometers in diameter), formed through cloud processing and efficient coagulation in the UT/LS. Compared to smaller particles in typical non-pyroCb smoke, these large particles increase outgoing radiation by 30 to 36%, substantially enhancing atmospheric radiative cooling. Climate models may greatly underestimate this cooling effect by assuming smaller aerosol sizes for pyroCb smoke. As pyroCb events become more frequent, accurately representing their aerosol properties is essential for improving climate projections.