Enhanced Radiative Cooling by Large Aerosol Particles from Pyrocumulonimbus

Large wildfires can generate pyrocumulonimbus (pyroCb) clouds that transport substantial amounts of smoke into the upper troposphere and lower stratosphere (UT/LS), perturbing aerosol budget and properties in these regions. Despite projections of increasing pyroCb events in the future, their climate impact, particularly the radiative forcing of smoke aerosols, remains poorly constrained, primarily due to limited direct measurements. Here we present aircraft measurements of aerosols and gases within 5-day-old pyroCb smoke plumes from a New Mexico wildfire, sampled at altitudes of 14-15 km. The aerosols, primarily organic biomass burning particles, exhibited an unusually large number-mode diameter of 500-600 nm. Microphysical simulations suggest that such large aerosols can form through combinations of cloud processing and coagulation in the relatively stable UT/LS environment. These large pyroCb aerosols increase outgoing radiation (aerosol-only perturbation) by 30-36% compared to typical non-pyroCb smoke aerosols with mode diameters of 200-300 nm, causing an instantaneous enhancement in cooling of the atmospheric column. Many climate models use smaller aerosol sizes for smoke than those observed for pyroCb aerosols, potentially underestimating the radiative cooling effects of pyroCb events. With a rising prevalence of pyroCb aerosols in a warming climate, accurately representing their size and optical properties in climate models is crucial.