Investigating Pyrocumulonimbus Smoke with In Situ Aircraft Measurements: Aerosol Properties and Direct Radiative Effects

Large wildfires can generate pyrocumulonimbus (pyroCb) clouds that inject a substantial amount of smoke into the upper troposphere and lower stratosphere (UT/LS), significantly perturbing the aerosol budget and properties in this region. Despite the predicted increase in frequency and intensity of pyroCb events due to climate change, the impact of pyroCb aerosols on climate remains uncertain, primarily due to the lack of direct measurements.

On 21 June 2022, NASA ER-2 high-altitude aircraft sampled pyroCb smoke in the uppermost troposphere between 14 and 15 km during the Dynamics and Chemistry of the Summer Stratosphere (DCOTSS) mission. Satellite and ground-based lidar observations confirmed that the sampled plumes originated from a New Mexico pyroCb event that occurred five days prior. In situ measurements revealed a significant enhancement in particle concentration, characterized by a distinct large size mode between 500 and 600 nm diameter. These particles were predominantly composed of organic material from biomass burning. Concurrent increases in gas-phase biomass burning tracers, such as carbon monoxide (CO) and carbon dioxide (CO₂), were also observed. The evolution of aerosol microphysics within the pyroCb smoke was further examined using an aerosol microphysical model constrained by the environmental conditions in the UT/LS. The measured aerosol properties were used to estimate the direct radiative forcing of pyroCb aerosols using a radiative transfer model. This study, based on in situ measurements of pyroCb smoke, provides valuable insights into the climate impacts of pyroCb aerosols in the UT/LS.