Insights into Pyrocumulus aerosol composition: black carbon content and organic vapor condensation†

Abstract

Megafires are increasingly generating Pyrocumulus clouds (PyroCus) through the interplay of atmospheric conditions such as stability and humidity, hot updrafts, and emitted aerosols from burning vegetation. As megafires become more frequent, the annual radiative influence of PyroCus on the climate is intensifying. In this study, we aim to quantify the aerosol mass and black carbon content that PyroCus injects into the stratosphere, which can persist for 3 to 15 months. Utilizing aircraft-sampled smoke plumes from both the Northern and Southern Hemispheres, our findings indicate that the mass fraction of black carbon within PyroCus remains consistent, ranging between 0.5 and 3%. This serves as a crucial constraint for incorporating source terms in climate models. Additionally, we provide evidence of the volatile vapor 1-nonene condensing in the updrafts, which is one of likely many organic vapors contributing to increased aerosol mass concentrations. To corroborate these findings, we conducted independent Large Eddy Simulations (LES) that demonstrate organic vapor condensation can double the aerosol mass in updrafts. These resolved LES serve as a valuable guide, directing future aircraft measurement locations and further development of PyroCus mechanisms in models.