Savannah and Boreal Biomass Burning as a Source for Cloud Condensation Nuclei

Abstract Aerosol particles emitted in wildfires can contribute to radiative forcing via aerosol‐cloud interactions and by directly interacting with solar radiation. Wildfire emissions increase the concentration of cloud condensation nuclei (CCN) in the atmosphere and thus affect cloud properties (e.g., cloud brightness). For this study, we conducted open laboratory burning experiments with less studied biomass types from South African savannah and European boreal forest, to quantify the particle hygroscopicity, an important factor affecting CCN concentrations. We also investigated the effect of burning conditions (characterized by the modified combustion efficiency (MCE)) and aging on the hygroscopicity of the particles. Considering all the experiments, the primary hygroscopicity parameter is constant (0.079 0.007 (mean STD) for supersaturation of 0.43%) with respect to MCE for particles from smoldering combustion and increases to a maximum of 0.19 with increasing contribution of flaming. With aging, the hygroscopicity increases (boreal forest biomass) or remains unchanged (savannah biomass) in smoldering experiments, while it decreases in experiments with more flaming‐dominated burning, decreasing the overall variability in . Using complementary chemical analysis methods, we provide insights into the markedly different CCN results under different burning conditions. Our results indicate that even in organic carbon‐dominated aerosol representative of wildfires, a small fraction of inorganic material can significantly influence the aerosol hygroscopicity, highlighting the impact of combustion efficiency on biomass burning aerosol CCN activity.