Physicochemical properties affect ice nucleating abilities of biomass burning derived charcoal aerosols at cirrus and mixed-phase cloud conditions

Abstract. Atmospheric aerosol particles play a key role for air pollution, health and climate. Particles from biomass burning emissions are an important source of ambient aerosols and have increased over the past, and are projected to further surge in the future as a result of climate and land use changes. Particles emitted from biomass combustion are often complex mixtures of inorganic and organic materials, with soot, ash and charcoal having previously been identified as main particle types emitted. Despite their importance for climate, their ice nucleation activities remain insufficiently understood, in particular for charcoal particles, whose ice nucleation activity has not been reported. Here, we present experiments of the ice nucleation activities of 400 nm size-selected charcoal particles, derived from two different biomass fuels, namely a grass charcoal and a wood charcoal. We find that the charcoal types investigated do not contribute to ice formation via immersion freezing in mixed-phase cloud conditions. However, our results reveal a considerable heterogeneous ice nucleation activity of both charcoal types at cirrus temperatures. Inspection of the ice nucleation results together with dynamic vapor sorption meas-urements indicates that cirrus ice formation proceeds via pore condensation and freezing. We find wood charcoal to be more ice-active than grass charcoal at cirrus temperatures. We attribute this to the enhanced porosity and water uptake capacity of the wood compared to the grass charcoal. In support of the results, we found positive correlation of the ice nu-cleation activity of the wood charcoal particles and their chemical composition, specifically the presence of mineral com-ponents, based on single-particle mass spectrometry measurements. Even though correlational in nature, our results corrobo-rate recent findings that ice-active mineral could largely govern the aerosol-cloud interactions of particles emitted from bio-mass burning emissions.