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

Atmospheric aerosol particles play a key role in air pollution, health, and climate. Particles from biomass burning emissions are an important source of ambient aerosols, have increased over the past few decades, and are projected to further surge in the future as a result of climate and land use changes. Largely as a result of the variety of organic fuel materials and combustion types, particles emitted from biomass burning are often complex mixtures of inorganic and organic materials, with soot, ash, and charcoal having previously been identified as main particle types being 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 the pyrolysis of two different biomass fuels, namely a grass charcoal and a wood charcoal. We find that the pyrolysis-derived charcoal types investigated do not contribute to ice formation via immersion freezing in mixed-phase cloud conditions. However, our results reveal considerable heterogeneous ice nucleation activity of both charcoal types at cirrus temperatures. An inspection of the ice nucleation results together with dynamic vapor sorption measurements 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 a positive correlation of the ice nucleation activity of the wood charcoal particles and their chemical composition, specifically the presence of (inorganic) mineral components, based on single-particle mass spectrometry measurements. Even though correlational in nature, our results corroborate recent findings that ice-active minerals could largely govern the aerosol–cloud interactions of particles emitted from biomass burning emissions.