Ice nucleation active potassium salt from biomass-burning smoke

The highest concentrations of ice nucleating particles (INPs) in biomass-burning smoke are observed during intense, flaming fires (Schaefer, 1952; Prenni, 2012). While evidence that INPs can be generated by the combustion process itself is sparse, their presence is commonly attributed to lofting of dust and soil from the ground by strong fire induced convection. Potassium containing particles are abundant in smoke plumes and serve as a marker for biomass burning. Inorganic potassium compounds include KCl, KNO₃, and K₂SO₄.  Fresh smoke from flaming fires contains crystalline KCl, which is converted to KNO₃ and K₂SO₄ through reactions with HNO₃ and H₂SO₄ during plume aging (Freney, 2009).

We present ice nucleation experiments on monodisperse potassium salt particles conducted using a modified version of the SPectrometer for Ice Nucleation (SPIN) chamber (Welti, 2020), in which the test particles are exposed to temperatures down to 208 K and well-defined humidity. Ice nucleation occurred at temperatures below 235 K, relevant for cirrus cloud formation. While KCl particles deliquesce at approx. 85% relative humidity and their solution droplets freeze homogeneously, the experiments demonstrate that crystalline K₂SO₄ can serve as INP below its deliquescence point. The absence of ice formation at higher temperatures suggests ice nucleation proceeds via homogeneous freezing within a thin layer of adsorbed water on the salt particle surface.

However, atmospheric observations show that most biomass-burning aerosol occur as mixed particles, with the potassium salt either coated by organics or attached to an organic particle (Freney, 2009). In mixed particles, ice formation could be inhibited by the uptake of water into the organic material, preventing the formation of a surface water layer. Together, these findings indicate that the ice nucleation potential of pyrogenic potassium compounds should be represented in atmospheric models in conjunction with their emission, chemical aging, and mixing state to improve the simulation of biomass-burning INPs.

This work was supported by the Academy of Finland, project MEDICEN (grant no. 345125), and the ACCC Flagship programme (grant no. 337552).

References:

Freney, E. J., et al.: Deliquescence and efflorescence of potassium salts relevant to biomass-burning aerosol particles, Aerosol Sci. Technol., 43, 799–807, 2009.

Prenni, A. J., et al.: Biomass burning as a potential source for atmospheric ice nuclei: Western wildfires and prescribed burns, Geophys. Res. Lett., 39, L11805, 2012.

Schaefer, V. J.: Relation of ice nuclei to forest fire smoke, Occasional Report, Project Cirrus, 35, 7–11, 1952.

Welti, A., et al.: SPIN modification for low-temperature experiments, Atmos. Meas. Tech., 13, 7059–7067, 2020.