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Abstract. Droplet freezing techniques (DFTs) have been used for half a century tomeasure the concentration of ice-nucleating particles (INPs) in the atmosphereand determine their freezing properties to understand the effects of INPs onmixed-phase clouds. The ice nucleation community has recently adopted dropletfreezing assays as a commonplace experimental approach. These dropletfreezing experiments are often limited by contamination that causesnonhomogeneous freezing of the “pure” water used to generate the dropletsin the heterogeneous freezing temperature regime that is being measured.Interference from the early freezing of water is often overlooked and notfully reported, or measurements are restricted to analyzing the moreice-active INPs that freeze well above the temperature of the backgroundwater. However, this avoidance is not viable for analyzing the freezingbehavior of less active INPs in the atmosphere that still have potentiallyimportant effects on cold-cloud microphysics. In this work we review a numberof recent droplet freezing techniques that show great promise in reducing theseinterferences, and we report our own extensive series of measurements usingsimilar methodologies. By characterizing the performance of differentsubstrates on which the droplets are placed and of different pure watergeneration techniques, we recommend best practices to reduce theseinterferences. We tested different substrates, water sources, dropletmatrixes, and droplet sizes to provide deeper insight into what methodologiesare best suited for DFTs. Approaches for analyzing droplet freezingtemperature spectra and accounting and correcting for the background “pure”water control spectrum are also presented. Finally, we propose experimentaland data analysis procedures for future homogeneous and heterogeneous icenucleation studies to promote a more uniform and reliable methodology thatfacilitates the ready intercomparison of ice-nucleating particles measured byDFTs. 

Significance Ice-nucleating particles significantly alter cloud properties and lifetime, causing large but poorly constrained climate impacts. Biomass-burning aerosol emitted by wildfires is a major and growing source of atmospheric pollution. Prior work suggested that ice-nucleating particles can sometimes be emitted by biomass combustion, but the production and characteristics of these particles are poorly understood. Here we show that mineral phases are a significant ice-active component of both biomass-burning aerosol and ash particles. These mineral phases are derived from plant inorganic material that decomposes and reforms as ice-active minerals during combustion; they form more commonly from tall grass versus wood fuels. Aerosolized mineral and ash are now understood as a major source of the ice-nucleating particles in biomass-burning smoke.