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Ice-nucleating particles (INPs) in biomass-burning aerosol (BBA) that affect cloud glaciation, microphysics, precipitation, and radiative forcing were recently found to be driven by the production of mineral phases. BBA experiences extensive chemical aging as the smoke plume dilutes, and we explored how this alters the ice activity of the smoke using simulated atmospheric aging of authentic BBA in a chamber reactor. Unexpectedly, atmospheric aging enhanced the ice activity for most types of fuels and aging schemes. The removal of organic carbon particle coatings that conceal the mineral-based ice-active sites by evaporation or oxidation then dissolution can increase the ice activity by greater than an order of magnitude. This represents a different framework for the evolution of INPs from biomass burning where BBA becomes more ice active as it dilutes and ages, making a larger contribution to the INP budget, resulting cloud microphysics, and climate forcing than is currently considered. 

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. 

Abstract. Some biological particles, such as Snomax, are very active ice nucleating particles, inducing heterogeneous freezing in supercooled water at temperatures above −15 and up to −2 °C. Despite their exceptional freezing abilities, large uncertainties remain regarding the atmospheric abundance of biological ice nucleating particles, and their contribution to atmospheric ice nucleation. It has been suggested that small biological ice nucleating macromolecules or fragments can be carried on the surfaces of dust and other atmospheric particles. This could combine the atmospheric abundance of dust particles with the ice nucleating strength of biological material to create strongly enhanced and abundant ice nucleating surfaces in the atmosphere, with significant implications for the budget and distribution of atmospheric ice nucleating particles, and their consequent effects on cloud microphysics and mixed-phase clouds. The new critical surface area g framework that was developed by Beydoun et al. (2016) is extended to produce a heterogeneous ice nucleation mixing model that can predict the freezing behavior of multicomponent particle surfaces immersed in droplets. The model successfully predicts the immersion freezing properties of droplets containing Snomax bacterial particles across a mass concentration range of 7 orders of magnitude, by treating Snomax as comprised of two distinct distributions of heterogeneous ice nucleating activity. Furthermore, the model successfully predicts the immersion freezing behavior of a low-concentration mixture of Snomax and illite mineral particles, a proxy for the biological material–dust (bio-dust) mixtures observed in atmospheric aerosols. It is shown that even at very low Snomax concentrations in the mixture, droplet freezing at higher temperatures is still determined solely by the second less active and more abundant distribution of heterogeneous ice nucleating activity of Snomax, while freezing at lower temperatures is determined solely by the heterogeneous ice nucleating activity of pure illite. This demonstrates that in this proxy system, biological ice nucleating particles do not compromise their ice nucleating activity upon mixing with dust and no new range of intermediary freezing temperatures associated with the mixture of ice nucleating particles of differing activities is produced. The study is the first to directly examine the freezing behavior of a mixture of Snomax and illite and presents the first multicomponent ice nucleation model experimentally evaluated using a wide range of ice nucleating particle concentration mixtures in droplets.