Ice nucleation and the resulting cloud glaciation are significant atmospheric processes that affect the evolution of clouds and their properties including radiative forcing and precipitation, yet the sources and properties of atmospheric ice nucleants are poorly constrained. Heterogeneous ice nucleation caused by ice-nucleating particles (INPs) enables cloud glaciation at temperatures above the homogeneous freezing regime that starts near −35 °C. Biomass burning is a significant global source of atmospheric particles and a highly variable and poorly understood source of INPs. The nature of these INPs and how they relate to the fuel composition and its combustion are critical gaps in our understanding of the effects of biomass burning on the environment and climate. Here we show that the combustion process transforms inorganic elements naturally present in the biomass (not soil or dust) to form potentially ice-active minerals in both the bottom ash and emitted aerosol particles. These particles possess ice-nucleation activities high enough to be relevant to mixed-phase clouds and are active over a wide temperature range, nucleating ice at up to −13 °C. Certain inorganic elements can thus serve as indicators to predict the production of ice nucleants from the fuel. Combustion-derived minerals are an important but understudied source of INPs in natural biomass-burning aerosol emissions in addition to lofted primary soil and dust particles. These discoveries and insights should advance the realistic incorporation of biomass-burning INPs into atmospheric cloud and climate models. These mineral components produced in biomass-burning aerosol should also be studied in relation to other atmospheric chemistry processes, such as facilitating multiphase chemical reactions and nutrient availability.

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.

The mineralogical and immersion-mode freezing properties of volcanic ashes from three volcanoes, Volcań de Fuego and Santiaguito in Guatemala, and Soufriere Hills Volcano, in Montserrat, were examined. All ashes (sieved to <37 μm) contained effective ice nuclei, typically freezing over the temperature range of −12 to −25 °C and possessing ice active site densities (n s ) spanning ∼10 1 to 10 5 cm −2 over this temperature range. The high freezing activity of the ashes was determined to likely originate from pyroxene minerals, and the ice nucleation properties of pyroxene minerals are also reported here for the first time for comparison. Ca-and Narich plagioclase feldspars also contributed to the observed freezing properties. Volcanic glass was present in all of the samples and is theorized to be a much weaker ice nucleant, effectively diluting the freezing ability of the crystalline mineral phases. Smaller particle size fractions of the Volcań de Fuego ash were observed to contain more active ice nucleating particles, attributed to an increase in the amount of pyroxene minerals with decreasing particle size fraction. The particle resuspension and size segregated collection process was also observed to increase the ice nucleating ability of all size fractions, likely due to mechanical ablation removing passivated surfaces and exposing fresher and more ice-active mineral surfaces.

The morphology and composition of laboratory-generated biomass-burning aerosol (BBA) and bottom ash particles from authentic fuels were determined using transmission and scanning electron microscopies (TEM/SEM) and single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-ICP-ToF-MS). BBA particles with mineral material identified through elemental analysis using SEM represented 3−25% of the individual BBA particle numbers analyzed. This percentage varied depending on the fuel, with BBA from grass fuels containing more mineral particles than BBA from ponderosa pine wood. TEM analysis showed that these particles typically consist of carbonaceous material and a small (50−500 nm) region rich in nonvolatile elements. We also performed SEM/EDX analysis on soil, mineral BBA, and ash particles to measure Si:Al:Fe ratios and show that each of these particle classes possesses a different average bulk composition. However, individual particles within each population possess varying Si:Al:Fe ratios that may not be sufficiently unique to consistently determine particle sources from single-particle analysis. Mineral regions in BBA particles were similar in composition to residual bottom ash particles but were more likely to contain mixtures of nonvolatile elements, suggesting that ash particles typically underwent more complete combustion and mineralization. Multielement sp-ICP-ToF-MS analysis confirmed the presence of mineral particles in BBA, ash, and soil samples, with the most prevalent elements being the common crustal elements Al, Si, and Fe. Zn-and Ti-bearing particles were identified in both ash and soil samples, with more Zn present in ash particles and more Ti present in soil particles, suggesting that both of these types of particles would be prevalent in ambient measurements of BBA and biomass-burning impacted air masses. The mineral phases present in combustion-derived mineral phases are likely distinct from those present in soil-derived particles and may significantly affect the bulk properties of biomass-burning smoke. Both mineral BBA particles and lofted ash are likely sources of bioavailable iron and phosphorus that have been measured in biomass-burning emissions. These combustion-generated mineral phases are also important sources of ice-nucleating particles that have recently been reported in biomass-burning aerosol and bottom ash.

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