Biomass combustion produces ice-active minerals in biomass-burning aerosol and bottom ash

Abstract

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.

Keywords: aerosol–cloud–climate interactions; atmospheric chemistry; climate change; heterogeneous ice nucleation; wildfires.

Fig. 1.

Droplet freezing temperature spectra (Top Left) and ice-active surface site density (n_s) spectra (Top Right) of biomass-burning ash (solid symbols) and aerosol (open symbols) generated from combustion of cutgrass (triangles), sawgrass (diamonds), and birch wood (squares). Also shown are a soil sample from a wetland in North Carolina (brown crosses), the soil exposed to combustion conditions (red crosses), and an average filtered water control (gray). Ash and soil suspensions were made at 0.01 weight %, except for birch ash, which was analyzed at 0.1 weight %. Aerosol spectra were acquired with 0.5-μL droplets and ash and soil were acquired with 0.1-μL droplets. Each spectrum is the composite of two to four separate arrays. Error bars on the pure water background represent a single SD around the average. A representative set of error bars calculated from the variability in the filtered water background freezing spectrum (64) is included for the n_s values of the sawgrass aerosol and ash and fired soil datasets; at many temperatures the error bars are smaller than the symbol size. A complete set of error bars is instead included in SI Appendix, Figs. S1, S2, and S5 due to space limitations. Also included here are n_s (INA) parameterizations for domestic ash produced from a mixture of hard and soft woods combusted in a domestic heating oven (57), coal fly ash collected from an electrostatic precipitator (57), quartz minerals (65), and highly ice-active potassium feldspar minerals (65). A more complete set of literature n_s parameterizations are shown (Lower), incorporating the above-mentioned freezing data and additional data from Umo et al. (57) and Grawe et al. (58). Data from Grawe et al. (58) are from dry-generated size-selected 300-nm bottom ash particles generated in a high-efficiency heating oven (symbols). Data from Umo et al. (57) are from the domestic ash and coal fly ash samples (magenta dashed lines). Together with our data this demonstrates that ash particles produced from biomass combustion are capable of nucleating ice over a wide temperature range similar to other mineral systems such as quartz.

Fig. 2.

XRD spectra to determine crystalline phase composition acquired from (A) birch, (B) sawgrass, (C) cutgrass, and (D) ponderosa pine bottom ashes. Note the different relative intensity (y-axis) for each spectrum. XRD spectra for switchgrass, wiregrass, and NC grass bottom ashes are shown in SI Appendix, Fig. S3. Spectra were acquired over the 2θ range of 10° to 70°. The spectra of the ashes all contain a baseline rise and broad feature at 2θ values less than ∼40°, indicative of amorphous material. Major spectral features corresponding to crystalline phases are denoted with individual letters as follows: C, calcite, CaCO₃; K, sylvite, KCl; M, potassium-sodium sulfate, (Na,K)SO₄; D, dolomite, (Ca,Mg)CO₃; O, potassium sulfate, K₂SO₄; Q, quartz, SiO₂.

Fig. 3.

TEM images of mineral phases in BBA. Images A–C are from ponderosa pine BBA and image D is from sawgrass BBA. The mineral and other particle regions were categorized by chemical composition based on EDX spectroscopy and morphology; these are labeled in the TEM images, with OC corresponding to organic carbon. The magnification and scale bars are variable as indicated. EDX spectra for the areas outlined in red are shown in SI Appendix, Fig. S4 and show that the mineral region in A is composed mostly of iron, silicon, and oxygen with smaller contributions from sulfur, sodium, potassium, magnesium, zinc, and manganese; the region in B is composed of iron and oxygen; the region in C is composed of magnesium, aluminum, silicon, and oxygen; and the region in D is composed of silicon, sodium, and oxygen.

Fig. 4.

TEM images (A and B) and corresponding selected area electron diffraction (SAED) patterns (C and D) for two aerosol samples generated from the combustion of ponderosa pine needles. The SAED patterns correspond to the areas in the upper images indicated by a red box. The regular and intense bright spots in the SAED pattern in D indicates the main particle is a single well-formed crystal, and the less-intense spots originate from a large agglomeration of small iron oxide particles around the upper right edge of the main particle in B. The many rings and spots in the SAED pattern in C indicate that this region is composed of disordered crystalline, polycrystalline, and/or amorphous phases.