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.

Atmospheric aging enhances the ice nucleation ability of biomass-burning aerosol

“…Note that the small <500-nm size of the mineral components suggests that these were formed during combustion, as the majority of minerals lofted by mechanical action from soil dust or biomass ash would be supermicrometer. In our previous work, the most crystalline mineral phases were present in all samples of both aerosol and ash from the tall grass fuels, which also contain more INPs and have higher INA than BBA from wood fuels ( 26 ). As most mineral particles in the BBA were found to be submicrometer, these potential INPs will have longer lifetimes versus gravitational settling, undergo atmospheric transport over longer distances, and exert more extensive effects on cloud microphysics over larger spatial scales than the much larger supermicrometer lofted ash particles would ( 26 ).…”

“…INA increased more after aging in BBA produced by the combustion of grassy fuels—which have a much higher mineral content—than other fuels like pine needles and woods that produce BBA with higher OA and BC content instead. These differences can be attributed to the observed changes in particle properties and composition that lead to an uncovering of preexisting ice-active surface sites that we recently demonstrated are mostly mineral phases produced by the biomass combustion itself ( 13 , 26 ). TEM images of the BBA collected in these experiments also show mineral phases and their mixtures with other particle components such as BC soot and salt phases that are often coated by organic carbon.…”

“…BBA is well known to contain OA of intermediate to low volatility (their saturation vapor pressure) that can experience considerable evaporation through smoke plume dilution ( 33 , 34 ), and vapor wall losses in Teflon chambers promote evaporation in laboratory experiments. The OA is heterogeneously distributed throughout the complex BBA and is often nonuniformly mixed in individual submicrometer particles that also contain soot, inorganic salts, and/or mineral phases ( 26 ). The evaporation of semivolatile OA observed here would reveal more of the ice-active surface sites already present in the BBA, allowing them to interact directly with water and nucleate ice that cannot occur when the sites are concealed.…”

“…This explains the large increase in ice activity observed in the aged BBA and the sharp increase in the ice-active site density ( n s ) over a narrow temperature range, which indicates that the ice nucleants in both fresh and aged BBA have similar properties. BBA produced by the combustion of ponderosa pine needles contains many fewer Si-containing mineral particles compared to the grass fuels, which could explain why an increase in INA was not observed for this fuel type ( 26 ).…”

“…This further supports the idea that ice-active sites in BBA active under immersion freezing conditions are not organic carbon based. Our previous analysis of the BBA and ash produced from these same fuels concluded that the production of new mineral phases from the biomass combustion itself is the major source of the ice nucleants, and that fuels that produced the most soot had the weakest or even unmeasurable INA at temperatures warmer than −25°C ( 26 ). These observations are supported by a recent study of prescribed burns and wildfires that estimated that BC contributed at most 10% to observed INP concentrations ( 44 ).…”