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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. 

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. 

We examined the reactive uptake of dinitrogen pentoxide (N 2 O 5 ) to authentic biomass-burning aerosol (BBA) using a small chamber reservoir in combination with an entrained aerosol flow tube. BBA was generated from four different fuel types and the reactivity of N 2 O 5 was probed from 30 to 70% relative humidity (RH). The N 2 O 5 reactive uptake coefficient, γ (N 2 O 5 ), depended upon RH, fuel type, and to a lesser degree on aerosol chloride mass fractions. The γ (N 2 O 5 ) ranged from 2.0 (±0.4) ×10 −3 on black needlerush derived BBA at 30% RH to 6.0 (±0.6) ×10 −3 on wiregrass derived BBA at 65% RH. Major N 2 O 5 reaction products were observed including gaseous ClNO 2 and HNO 3 and particulate nitrate, and used to create a reactive nitrogen budget. Black needlerush BBA had the most particulate chloride, and the only measured ClNO 2 yield > 1%. The ClNO 2 yield on black needlerush decayed from an initial value of ∼100% to ∼30% over the course of the burn experiment, suggesting a depletion of BBA chloride over time. Black needlerush was also the only fuel for which the reactive nitrogen budget indicated other N-containing products were generated. Generally, the results suggest limited chloride availability for heterogeneous reaction for BBA in the RH range probed here, including BBA with chloride mass fractions on the higher end of previously reported values (∼17–34%). Though less than fresh sea spray aerosol, ∼50%. We use these measured quantities to discuss the implications for nocturnal aerosol nitrate formation, the chemical fate of N 2 O 5 (g), and the availability of particulate chloride for activation in biomass burning plumes.