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

Abstract

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.

Fig. 1. Ice-active surface site density (n_s) plotted versus freezing temperature.

Each panel shows one type of aging of BBA produced from combustion of cutgrass, sawgrass, or ponderosa pine. Fresh samples (before external perturbation or time aging) are shown in green, and aged samples following several hours of chamber aging are shown in purple. (A) Time aging experiments revealed substantial increases in INA along with evaporation of organic carbon aerosol. (B) Hydroxyl radical photooxidative aging caused slight increases in INA (additional experiments shown in fig. S2). (C) Thermal evaporation of the BBA followed by hydroxyl radical photooxidation revealed mixed effects on INA. (D) Ozonolysis without photochemistry resulted in no observed changes or a prominent decrease in INA in one case, along with substantial increases in the OA mass loading. A subset of these experiments shown with 95% confidence intervals is provided in fig. S3, and the ice-active site density normalized to the mass concentration of BC aerosol (a conserved nonvolatile tracer) rather than aerosol surface area is provided in fig. S9.

Fig. 2. Exemplary evolution of submicrometer aerosol chemical composition in the four types of simulated atmospheric aging explored.

Each panel shows SP-AMS chemical composition measured for one type of aging. Mass concentration (trace color corresponds to chemical component) in SP-mode is plotted in the lower portion; the OA:BC ratio—a measure of the gain or loss of OA versus the conserved BC tracer that only undergoes chamber wall loss—is plotted on the upper left axis. A measure of OA oxidation state, the O:C atomic ratio from EI-mode measurements, is plotted on the upper right axis. OA:BC and O:C are unreliable during the first few minutes of the experiment while the chamber is filling. Collection times of fresh and aged filters for INP analysis are shown by green and purple bars, respectively, on the top of each panel. (A) Cutgrass time aging experiment with considerable evaporation of OA observed in the OA:BC ratio. (B) Sawgrass photooxidation experiment; HONO injection is shown by the orange bar and UV illumination by the purple bar. Sudden decreasing mass concentration and increasing O:C occurred when the aerosol particles were passed through a heated thermodenuder before entering the SP-AMS. (C) Sawgrass experiment where the BBA was subjected to thermal desorption at 250°C before injection into chamber. HONO injection is shown by the orange bar, and UV illumination is shown by the purple bar. (D) Cutgrass dark ozonolysis experiment, with 350 ppb of ozone injected at the labeled time. The formation of SOA is indicated through the increase in OA:BC following ozone injection.

Fig. 3. TEM images of fresh BBA.

(A) and (B) are from ponderosa pine needles and (C) is from sawgrass BBA collected on substrates indicating the presence of OA coatings around mineral-containing particles. Particle (A) is a fractal soot particle agglomerated with an iron-based mineral in the region indicated by the box. Particle (B) has a mixed core in region 1 composed of KCl, minerals, and carbonaceous material, with additional KCl in region 2 and a carbonaceous OA coating that appears gray surrounding the entire particle. Particle (C) contains several inorganic salt phases in regions 1 and 3 and is also surrounded by an OA coating that appears gray in regions 2 and 4. EDX spectra of the boxed regions are provided in fig. S6.

Fig. 4. Schematic representation of the atmospheric coevolution of BBA composition and INA.

The particle contains minerals depicted in shades of brown, soot depicted in black, and OA depicted in green. Evaporation of OA leads to increased availability of ice-active sites. Oxidation of OA changes its chemical composition to be more oxidized and water-soluble organic carbon such that, when the particle is immersed in a droplet, the OA can dissolve more readily to reveal ice-active sites. SOA condensation from oxidation and condensation of organic carbon onto the particle conceals ice-active sites. This schematic represents the different BBA aging processes that may occur in the atmosphere, which are not ordinarily decoupled as depicted and can occur simultaneously or in competition to varying degrees.