Increased Viscosity of Submicron Biomass-Burning Organic Aerosol after Photochemical Aging

Abstract

Biomass burning (BB) is a major global source of atmospheric aerosols. BB primary organic aerosol (BB-POA) particles vary based on fuel and burning conditions, leading to differences in particle-to-particle composition, even from a single burning event. In addition to BB-POA particles, BB emits volatile organic compounds (VOCs), which can be oxidized to form lower-volatility species that condense onto or react with POA to form secondary organic aerosol (SOA). Aerosol viscosity impacts aging processes by changing mixing time scales and is highly dependent on chemical composition, but little is known about BBOA viscosity at the single-particle level and the impacts of BB-SOA formation. We generated BB-POA and BB-SOA using a wood-burning stove connected to an atmospheric simulation chamber. Particle viscosity was calculated from direct measurements of individual particle melting temperatures (T_m) using nanothermal analysis (NanoTA), an atomic force microscopy (AFM)-based thermal technique. Additionally, bulk BBOA composition was determined by extractive electrospray ionization time-of-flight mass spectrometry (EESI-ToF) and used to calculate predicted T_m, viscosity, and mixing time scales. Overall, BBOA after photochemical aging (BB-POA+SOA) had significantly higher measured T_m values than BB-POA, leading to higher viscosity predictions. AFM-photothermal infrared (AFM-PTIR) spectra indicated both inter- and intraparticle chemical heterogeneity. Viscosities predicted from EESI-ToF measurements also increased after photochemical aging but underestimated the measurement-derived viscosities under both conditions. Our results indicate that aged particles within biomass burning plumes in the atmosphere may be more viscous than anticipated during much of their atmospheric lifetimes with implications for aerosol processes over large spatial scales.