Secondary Organic Aerosol from Reactions of Biomass Burning Phenolic Compounds with Nitrate Radicals (NO3) Can Be Highly Viscous and Even Glassy over a Wide Relative Humidity Range due to Limited Hygroscopicity
SEPEHR NIKKHO, Bin Bai, Fabian Mahrt, Julia Zaks, Long Peng, Kristian Kiland, Pengfei Liu, Allan K. Bertram, University of British Columbia
Abstract Number: 582
Working Group: Aerosol Physics
Abstract
Biomass burning events, including wildfires, can emit large amounts of phenolic compounds, such as guaiacol, into the atmosphere. Once in the atmosphere, phenolic compounds can be oxidized by nitrate radicals (NO3) to form secondary organic aerosol (SOA). Viscosity and hygroscopicity are two important properties of SOA since they influence several atmospheric processes including particle reactivity, particle growth, and cloud nucleating ability. However, the viscosity and hygroscopicity of SOA generated from phenolic compounds and NO3 have not been quantified. In the following, we use the poke-flow technique to measure the RH-dependent viscosity of SOA particles generated by reacting guaiacol and NO3, referred to as guaiacol-NO3 SOA. We also measured the hygroscopicity of the guaiacol-NO3 SOA to better understand the RH-dependent viscosity measurements. We found that the viscosity of guaiacol-NO3 SOA is high (≳ 5×107 Pa s) at RH values ≲ 70%. For reference, the viscosity of tar pitch is ~108 Pa s. Even at 80% RH, the viscosity is still 6×105 Pa s. In addition, the viscosity of guaiacol-NO3 SOA is drastically higher than the other types of SOA studied with the poke-flow technique at ≳ 40% RH. The difference between the viscosities of guaiacol-NO3 SOA and the other types of SOA can be explained, at least in part, by a low hygroscopicity parameter for guaiacol-NO3 SOA. We extrapolated our results to lower temperatures using the Vogel-Fulcher-Tammann equation. Based on this extrapolation, guaiacol-NO3 SOA is most often in a semi-solid state in the planetary boundary layer (≲ 1 km in altitude) and a glass state in the free troposphere (1km ≲ altitude ≲ 18 km). The liquid state is only predicted for a very small region of the planetary boundary layer (≳ 80 ° latitude).