Secondary Organic Aerosol from Biomass Burning Phenolics Could Increase Brown Carbon Lifetimes, Seed Ice Clouds, and Transport Pollutants

Biomass burning events emit large amounts of phenolic compounds, which are oxidized in the atmosphere and form secondary organic aerosol (SOA). Using the poke-flow technique, we measured room-temperature and relative humidity (RH)-dependent viscosities of SOA generated by the oxidation of three biomass burning phenolic compounds: catechol, guaiacol, and syringol. All systems had viscosity < 3 × 10³ Pa s at RH ⪆ 40% and > 2 × 10⁸ Pa s at RH ⪅ 3%. At RH values of 0-10%, the viscosities of these SOA were at least 2 orders of magnitude higher than the viscosity of primary organic aerosol (POA) from biomass burning. These results suggest that mixing biomass burning SOA and POA may extend the lifetime of the brown carbon in the atmosphere. Based on an extrapolation of our results to tropospheric temperature and RH values, phenolic SOA is in a glassy state (𝜂 > 10¹² Pa s) above ∼6 km in the troposphere, potentially acting as heterogeneous ice nuclei in clouds, thereby influencing climate. Furthermore, the mixing time of organic molecules in a 200 nm phenolic SOA particle exceeds 1 h above 3 km in the troposphere, which has implications for the long-range transport of pollutants.