Surprising Crystallinity of Biomass Burning Secondary Organic Aerosol from Catechol and Nitrate Radical Reactions: Evidence and Possible Implications

Biomass burning events, including wildfires, emit large amounts of phenolic compounds such as catechol. These compounds can react with nitrate radicals (NO₃), a key nighttime oxidant, to form secondary organic aerosol (SOA). Although SOA is traditionally assumed to be noncrystalline, we present surprising evidence from X-ray diffraction that SOA formed from catechol + NO₃ in an atmospheric simulation chamber contains crystalline material. In addition, the diffraction pattern and mass spectrum of this SOA closely resemble those of nebulized crystalline 4-nitrocatechol (4-NC), suggesting the presence of crystalline 4-NC within the SOA. These findings help explain unusual size distributions of catechol + NO₃ SOA observed in prior studies and conflicting measurements of 4-NC's effective saturation vapor concentrations. Calculations of 4-NC's melting temperature as a function of its mole fraction in SOA, combined with observations of ambient 4-NC concentrations, suggest that 4-NC can exist in a solid crystalline phase state at temperatures below 288 K in wildfire plumes in the atmosphere. The presence of crystalline 4-NC and crystalline SOA in wildfire plumes may affect particle size distributions, cloud formation, and heterogeneous and photolytic reaction rates, with potentially important implications for atmospheric chemistry, air quality, and climate, warranting additional studies on this topic.