Secondary organic aerosol (SOA) physicochemical properties such as hygroscopicity, degree of oxidation, phase state, pH, and viscosity influence important processes that ultimately impact air quality and climate. Due in part to the experimental challenge of in situ characterization, these physicochemical properties remain incompletely understood. Here, an aerosol fluorescent labeling methodology in conjunction with a fluorescence aerosol flow tube (F-AFT) was used for direct, in situ physicochemical characterization of α-pinene and toluene SOA. The wavelength of maximum emission λmax of the fluorescent probe Prodan was shown to depend on the hygroscopicity, degree of oxidation, and relative humidity (RH)-dependent phase transitions of the SOA. At varying RH, λmax was linearly correlated with % water content with an R2 value of 0.95. For fixed RH and varying SOA oxidation degree, λmax was linearly correlated with the oxygen-to-carbon ratio (O:C) with an R2 value of 0.95. Finally, the λmax values measured from mixed organic/inorganic particles indicated that ammonium sulfate seeded toluene SOA exhibited liquid–liquid phase separation (LLPS) at 80% RH and below while remaining homogeneous at 90% RH. Overall, this study demonstrates the power and versatility of this approach for direct, online analysis of a range of physicochemical properties in SOA.
Boadu et al. (2026) studied this question.