A Novel Short-Pathlength Photoreactor to Study Aqueous-phase Photochemistry: Application to Biomass-Burning Phenols

Abstract. Aqueous-phase oxidation of biomass-burning phenols is a significant but poorly constrained source of secondary organic aerosol (SOA) in the atmosphere. Laboratory studies replicating aerosol liquid water (ALW) – characterized by high solute and chromophore concentrations – remain scarce due to strong light attenuation and thermal gradients in conventional photoreactors. To address these limitations, we developed a short-pathlength photoreactor (SPP) that minimizes optical screening and provides precise control of temperature, humidity, and illumination conditions. Using guaiacyl acetone (GA) as a model phenol compound and 3,4-dimethoxybenzaldehyde (DMB) as a triplet precursor (³C*), the SPP successfully reproduced SOA yields from established photoreactors under dilute conditions and further enabled experiments under strongly light-absorbing ALW regimes. The system maintained stable temperature and relative humidity, consistent photon flux, and reproducible photochemical performance. Positive matrix factorization (PMF) of high-resolution aerosol mass spectra resolved distinct stages of GA-derived aqueous SOA (aqSOA) evolution across a wide range of ionic strengths. The analysis further revealed the formation of GA dimers through photosensitized coupling pathways, with dimer formation rates increasing significantly with ionic strength. Overall, the SPP provides a validated and versatile platform for investigating aqSOA formation and transformation processes under atmospherically relevant droplet and ALW conditions.