Revisiting the Effects of Seed Liquid Water Content on Isoprene Secondary Organic Aerosol Growth

Aerosol liquid water content (ALWC) has been shown to enhance partitioning of water-soluble oxygenated organic vapors into the aerosol particles and facilitate aqueous chemistry, impacting secondary organic aerosol (SOA) formation, composition, and physicochemical properties. This study examines the effect of ALWC on isoprene SOA formation using effloresced and deliquesced ammonium sulfate ((NH4)2SO4) and sodium chloride (NaCl) seed particles in an environmental chamber with photo-oxidation extending over hours under moderate relative humidity (RH) (50%–60%). Our findings indicate that isoprene SOA mass yield is not significantly influenced (p value ≫ 0.05) by the phase state or the chemical identity of the seed particles within a 3–4 h time scale. Detailed molecular analysis using advanced high-resolution mass spectrometry indicates that while both effloresced and deliquesced seed types produced similar molecular formula distributions, deliquesced (NH4)2SO4 slightly favored the production of less volatile compounds. Depth-profiling analysis by single particle mass spectrometry confirmed that isoprene SOA was predominantly located on the surfaces of particles with core–shell morphologies and showed partial dissolution of the effloresced inorganic salt core into the organic shell, even though the chamber RH was below the deliquescence relative humidity of the inorganic salt. This finding suggests that assuming dry salt core particles do not interact with SOA coatings, based on salt deliquescence points, may overlook important aqueous chemistry. These results emphasize the critical role of reaction time scale and mixing state on SOA formation and call for further research to understand the effects of LWC on SOA from different precursors.