Ionic strength effects on the photochemical degradation of acetosyringone in atmospheric deliquescent aerosol particles

Abstract A number of laboratory experimental investigations, field campaigns, and modeling results have emphasized the role of aqueous-phase photochemical reactions in the formation of secondary organic aerosols (SOA). However, investigations focused on aqueous-phase reactions under high ionic-strength conditions are scarce. Here we study the photochemical behavior of a lignin-derived compound, acetosyringone (AcS), upon addition of an inert salt (NaClO 4 ). The increase in the ionic strength modifies the acidic constant of AcS, enhancing its deprotonation. As a consequence, the UV-VIS absorption spectra of AcS undergo modifications due to red shifts at high ionic strength of the electronic transitions n → π* (from λ max  = 297 nm to λ max  = 354 nm) and π → π* (from λ max  = 214 nm to λ max  = 247 nm). At fixed pH = 4, representative of moderately acidic atmospheric aerosol deliquescent particles, the pseudo-first-order rate constants (k 1st ) of AcS increased by ∼6 times from a dilute aqueous phase to a solution with an effective ionic strength I eff.  = 0.46 M. The rate constant then followed a saturation trend at elevated ionic strength up to I eff.  = 3.1 M. A similar saturation effect of the observed rate constants with ionic strength was observed in presence of NaCl and Na 2 SO 4 . Differential absorption spectroscopy (DAS) methodology was applied to examine the changes in absorption spectra of AcS upon prolonged light irradiation. The very subtle pH-induced changes of the absorption spectra of irradiated AcS are due to the formation of acidic compounds emerged upon photochemical transformation of AcS.