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

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 (NaClO4). 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 λmax=297 nm to λmax = 354 nm) and π → π* (from λmax=214 nm to λmax=247 nm). At fixed pH=4, representative of moderately acidic atmospheric aerosol deliquescent particles, the pseudo-first-order rate constants (k1st) of AcS increased by ∼6 times from a dilute aqueous phase to a solution with an effective ionic strength Ieff.=0.46 M. The rate constant then followed a saturation trend at elevated ionic strength up to Ieff.=3.1 M. A similar saturation effect of the observed rate constants with ionic strength was observed in presence of NaCl and Na2SO4. 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.