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. Highlights • The increase of ionic strength leads to a red shift of n→π* absorption band of AcS. • The pKa of AcS, hence acidity is affected with increasing ionic strength. • The decay rate of AcS increases by 6 times from I = 0 M to I = 0.5 M. • Photochemical evolution of AcS leads to acidification of the aerosol particles. [ABSTRACT FROM AUTHOR]