Hygroscopic behavior and chemical reactivity of aerosols generated from mixture solutions of low molecular weight dicarboxylic acids and NaCl

Abstract

Ambient sea spray aerosols (SSAs) have been reported to undergo reactions with low molecular weight dicarboxylic acids (LMW DCAs). In the present study, the hygroscopic behavior of aerosols generated from NaCl–LMW DCA mixture solutions with different mixing ratios was explained. In situ Raman microspectrometry (RMS) was used to simultaneously monitor the alterations in chemical composition, size, and phase as a function of the relative humidity (RH) for individual aerosols. The observation of individual mixture aerosols revealed chemical reactions on the timescale of one hour in the aqueous phase, mostly during the dehydration process, leading to the formation of sodium salts of DCAs with distinct reactivities among different DCAs and mixing ratios, which in turn exhibited diverse hygroscopic behaviors. The NaCl–DCA mixture aerosols were either in a ternary NaCl–DCA–DCA sodium salt system or a binary NaCl–DCA sodium salt or DCA–DCA sodium salt system, instead of a binary NaCl–DCA system when experiencing the hygroscopic process. The chemical compositional evolution of the NaCl–DCA aerosols during the hygroscopic measurements was examined based on the Raman spectra acquired for aqueous, amorphous, and/or crystalline pure standard aerosols at specific RHs. The different reactivity observed among the DCAs with different mixing ratios suggests that the reactivity driven by the irreversible liberation of HCl is governed mainly by the available aqueous H⁺ because Cl⁻ is always available in the aqueous NaCl–DCA aerosols until the complete consumption of NaCl.