Chemical reaction between sodium pyruvate and ammonium sulfate in aerosol particles and resultant sodium sulfate efflorescence

The hygroscopicity of aerosols is dependent upon their chemical composition. When their chemical compositions are altered, the water content in aerosols often changes, which may further modify phase behaviour. However, the study of phase behaviour dependence on chemical reactions is still limited. In this work, internally mixed sodium pyruvate (SP)/ammonium sulfate (AS) droplets were studied using an in-situ ATR-FTIR spectrometer. FTIR spectral analysis showed that solid sodium sulfate (SS) formed during the dehydration process, indicating a chemical reaction between SP and AS. In addition, the water content decreased after a dehydration-hydration process despite organic salt (SS) to inorganic salt (AS) mole ratios (OIRs) During the second relative humidity (RH) cycle, the water content remained constant, however, the efflorescence relative humidity (ERH) was lower than that in the first dehydration. The crystal relative humidities (CRHs) of SS are 66.7–53.1%, 66.0–58.2%, 62.2–57.1% and 49.6–43.6% for OIRs of 3:1, 2:1, 1:1 and 1:3, respectively, suggesting the crystallization of SS was favoured by higher SP content. For 2:1 OIRs, the solid SS was the greatest and an excess of either SP or AS blocked the solid SS formation. At a constant 80% RH, depletion of reagents was ∼0.97, and water loss was ∼0.6 in ∼40 min. After 90 min, solid SS formed. The chemical reaction was faster than water loss; furthermore, water loss from the chemical reaction led to solid SS above the ERH of pure SS particles (∼75% RH). When the RH changed rapidly, the reaction was slow and solid SS decreased.