pH effect on the release of NH₃ from the internally mixed sodium succinate and ammonium sulfate aerosols

The pH value is an important parameter of atmospheric aerosol. It affects the concentration of conjugate acid-base pair through the acid-base equilibrium and thus determines the gas–particle partitioning of acids or bases with volatility. Our recent report shows that there is a substitution of weak base for strong base in the aerosols of internally mixed water-soluble organic acid salt/ammonium sulfate. However, the acidity effect on the substitution process still remains ambiguous. In this work, the aerosols generated from sodium succinate/ammonium sulfate solutions with different pHs were studied in detail by using ATR-FTIR technique. The effects of relative humidity (RH) and acidity (pH) on the composition evolution, hygroscopic property and phase change were monitored. At a constant RH for a given pH, there were continuous depletions of NH₄⁺, COO⁻ and water content accompanying occurrence of (CH₂COOH)₂ at initial stage, and then followed by Na₂SO₄ efflorescence, and at last participation of (CH₂COOH)₂. Lower RH was conductive to faster chemical composition evolution and resultant Na₂SO₄ crystallization. Higher pH promoted the composition evolution process and solid phase formation process. The consumptions of COO⁻ and NH₄⁺ increased with increasing pH, showing that the dissolution of NH₄⁺ to release H⁺ in aerosols and NH₃ to gas phase led to water loss, in turn, Na₂SO₄ and succinic acid efflorescence. Water loss was more sensitive to Na₂SO₄ efflorescence than succinic acid. When a RH cycle was experienced, Efflorescence RHs of sodium succinate/ammonium sulfate aerosols almost kept unchanged and deliquscence RHs increased obviously with pH, while no deliquescence for pH 7.62. To our knowledge, it is the first time to investigate the pH effect on chemical process about composition evolution.