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

摘要

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 RI-Is 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.