Hygroscopic properties of NaCl and NaNO_3 mixture particles as reacted inorganic sea-salt aerosol surrogates

摘要

NaCl in fresh sea-salt aerosol (SSA) particles can partially or fully react with atmospheric NO_x/HNO_3, so internally mixed NaCl and NaNO_3 aerosol particles can co-exist over a wide range of mixing ratios. Laboratorygenerated, micrometer-sized NaCl and NaNO_3 mixture particles at 10 mixing ratios (mole fractions of NaCl (X_(NaCl)) = 0:1 to 0.9) were examined systematically to observe their hygroscopic behavior, derive experimental phase diagrams for deliquescence and efflorescence, and understand the efflorescence mechanism. During the humidifying process, aerosol particles with the eutonic composition (X_(NaCl)) = 0:38) showed only one phase transition at their mutual deliquescence relative humidity (MDRH) of 67.9 (±0.5) %. On the other hand, particles with other mixing ratios showed two distinct deliquescence transitions; i.e., the eutonic component dissolved at MDRH, and the remainder in the solid phase dissolved completely at their DRHs depending on the mixing ratios, resulting in a phase diagram composed of four different phases, as predicted thermodynamically. During the dehydration process, NaCl-rich particles (X_(NaCl) > 0.38) showed a two stage efflorescence transition: the first stage was purely driven by the homogeneous nucleation of NaCl and the second stage at the mutual efflorescence RH (MERH) of the eutonic components, with values in the range of 30.0-35.5 %. Interestingly, aerosol particles with the eutonic composition (X_(NaCl) = 0:38) also showed two-stage efflorescence, with NaCl crystallizing first followed by heterogeneous nucleation of the remaining NaNO_3 on the NaCl seeds. NaNO_3-rich particles (X_(NaCl) ≤ 0.3) underwent singlestage efflorescence transitions at ERHs progressively lower than the MERH because of the homogeneous nucleation of NaCl and the almost simultaneous heterogeneous nucleation of NaNO_3 on the NaCl seeds. SEM/EDX elemental mapping indicated that the effloresced NaCl-NaNO_3 particles at all mixing ratios were composed o