Displacement of Strong Acids or Bases by Weak Acids or Bases in Aerosols: Thermodynamics and Kinetics

Depletion of chloride, nitrate, or ammonium from atmospheric aerosols represents a unique class of displacement reactions in which strong acids (HCl and HNO3) or bases (NH3) are substituted by weaker ones (i.e., dicarboxylic acids or dicarboxylate salts, respectively). These reactions alter the aerosol composition and perturb the atmospheric cycle of volatile compounds, thereby affecting environmental systems and climate. Despite the profound implications, the mechanism of these unique displacement reactions remains unclear. Here, we elucidate the thermodynamics and kinetics of these reactions using the multiphase buffer theory and a diffusion-controlled mass-transfer function, respectively. On the thermodynamic aspect, we find that the effective dissociation constants of the strong acids and bases in aerosols are 2 to 10 orders of magnitude lower than those in bulk solutions. On the kinetic aspect, we find that displacement reactions occur rapidly in aerosol microdroplets with a radius below 10 μm. Within this size range, the characteristic reaction time is always shorter than the lifetime of the aerosols in the atmosphere. Our findings suggest that the unique displacement reactions can significantly modify the composition of atmospheric aerosols, and consequentially, these aerosols may manifest distinct properties unforeseen by the chemistry of homogeneous bulk systems.