Size-dependent acidity of aqueous nano-aerosols

Understanding the accurate acidity of nano-aerosols is important for the research on atmospheric chemistry. Herein, we propose the contributions from both the aerosol size and multiphase buffer effect to the steady-state acidity of nano-aerosols at a constant aerosol water content (AWC) through molecular simulations. As increasing of the aerosol size, the solvation free energy (SFE, ΔG_s) became more negative (decreasing by 3-130 kcal mol^-1 for different types of species) and Henry's law constant (H) apparently increased (from e⁶ to e¹⁶ mol m^-3 Pa^-1) in the nano-aerosols compared to that in bulk solutions. The lower SFE led to lower solute pK_a and pK_b values; thus, the acidity of HSO₄^- and HNO₃ and the alkalinity of NH₃ showed positive relations with the aerosol size. The lower H also increased the pK_a of gaseous solutes, leading to a decrease in the acidity of HNO₃ and a shift from alkaline to acidic for the NH₄⁺/NH₃ buffer pair in the nano-aerosols. The present study revealed the relationship between aerosol acidity and solvent size from a microscopic perspective. Specifically, the acidity of aerosols containing HSO₄^-/SO₄^2- and HNO₃/NO₃^- decreased with an increase in their radii, whereas aerosols containing NH₄⁺/NH₃ exhibited an opposite trend. This phenomenon can be attributed to the disappearance of the interfacial effect with an increase in the size of the aerosols. The above conclusions are of great significance for studying the pH-dependent multi-phase chemical processes in aerosols.