Hydration of the Sulfuric Acid-Methylamine Complex and Implications for Aerosol Formation

The binary H2SO4-H2O nucleation is one of the most important pathways by which aerosols form in the atmosphere, and the presence of ternary species like amines increases aerosol formation rates. In this study, we focus on the hydration of a ternary system of sulfuric acid (H2SO4), methylamine (NH2CH3), and up to six waters to evaluate its implications for aerosol formation. By combining molecular dynamics (MD) sampling with high-level ab initio calculations, we determine the thermodynamics of forming H2SO4(NH2CH3)(H2O), where n = 0-6. Because it is a strong acid-base system, H2SO4-NH2CH3 quickly forms a tightly bound HSO4--NH3CH3+ complex that condenses water more readily than H2SO4 alone. The electronic binding energy of H2SO4-NH2CH3 is -21.8 kcal mol(-1) compared with -16.8 kcal mol(-1) for H2SO4-NH3 and -12.8 kcal mol(-1) for H2SO4-H2O. Adding one to two water molecules to the H2SO4-NH2CH3 complex is more favorable than adding to H2SO4 alone, yet there is no systematic difference for n >= 3. However, the average number of water molecules around H2SO4-NH2CH3 is consistently higher than that of H2SO4, and it is fairly independent of temperature and relative humidity.