Large nucleating ions (H2SO4)n(HSO4-)(NH3)m : Formation mechanism and appearance in the earth’s atmosphere

Ammonium bisulfate (HSO4-)(NH3) ion is extremely unstable thermodynamically. However, larger stable (H2SO4)n(HSO4)(NH3)m clusters have been observed in laboratory experiments under atmospherically relevant conditions. In the present paper, we investigate stabilization of (H2SO4)n(HSO4-)(NH3)m clusters by NH3 using the Density Functional Theory (DFT), discuss structural trends, elucidate the stabilization mechanism, report new thermochemical data and explore their implications. In particular, we show that stable (H2SO4)n(HSO4-)(NH3)m clusters in the atmosphere are formed via addition of NH3 to (H2SO4)n(HSO4-) at n >2 and that the most abundant (H2SO4)n(HSO4-)(NH3)m ions contain one and two NH3 molecules, respectively.