Fragmentation energetics of clusters relevant to atmospheric new particle formation.

The exact mechanisms by which small clusters form and grow in the atmosphere are poorly understood, but this process may significantly impact cloud condensation nuclei number concentrations and global climate. Sulfuric acid is the key chemical component to new particle formation (NPF), but basic species such as ammonia are also important. Few laboratory experiments address the kinetics or thermodynamics of acid and base incorporation into small clusters. This work utilizes a Fourier transform ion cyclotron resonance mass spectrometer equipped with surface-induced dissociation to investigate time- and collision-energy-resolved fragmentation of positively charged ammonium bisulfate clusters. Critical energies for dissociation are obtained from Rice-Ramsperger-Kassel-Marcus/quasi-equilibrium theory modeling of the experimental data and are compared to quantum chemical calculations of the thermodynamics of cluster dissociation. Fragmentation of ammonium bisulfate clusters occurs by two pathways: (1) a two-step pathway whereby the cluster sequentially loses ammonia followed by sulfuric acid and (2) a one-step pathway whereby the cluster loses an ammonium bisulfate molecule. Experimental critical energies for loss of an ammonia molecule and loss of an ammonium bisulfate molecule are higher than the thermodynamic values. If cluster growth is considered the reverse of cluster fragmentation, these results require the presence of an activation barrier to describe the incorporation of ammonia into small acidic clusters and suggest that kinetically (i.e., diffusion) limited growth should not be assumed. An important corollary is that models of atmospheric NPF should be revised to consider activation barriers to individual chemical steps along the growth pathway.