Molecular cluster ions H(+)(H(2)O)(n), H(+)(pyridine)(H(2)O)(n), H(+)(pyridine)(2)(H(2)O)(n), and H(+)(NH(3))(pyridine)(H(2)O)(n) (n = 16-27) and their reactions with ammonia have been studied experimentally using a quadrupole-time-of-flight mass spectrometer. Abundance spectra, evaporation spectra, and reaction branching ratios display magic numbers for H(+)(NH(3))(pyridine)(H(2)O)(n) and H(+)(NH(3))(pyridine)(2)(H(2)O)(n) at n = 18, 20, and 27. The reactions between H(+)(pyridine)(m)(H(2)O)(n) and ammonia all seem to involve intracluster proton transfer to ammonia, thus giving clusters of high stability as evident from the loss of several water molecules from the reacting cluster. The pattern of the observed magic numbers suggest that H(+)(NH(3))(pyridine)(H(2)O)(n) have structures consisting of a NH(4)(+)(H(2)O)(n) core with the pyridine molecule hydrogen-bonded to the surface of the core. This is consistent with the results of high-level ab initio calculations of small protonated pyridine/ammonia/water clusters.

We present the first observation of airborne organic and inorganic arsenic cations, detected in real time within the boreal forest in Hyytiälä, Finland, and over nearby Lake Kuivajärvi. The technique of atmospheric-pressure interface time-of-flight mass spectrometry provides online, in situ monitoring as well as chemical information about the arsenic species, identified as protonated trimethylarsine oxide (AsC3H10O+) and AsO­(H2O) n + clusters (n = 0–4). Quantum chemical calculations confirm that the proposed cations are stable under atmospheric conditions. Our most remarkable discovery is that minimal arsenic appeared during spring 2011 until after the ground began to thaw, triggering a sharp increase in airborne arsenic levels as snowmelt flooded the soil with water and stimulated microbial activity. These findings reveal that volatile arsenic species, detected here as atmospheric ions, link the biogeochemical cycling of arsenic through air, soil, water, and living organisms.

Abstract. The optimized structures and proton affinities of a total of 81 nitrogen-containing bases, chosen based on field measurements of ambient positive ions, were studied using the CBS-QB3 quantum chemical method. The results were compared to values given in the National Institute of Standards and Technology (NIST) Chemistry WebBook in cases where a value was listed. The computed values show good agreement with the values listed in NIST. Grouping the molecules based on their molecular formula, the largest calculated proton affinities for each group were also compared with experimentally observed ambient cation concentrations in a boreal forest. This comparison allows us to draw qualitative conclusions about the relative ambient concentrations of different nitrogen-containing organic base molecules.