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Abstract.A new aerosol microphysical module MATRIX, the Multiconfiguration Aerosol TRacker of mIXing state, and its application in the Goddard Institute for Space Studies (GISS) climate model (ModelE) are described. This module, which is based on the quadrature method of moments (QMOM), represents nucleation, condensation, coagulation, internal and external mixing, and cloud-drop activation and provides aerosol particle mass and number concentration and particle size information for up to 16 mixed-mode aerosol populations. Internal and external mixing among aerosol components sulfate, nitrate, ammonium, carbonaceous aerosols, dust and sea-salt particles are represented. The solubility of each aerosol population, which is explicitly calculated based on its soluble and insoluble components, enables calculation of the dependence of cloud drop activation on the microphysical characterization of multiple soluble aerosol populations.A detailed model description and results of box-model simulations of various aerosol population configurations are presented. The box model experiments demonstrate the dependence of cloud activating aerosol number concentration on the aerosol population configuration; comparisons to sectional models are quite favorable. MATRIX is incorporated into the GISS climate model and simulations are carried out primarily to assess its performance/efficiency for global-scale atmospheric model application. Simulation results were compared with aircraft and station measurements of aerosol mass and number concentration and particle size to assess the ability of the new method to yield data suitable for such comparison. The model accurately captures the observed size distributions in the Aitken and accumulation Correspondence to: S. E. Bauer (sbauer@giss.nasa.gov) modes up to particle diameter 1 µm, in which sulfate, nitrate, black and organic carbon are predominantly located; however the model underestimates coarse-mode number concentration and size, especially in the marine environment. This is more likely due to oversimplifications of the representation of sea salt emissions -sea salt emissions are only calculated for two size classes -than to inherent limitations of MA-TRIX.

Benzene clusters are generated by pulsed supersonic beam expansion, ionized by electron impact, mass-selected and then injected into a drift cell for ion mobility measurements in a helium buffer gas. The measured collision cross sections and theoretical calculations are used to determine the structures of the cluster cations (C(6)H(6))(n)(+) with n = 2-6. Density functional theory calculation, at an all-electron level and without any symmetry constraint, predicts that the dimer cation has two nearly degenerate ground state structures with the sandwich configuration more stable than the T-configuration by only 0.07 eV. The ion mobility experiment indicates that only one structure is observed for the mass-selected dimer cation at room temperature. The calculated cross section for the sandwich structure agrees very well (within 2.4%) with the experimental value. For the n = 3-6 clusters, the experiments suggest the presence of at least two structural isomers for each cluster. A Monte Carlo minimum-energy search technique using the 12-site OPLS potential for benzene is used to determine the structures of the lowest-energy isomers. The calculated cross sections for the two lowest-energy isomers of the n = 3-6 clusters agree well with the experimental results. The clusters' structures reveal two different growth patterns involving a sandwich dimer core or a pancake trimer stack core. The lowest-energy isomers of the n = 3-6 clusters incorporate the pancake trimer stack as the cluster's core. The trimer stack allows the charge to hop between two dimers, thus maximizing charge resonance interaction in the clusters. For larger clusters, the appearance of magic numbers at n = 14, 20, 24, 27, and 30 is consistent with the incorporation of a sandwich dimer cation within icosahedral, double icosahedral, and interpenetrating icosahedral structures. On the basis of the ion mobility results and the structural calculations, the parallel-stacked motif among charged aromatic-aromatic interactions is expected to play a major role in determining the structures of multi aromatic components. This conclusion may provide new insights for experimental and theoretical studies of molecular design and recognition involving aromatic systems.

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