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The Tandem Differential Mobility Analyzer (TDMA) technique coupled with aerosol humidification has been widely used for studying aerosol hygroscopicity. In this study, we evaluate the performance of a commercial Humidified TDMA (BMI HTDMA, Model 3002) with respect to DMA sizing, relative humidity (RH) control, and growth factor (GF) measurements. Unique features of this particular HTDMA include a diffusionbased particle humidifier, a DMA design allowing selection of particles up to 2 mm diameter at only 5600 volts, and the ability to study the complete deliquescence and efflorescence cycle. The sizing agreement between DMA 1 and 2 was within 2% over the 35 to 500 nm diameter range. The measured TDMA responses agreed well with theoretical calculations. The RH control and stability were tested at a suburban field site in Hong Kong. The system achieved RH equilibrium in less than 4 min when changing the RH set point. With indoor temperature changes of less than 1 C per hour, the RH control of the system was very stable at 90%, within 1% RH deviation, as confirmed by GF measurements on ammonium sulfate (AS) aerosols performed on separate days. The hygroscopic properties of various pure aerosols were examined and the results agreed well with model predictions. The application of the BMI HTDMA for field measurements was also demonstrated. Two modes were resolved from the GF distributions at 90% RH and variable hygroscopic growth with changing RH was observed.

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Mimicking trimeric interactions in the aromatic side chains of the proteins: A gas phase study of indole…(pyrrole)2 heterotrimer JCP: BioChem. Phys. 6, 05B601 (2012) Mimicking trimeric interactions in the aromatic side chains of the proteins: A gas phase study of indole…(pyrrole)2 heterotrimer J. Chem. Phys. 136, 174302 (2012) Ligand migration in myoglobin: A combined study of computer simulation and x-ray crystallography JCP: BioChem. Phys. 6, 04B620 (2012) Ligand migration in myoglobin: A combined study of computer simulation and x-ray crystallography J. Chem. Phys. 136, 165101 (2012) Deoxycholate induced tetramer of αA-crystallin and sites of phosphorylation: Fluorescence correlation spectroscopy and femtosecond solvation dynamics JCP: BioChem. Phys. 6, 04B614 (2012) Additional information on J. Appl. Phys. Whispering gallery mode (WGM) optical biosensors are capable of extraordinarily sensitive specific and nonspecific detection of species suspended in a gas or fluid. Recent experimental results suggest that these devices may attain single-molecule sensitivity to protein solutions in the form of stepwise shifts in their resonance wavelength, k R , but present sensor models predict much smaller steps than were reported. This study examines the physical interaction between a WGM sensor and a molecule adsorbed to its surface, exploring assumptions made in previous efforts to model WGM sensor behavior, and describing computational schemes that model the experiments for which single protein sensitivity was reported. The resulting model is used to simulate sensor performance, within constraints imposed by the limited material property data. On this basis, we conclude that nonlinear optical effects would be needed to attain the reported sensitivity, and that, in the experiments for which extreme sensitivity was reported, a bound protein experiences optical energy fluxes too high for such effects to be ignored. V C 2012 American Institute of Physics.