| Abstract
| - High signal-to-noise ratio (S/N) Raman spectra of (NH4)2SO4 droplets deposited on a quartz substrate wereobtained from dilute to supersaturated states upon decreasing the relative humidity (RH). When the molarwater-to-solute ratio (WSR) decreases from 16.8 to 3.2, the v1-SO42- band changes very little, that is, showinga red-shift of only about 1 cm-1 (from 979.9 to 978.8 cm-1) and an increase of its full width at half-maximum(fwhm) from 8.3 to 9.8 cm-1. Other vibration modes such as v2- and v4-SO42- bands appear almost constantlyat 452 and 615 cm-1. Such kind of a spectroscopic characteristic is different from previous observation onother cations, indicating that the interactions between SO42- and NH4+ in supersaturated states are similar tothose between SO42- and H2O in dilute states. After fitting the Raman spectra with Gaussian functions in thespectral range of 2400−4000 cm-1, we successfully extracted six components at positions of 2878.7, 3032.1,3115.0, 3248.9, 3468.4, and 3628.8 cm-1, respectively. The first three components are assigned to the secondovertone of NH4+ umbrella bending, the combination band of NH4+ umbrella bending and rocking vibrations,and the NH4+ symmetric stretching vibration, while the latter three components are from the strongly, weakly,and slightly hydrogen-bonded components of water molecules, respectively. With a decrease of the RH, theproportion of the strongly hydrogen-bonded components increases, while that of the weakly hydrogen-bondedcomponents decreases in the droplets. The coexistence of strongly, weakly, and slightly hydrogen-bondedwater molecules must hint at a similar hydrogen-bonding network of NH4+, SO42-, and H2O to that of pureliquid water in supersaturated (NH4)2SO4 droplets.
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