Raman observation of the interactions between NH₄⁺, so₄^2-, and H₂O in supersaturated (NH ₄)₂so₄ droplets

High signal-to-noise ratio (S/N) Raman spectra of (NH₄) ₂SO₄ droplets deposited on a quartz substrate were obtained from dilute to supersaturated states upon decreasing the relative humidity (RH). When the molar water-to-solute ratio (WSR) decreases from 16.8 to 3.2, the ν₁-SO₄^2- band changes very little, that is, showing a 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 ν₂- and ν₄-SO₄^2- bands appear almost constantly at 452 and 615 cm^-1. Such kind of a spectroscopic characteristic is different from previous observation on other cations, indicating that the interactions between SO₄^2- and NH₄⁺ in supersaturated states are similar to those between SO₄ ^2- and H₂O in dilute states. After fitting the Raman spectra with Gaussian functions in the spectral 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 second overtone of NH ₄⁺ umbrella bending, the combination band of NH ₄⁺ 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, the proportion of the strongly hydrogen-bonded components increases, while that of the weakly hydrogen-bonded components decreases in the droplets. The coexistence of strongly, weakly, and slightly hydrogen-bonded water molecules must hint at a similar hydrogen-bonding network of NH₄⁺, SO ₄^2-, and H₂O to that of pure liquid water in supersaturated (NH₄)₂SO₄ droplets.