Osmotic coefficient and mean ionic activity coefficient of supersaturated ammonium nitrate solutions measured by laser trapping and Raman spectroscopy

We report thermodynamic measurements for supersaturated aqueous ammonium nitrate (NH₄NO₃) solutions in single micrometer-sized droplets levitated in air by laser trapping at 298 K. Water activity (aw) was obtained directly from the gas-phase relative humidity as the humidity was systematically reduced. Droplet composition was quantified by in situ Raman spectroscopy: spectral deconvolution was used to determine the NO₃⁻/H₂O peak-area ratio, which was converted to nitrate concentration using a calibration curve constructed from bulk solutions of known concentration. This approach enables access to ultra-high molalities up to m = 215 mol kg⁻¹, corresponding to a total ionic mole fraction xI = 0.89. The aw data agree with literature values over xI = 0.20-0.67, whereas systematic differences relative to electrodynamic-balance datasets appear at higher xI, where volatilization of NH₄NO₃ (via NH₃ and HNO₃ loss) and other non-ideal processes may become increasingly important. Using the measured composition - aw relationship, we derive the osmotic coefficient (ϕ) and the mean molal ionic activity coefficient (γ±) across the full concentration range, providing new benchmarks for modeling highly concentrated NH₄NO₃ relevant to aerosol thermodynamics.