Hygroscopicity measurement of sodium carbonate, β-alanine and internally mixed β-alanine/Na₂CO₃ particles by ATR-FTIR

Water-uptakes of pure sodium carbonate (Na₂CO₃), pure β-alanine and internally mixed β-alanine/Na₂CO₃ aerosol particles with different mole ratios are first monitored using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) technique. For pure Na₂CO₃ aerosol particles, combining the absorptions at 877 and 1422 cm⁻¹ with abrupt water loss shows the efflorescence relative humidity (ERH) of 62.9%–51.9%. Upon humidifying, solid Na₂CO₃ firstly absorbs water to from Na₂CO₃·H₂O crystal at 72.0% RH and then deliquesces at 84.5% RH (DRH). As for pure β-alanine particles, the crystallization takes place in the range of 42.4%–33.2% RH and becomes droplets at ~ 88.2% RH. When β-alanine is mixed with Na₂CO₃ at various mole ratios, it shows no efflorescence of Na₂CO₃ when β-alanine to Na₂CO₃ mole ratio (OIR) is 2:1. For 1:1 and 1:2 β-alanine/Na₂CO₃ aerosols, the ERHs of Na₂CO₃ are 51.8%–42.3% and 57.1%–42.3%, respectively. While β-alanine crystal appears from 62.7% RH for 2:1 and 59.4% RH for both 1:1 and 1:2 particles and lasts to driest state. On hydration, the DRH is 44.7%–75.2% for Na₂CO₃ with the OIR of 1:1 and 44.7%–69.0% for 1:2 mixture, and those of β-alanine are 74.8% for 2:1 mixture and 68.9% for two others. After the first dehumidification–humidification, all the water contents decrease despite of constituent fraction. And at ~ 92% RH, the remaining water contents are 92%, 89% and 82% at ~ 92% RH, corresponding to OIR of 2:1, 1:1 and 1:2 mixed system, respectively.