Nucleation Kinetics in Mixed NaNO3/Glycerol Droplets Investigated with the FTIR-ATR Technique

Area of Science:

• Atmospheric chemistry
• Physical chemistry
• Spectroscopy

Background:

• Sodium nitrate (NaNO3) is a key component in atmospheric aerosols.
• Understanding its phase transitions and crystallization is crucial for atmospheric modeling.
• Glycerol is a common organic component in atmospheric aerosols, influencing inorganic salt behavior.

Purpose of the Study:

• To investigate the efflorescence behavior and nucleation mechanisms of sodium nitrate (NaNO3) in mixed NaNO3/glycerol droplets.
• To determine the effect of varying organic-to-inorganic molar ratios (OIR) on NaNO3 crystallization.
• To elucidate the role of glycerol in modifying the efflorescence relative humidity (ERH) and nucleation kinetics of NaNO3.

Main Methods:

• In situ infrared spectra collection using Fourier transform infrared attenuated total reflection (FTIR-ATR) on a ZnSe substrate.
• Controlled decrease in relative humidity (RH) to observe efflorescence processes.
• Analysis of spectral bands (829 cm⁻¹ for liquid, 836 cm⁻¹ for crystal) to track NaNO3 phase transformation.
• Calculation of nucleation ratios and rates based on spectral absorbance.

Main Results:

• Efflorescence of NaNO3 droplets was observed with characteristic spectral shifts from liquid to crystal phases.
• Glycerol significantly lowered the ERH of NaNO3, from 62.5% for pure NaNO3 to 15% for OIR = 2:1.
• Heterogeneous nucleation rate of NaNO3 was suppressed with increasing OIR.
• Mixed droplets (OIR ≤ 1:1) showed dual nucleation mechanisms: homogeneous at high supersaturation and heterogeneous at low supersaturation.
• Droplets with OIR = 2:1 exclusively exhibited homogeneous nucleation.

Conclusions:

• Glycerol acts as a significant efflorescence retardant for NaNO3 but also inhibits heterogeneous nucleation.
• The presence of glycerol introduces complex nucleation pathways in mixed NaNO3/glycerol aerosols.
• NaNO3 crystallization in mixed droplets is dependent on both RH and the OIR, influencing atmospheric aerosol behavior.