In Situ Observation on the Dynamic Process of Evaporation and Crystallization of Sodium Nitrate Droplets on a ZnSe Substrate by FTIR-ATR

Qing-Nuan Zhang, Yun Zhang, Chen Cai, Yu-Cong Guo, Jonathan P. Reid, Yun-Hong Zhang*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

35 Citations (Scopus)

Abstract

Sodium nitrate is a main component of aging sea salt aerosol, and its phase behavior has been studied repeatedly with wide ranges observed in the efflorescence relative humidity (RH) in particular. Studies of the efflorescence dynamics of NaNO3 droplets deposited on a ZnSe substrate are reported, using an in situ Fourier transform infrared attenuated total reflection (FTIR-ATR) technique. The time-dependence of the infrared spectra of NaNO3 aerosols accompanying step changes in RH have been measured with high signal-to-noise ratio. From the IR difference spectra recorded, changes of the time-dependent absorption peak area of the O-H stretching band (v-OH, similar to 3400 cm(-1)) and the nitrate out-of-plane bending band (v(2)-NO3-, similar to 836 cm(-1)) are obtained. From these measurements, changes in the IR signatures can be attributed to crystalline and solution phase nitrate ions, allowing the volume fraction of the solution droplets that have crystallized to be determined. Then, using these clear signatures of the volume fraction of droplets that have yet to crystallize, the homogeneous and heterogeneous nucleation kinetics can be studied from conventional measurements using a steady decline in RI-I. The nucleation rate measurements confirm that the rate of crystallization in sodium nitrate droplets is considerably less than in ammonium sulfate droplets at any particular degree of solute supersaturation, explaining the wide range of efflorescence RI-Is observed for sodium nitrate in previous studies. We demonstrate that studying nucleation kinetics using the FTIR-ATR approach has many advantages over brightfield imaging studies on smaller numbers of larger droplets or measurements made on single levitated particles.

Original languageEnglish
Pages (from-to)2728-2737
Number of pages10
JournalJournal of Physical Chemistry A
Volume118
Issue number15
DOIs
Publication statusPublished - 17 Apr 2014

Keywords

  • AMMONIUM-SULFATE PARTICLES
  • LIQUID PHASE-SEPARATION
  • SEA-SALT PARTICLES
  • AEROSOL-PARTICLES
  • HYGROSCOPIC PROPERTIES
  • CONFOCAL RAMAN
  • SUPERSATURATED STRUCTURES
  • HETEROGENEOUS CHEMISTRY
  • CHLORIDE DROPLETS
  • QUARTZ SUBSTRATE

Cite this