Analytical Sciences

In situ Quantification of Ammonium Sulfate in Single Aerosol Droplets by Means of Laser Trapping and Raman Spectroscopy

Shoji ISHIZAKA,*,** Kunihiro YAMAUCHI,*** and Noboru KITAMURA***

*Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-hiroshima 739-8526, Japan
**PRESTO-JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
***Department of Chemistry, Faculty of Science and Department of Chemical Sciences and Engineering, Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan

Noncontact levitation in air of single micrometer-sized water droplets containing ammonium sulfate was successful by a laser trapping technique. The trapping laser beam was also used simultaneously as an excitation light source for the Raman spectroscopy of trapped droplets. Raman scattering of the symmetric stretching vibration of the SO₄²⁻ anion and the OH stretching vibrations of H₂O were observed at 980 and 3420 cm⁻¹, respectively. The intensity ratio of these two Raman peaks was linearly proportional to the ammonium sulfate concentration in water. Therefore, the in situ quantification of ammonium sulfate in single aerosol droplets was achieved by means of laser trapping and Raman spectroscopy. To the best of our knowledge, this study is the first experimental observation of the independence of ammonium sulfate concentrations of aerosol water droplets to those of the mother solutions under constant relative humidity conditions.

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