Study of Hygroscopic Properties of Aminium Sulfate Particles Using Micro-Raman Spectroscopy

Abstract:

As one of the organic nitrogen species, amines are alkaline compounds ubiquitously detected in the atmosphere. Amines have been found to have the potential to enhance atmospheric new particle formation via forming aminium sulfate salts as much as 1000-fold when compared with ammonia, albeit at relatively low atmospheric contents (Almeida et al., 2013). Hygroscopicity of aminium sulfate particles can affect their cloud condensation nuclei (CCN) activity, light scattering or absorption and reactivity in the atmosphere. However, aerosol thermodynamic models such as Extended Aerosol Thermodynamics Model (E-AIM) assume that aminium ions behave similarly to ammonium ions, thus resulting in inaccuracy in predictinghygroscopicity of such compounds. In this study, Micro-Raman spectroscopy was applied to investigate the hygroscopicity of mono-methylaminium sulfate (MMAS), di-methylaminium sulfate (DMAS), tri-methylaminium sulfate (TMAS) and mono-ethylaminium sulfate (MEAS) particles. Since amine can evaporate from stock solutions, Ion Chromatography was used to determine aminium-to-sulfate ion molar ratio (A/S) of studied particles. Results were compared with literature data (Clegg et al., 2013). Due to amine evaporation upon mixing, A/S of stock solutions for particle generation can be below 2.0. In fact, initial A/S were 1.59, 1.91, 1.33 and 2.00 of MMAS, DMAS, TMAS and MEAS particles respectively. Ultimately A/S decreased to 1.5 and 1.0 for DMAS and TMAS, suggesting di-methylamine (DMA) and tri-methylamine (TMA) evaporated from corresponding particles. A/S of MMAS and MEAS kept relatively constant during the relative humidity (RH) cycle in hygroscopic measurements. Unlike (NH₄)₂SO₄, all studied aminium sulfate particles can take up water at low RH. MMAS and MEAS particles underwent phase transition at around 45% and 20% RH respectively, while particles of other aminium sulfates showed continuous hygroscopic trends.

References:

Almeida, J. et al., 2013. Nature 502, 359-363.

Clegg, S. L. et al., 2013. Atmospheric Environment 73, 145-158.

Acknowledgement:

The financial support of the Research Grants Council (GRF 600112) and the Hong Kong RGC PhD Fellowship Scheme (HKPFS) is gratefully acknowledged.