Ling Tsz Yan, Chan Chak K
Department of Chemical Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong.
Environ Sci Technol. 2007 Dec 1;41(23):8077-83. doi: 10.1021/es071419t.
Ammonium nitrate (AN) and ammonium sulfate (AS) are ubiquitous components of atmospheric aerosols. Thermodynamic models predict formation of pure (AN and AS) and double salts (3AN. AS and 2AN. AS) for the AN/AS system. Because of the high supersaturation at which a droplet crystallizes, metastable crystal formation is possible. In this study, the identity of the crystals formed from the crystallization of equimolar AN/AS mixed droplets was investigated in an electrodynamic balance coupled with a Raman spectroscopic system. Raman spectra of bulk AN/AS double salts possibly formed in this system are first reported for comparison with the single particle Raman results. The double-salt 3AN. AS, not predicted from thermodynamics, was observed in the freshly crystallized single particles. The degree of metastability can be different among several crystallization processes of the same particles. The metastable salt 3AN. AS gradually transformed into stable 2AN. AS, and the rate of such transformation increased with increasing relative humidity. This study illustrates the possibility of occurrence of metastable salts in atmospheric aerosols.
硝酸铵（AN）和硫酸铵（AS）是大气气溶胶中普遍存在的成分。热力学模型预测了AN/AS体系中纯盐（AN和AS）和复盐（3AN·AS和2AN·AS）的形成。由于液滴结晶时的高过饱和度，亚稳晶体的形成是可能的。在本研究中，利用电动平衡结合拉曼光谱系统，研究了等摩尔AN/AS混合液滴结晶形成的晶体的特性。首次报道了该体系中可能形成的块状AN/AS复盐的拉曼光谱，以便与单颗粒拉曼结果进行比较。在新结晶的单颗粒中观察到了热力学未预测到的复盐3AN·AS。同一颗粒的几个结晶过程中亚稳程度可能不同。亚稳盐3AN·AS逐渐转变为稳定的2AN·AS，且这种转变速率随相对湿度的增加而增大。本研究说明了大气气溶胶中出现亚稳盐的可能性。
