BIC-ESAT and SKL-ESPC, College of Environmental Sciences and Engineering, Peking University, Beijing, 100871, China.
State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Protection and Resources Utilization, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, 510640, China.
Anal Chim Acta. 2021 Nov 22;1186:339089. doi: 10.1016/j.aca.2021.339089. Epub 2021 Sep 23.
Atmospheric aerosol acidity impacts numerous physicochemical processes, but the determination of particle pH remains a significant challenge due to the nonconservative nature of the H concentration ([H]). Traditional measurements have difficulty in describing the practical state of an aerosol because they comprise chemical components or hypotheses that change the nature of the particles. In this work, we present a direct pH measurement that uses water as a general probe to detect [H] in individual particles by micro-Raman spectroscopy. Containing the vibrational bands of ions and water influenced by ions, the spectra of hydrated ion were decomposed from the solution spectra as standard spectra by multivariate curve resolution analysis. Meanwhile, ratios of hydrated ions were calculated between the Raman spectra and standard spectra to evaluate concentration profiles of each ion. It demonstrated that good quantitative models between the ratio and concentration for all ions including H can be built with correlation coefficients (R) higher than 0.95 for the solutions. The method was further applied to individual particle pH measurement. The pH value of sulfate aerosol particles was calculated, and the standard error was 0.09 using pH values calculated from the [HSO]/[SO] as a reference. Furthermore, the applicability of the method was proven by detecting the pH value of chloride particles. Therefore, utilizing water, the most common substance, as the spectroscopic probe to measure [H] without restriction of the ion system, this method has potential to measure the pH value of atmospheric particles with various compounds, although more work needs to be done to improve the sensitivity of the method.
大气气溶胶的酸度会影响许多物理化学过程，但由于 H 浓度 ([H]) 的非保守性质，颗粒 pH 值的测定仍然是一个重大挑战。传统的测量方法在描述气溶胶的实际状态方面存在困难，因为它们包含改变颗粒性质的化学组分或假设。在这项工作中，我们提出了一种直接的 pH 值测量方法，该方法使用水作为一般探针，通过微拉曼光谱来检测单个颗粒中的 [H]。由于含有受离子影响的离子和水的振动带，因此通过多元曲线分辨分析，将溶液光谱中的水合离子光谱分解为标准光谱。同时，通过将拉曼光谱与标准光谱之间的水合离子比计算出来，评估每个离子的浓度分布。结果表明，对于所有离子（包括 H），可以建立具有高于 0.95 的相关系数 (R) 的定量模型，以建立比率与浓度之间的良好定量模型。该方法进一步应用于单个颗粒 pH 值测量。硫酸盐气溶胶颗粒的 pH 值被计算出来，并且使用 [HSO]/[SO] 计算的 pH 值作为参考，标准误差为 0.09。此外，通过检测氯离子颗粒的 pH 值证明了该方法的适用性。因此，利用最常见的物质水作为光谱探针来测量 [H]，而不受离子体系的限制，该方法有可能测量具有各种化合物的大气颗粒的 pH 值，尽管需要做更多的工作来提高方法的灵敏度。
