三维荧光光谱(3DEEM)是表征大气颗粒物中水溶性有机物(WSOC)化学组成与官能团的有效工具, 目前所报道的应用3DEEM分析WSOC仅局限于荧光强度及其指标研究。 由于大气颗粒物pH值变化范围大(0~9), WSOC形成、 传输和转化过程易受pH值的影响。 因此, 研究pH值对WSOC荧光特性的影响, 对于加深了解WSOC的环境效应以及化学结构组成具有重要意义, 然而关于pH值对大气颗粒物中WSOC荧光特性影响的研究还未见报道。 在此基础上, 采用三维荧光光谱研究北京市大气细粒子中WSOC荧光特征, 并着重考察WSOC在不同pH值(2~8)条件下荧光光谱特征的变化情况。 研究表明, 大气细粒子中WSOC荧光主要成分包括类腐殖质、 类色氨酸和新生成有机物质。 进一步研究发现, WSOC荧光峰位置、 荧光强度和荧光指数对pH取值敏感。 WSOC荧光强度在pH值为3左右时达到最大值, 推测是羧基质子化达到最强的结果, 然而, 当pH值大于3时, 荧光强度变化则呈现相反趋势, 随pH值增大明显下降, 可能是由于分子间与分子内的氢键作用增强造成的。 另外, 夏季WSOC类色氨酸荧光峰随pH值升高略有红移现象, 表明分子结构中苯环增多, 共轭度增加。 相对于其他荧光物质, 类腐殖质受溶液pH值影响更显著。 通过对pH值与荧光指标进行相关性分析发现, WSOC荧光基团结构具有季节性差异。 pH值与自生源指数(BIX)呈显著负相关性, 随pH值的增大明显下降。 而pH值与Peak T/C(荧光峰T与荧光峰C的荧光强度比值)呈显著正相关性(p<0.01), 随pH值的增大明显升高, 说明低pH值不利于有机物的生物降解。 腐殖化指数(HIX)和荧光指数(FI)随pH值增大先升高后降低, 其值变化范围分别为1.6~3.64和0.8~1.94。 通过对三维荧光光谱分析发现pH值是影响WSOC荧光特性的一个重要因素, 在表征WSOC光学特性或其他性质时, 还应考虑pH值对WSOC的影响。

Three dimensional excitation-emission matrix spectra (3DEEM) is an effective tool for characterizing the chemical composition and functional groups of water-soluble organic compounds (WSOC) in atmospheric particulate, at present, the report of WSOC was analyzed by 3DEEM just in fluorescence intensity and its indices. Due to the wide range of pH value of atmospheric particulate (0~9), the formation, transmission and transformation of WSOC in atmospheric particulate are easily influenced by pH. Therefore, studying the effect of pH on the fluorescence characteristics of WSOC is important for further understanding the environmental effects and chemical composition of WSOC, however, the effect of pH on the fluorescence characteristics of WSOC has not been reported. On this basis, 3DEEM was used to investigate the fluorescence characteristics of WSOC in PM2.5 in Beijing, and focused on the changes of fluorescence characteristics of WSOC under different pH (2~8). Humic-like, tryptophan-like and freshly produced organic matter were found to be the main fluorescence substances of WSOC in PM2.5. The further studying showed that the fluorescence peak position, fluorescence intensity and fluorescence indices of WSOC were sensitive to pH. The fluorescence intensity reached the maximum when pH was about 3, which may be the result of the maximum carboxyl protonation, the fluorescence intensity showed an opposite trend when the pH was more than 3, and decreased significantly with the increasing pH, which was probably caused from enhanced of hydrogen bonding between intermolecular and intramolecular. In addition, the fluorescence peak of tryptophan-like appeared a slightly red shift with the increasing of pH in summer, suggested the increasing benzene ring and degree of conjugation in the molecular structure of WSOC. Comparing to other fluorescent substances, the humic-like substance was more easily influenced by pH. The statistical correlation analysis between pH and fluorescence indices exhibited seasonal differences in fluorophore structure of WSOC. Biological index (BIX) had a significant negative correlation with pH, and decreased significantly with the increasing of pH. While pH was a significant positive correlation with Peak T/C (the intensity ratio of peak T to peak C) (p<0.01), and Peak T/C increased significantly with the increasing of pH, it indicated that low pH was not conducive to the biodegradation of organic matter. Fluorescence index (FI) and humification index (HIX) increased first and then decreased with increasing of pH, and the range of values varied from 1.6~3.64 and 0.8~1.94, respectively. Through the analysis of 3DEEM, it was found that pH was an important factor affecting the fluorescence characteristics of WSOC, the effect of pH should be considered when characterizing the optical properties or other properties of WSOC.