Research on Respond of Water-Soluble Organic Compounds in Atmospheric Particulate to pH Based on Three Dimensional Excitation-Emission Matrix

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 similar to 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 similar to 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 similar to 3. 64 and 0. 8 similar to 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.