Ph-Dependent Spectra of Particulate Water-Soluble Organic Carbon (WSOC) from Typical Emission Sources Using EEM-PARAFAC and 2D-COS

Water-soluble organic carbon (WSOC) plays a significant role in the environmental quality, climate, and human health, yet its sources, formation, and aging in the atmosphere remain poorly understood. In this study, we systematically investigated how pH (3–10) affects the fluorescence characteristics (spectral contour, intensity, components, and energy state) of particulate WSOC from four typical emission sources (coal combustion (CC), biomass burning (BB), vehicle exhaust (VE), and dust emission (DE)) by the excitation-emission matrix (EEM) fluorescence spectroscopy and two-dimensional correlation spectroscopy (2D-COS). The EEM spectra of WSOC from different sources showed a decrease in intensity as pH increased from 3 to 10, which might be related to the dissociation or protonation of carboxylic and phenolic or amino groups. Meanwhile, the apparent quantum yield (AQY) of CC and VE decreased with increasing pH but turned upwards at pH 7, and decreased with increasing pH for BB and DE samples, which could be a consequence of the increase in the rate of non-radiative transition. The Stokes shift of BB, VE, and DE generally increased with increasing pH, indicating that the energy gap was also susceptible to pH. 2D-COS revealed that protein-like compounds in BB and less-oxygenated aromatic humic-like (less-oxygenated HULIS) compounds in VE exhibited much stronger susceptibility to pH. These findings suggested that the specificity of pH should be considered when investigating WSOC in these typical emission source samples.