Insight into the interaction between heavy metals and water-soluble organic compounds in PM_2.5 affected by heavy haze using ultraviolet–visible and fluorescence spectra combined with two-dimensional correlation spectroscopy

Two-dimensional correlation spectroscopy (2D-COS) on ultraviolet–visible and fluorescence spectra were used to comprehensively investigate the binding properties of atmospheric heavy metals (AHMs) with water-soluble organic compounds (WSOC) in PM_2.5 during heavy haze. The results showed that the interaction between AHMs and the chromophores increased the light absorption capacity of the chromophores. 2D-absorption-COS indicated that the chromophores exhibited higher sensitivity in the shorter wavelength spectral area (208 nm for Cu(Ⅱ) and 209–212 nm for Pb(Ⅱ)) than in the longer wavelength spectral area. However, with the deterioration of air quality, the spectral region of chromophores sensitive to the addition of Cu(Ⅱ) was similar. In contrast, the spectral region of chromophores sensitive to the addition of Pb(Ⅱ) was shifted to longer wavelengths. Four independent fluorescent components including fulvic-like, tryptophan-like, terrestrial humic-like, and microbial humic-like substances were separated by parallel factor analysis (PARAFAC). Humic-like substances (fulvic-like, terrestrial humic-like, and microbial humic-like substances), especially microbial humic-like substances, were the main fluorescent components binding with AHMs. Furthermore, 2D-synchronous fluorescence-COS indicated that the binding ability of fluorescent substances to AHMs was enhanced during air pollution and the fluorescent substances that preferentially bind to two AHMs gradually changed from fulvic-like substances to humic-like substances. The enhanced complexation between AHMs and WSOC during the haze period might have long-term effects on the biogeochemistry of AHMs, the atmospheric environment, and human health. The combination of spectroscopy technology with 2D-COS and PARAFAC was a powerful tool to reveal the interaction mechanism between AHMs and WSOC. The results contributed to our understanding of the complexation behavior of WSOC and AHMs during air pollution.