Raman-Polarization-Fluorescence Spectroscopic Lidar for Real-Time Detection of Humic-like Substance Profiles

Humic-like substances (HULIS) widely exist in the atmosphere and may strongly affect human health, environment, and climate. However, there are still no accurate methods for detecting the vertical distribution of HULIS. Here, a Raman-Polarization-Fluorescence Spectroscopic Lidar (RPFSL) was developed to simultaneously measure 64-channel broad fluorescence spectra (370-710 nm) of atmospheric aerosols at an excitation wavelength of 355 nm. The study revealed that dust could be coated by abundant fluorescent substances, with a maximum fluorescence efficiency reaching 0.15. Moreover, the fluorescent spectra of air pollutants exhibited a unimodal structure, while the spectra of dust exhibited three peaks, suggesting that they may be useful for highly accurate identification of dust aerosols from other aerosols. The findings in this study were confirmed by near-ground air sampling analysis based on fluorescence excitation-emission matrix-parallel factor (EEM-PARAFAC) methods; we demonstrated that HULIS and protein-like organic matter (PLOM) were the main components of fluorescent aerosols during the study period. During air pollution events, the number concentration of HULIS reached up to 9699 particles·m^-3. For the first time, this study proposes a real-time, high-resolution method for detecting height-resolved HULIS, significantly helping to evaluate the environmental and health implications of HULIS.