Measurement report: Particle-size-dependent fluorescence properties of water-soluble organic compounds (WSOCs) and their atmospheric implications for the aging of WSOCs

Abstract

Abstract. Water-soluble organic compounds (WSOCs) play important roles in atmospheric particle formation, migration, and transformation processes. Size-segregated atmospheric particles were collected in a rural area of Beijing. Three-dimensional fluorescence spectroscopy was used to investigate the optical properties of WSOCs as a means of inferring information about their atmospheric sources. Sophisticated analysis on fluorescence data was performed to characteristically estimate the connections among particles of different sizes. WSOC concentrations and the average fluorescence intensity (AFI) showed a monomodal distribution in winter and a bimodal distribution in summer, with the dominant mode in the 0.26–0.44 µm size range in both seasons. The excitation–emission matrix (EEM) spectra of WSOCs varied with particle size, likely due to changing sources and/or the chemical transformation of organics. Size distributions of the fluorescence regional integration (regions III and V) and humification index (HIX) indicate that the humification degree or aromaticity of WSOCs was the highest in the particle size range of 0.26–0.44 µm. The Stokes shift (SS) and the harmonic mean of the excitation and emission wavelengths (WH) reflected that π-conjugated systems were high in the same particle size range. The parallel factor analysis (PARAFAC) results showed that humic-like substances were abundant in fine particles (< 1 µm) and peaked at 0.26–0.44 µm. All evidence supported the fact that the humification degree of WSOCs increased with particle size in the submicron mode (< 0.44 µm) and then decreased gradually with particle size, which implied that the condensation of organics occurred in submicron particles, resulting in the highest degree of humification in the particle size range of 0.26–0.44 µm rather than in the < 0.26 µm range. Synthetically analyzing three-dimensional fluorescence data could efficiently reveal the secondary transformation processes of WSOCs.

pH-Dependent Chemical Transformations of Humic-Like Substances and Further Cognitions Revealed by Optical Methods

“…The peaks of EEM spectra of HULIS coincided with those of WSOC reported in our previous research , with their fluorescence peaks mainly produced at regions II–V. The fluorescence efficiency (represented by AFI/TOC) of HULIS was higher than that of WSOC with AFI/TOC HULIS being 1.34 times of AFI/TOC WSOC in winter and 1.08 times in summer.…”

Chemical Composition, Optical Properties and Sources of PM2.5 From a Highly Urbanized Region in Northeastern Mexico

“…The maximum wavelength intensity of the spectra from the winter and summer samples ranged from 440 to 464 nm and 439-449 nm, respectively. Similar ranges have been reported by other researchers for highly oxygenated carbonaceous and humic-like substances (HULIS) (McKnight et al, 2001;Chen et al, 2016b;Qin et al, 2021). Barsotti et al (2016) reported that HULIS derived from the oligomerization of phenolic compounds exhibited a strong fluorescence emission in the vicinity of 450 nm.…”

Review of Brown Carbon Aerosols in China: Pollution Level, Optical Properties, and Emissions

“…Similarly, the measurements in Beijing, China showed that HULIS was rich in fine particles and protein-like compounds were rich in coarse particles (J. Qin et al, 2022). The results by Wu et al (2020) showed a monomodal size distribution of BrC absorption in winter with a peak in the fine mode (<2.1 μm) but a bimodal distribution in summer with two peaks in the fine and coarse modes.…”