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.
Highlights
The environmental, health, and climate effects of atmospheric aerosol particles have been reiterated for many years (Pósfai and Buseck, 2010; Burnett et al, 2018; Yan et al, 2020; Fan et al, 2020)
The Water-soluble organic compounds (WSOCs)/OC ratios were higher in particles with an aerodynamic diameter smaller than 1.4 μm compared with the coarse mode (PM2.5–10), which is accordant with findings previously reported for clear days in Beijing (Tian et al, 2016)
The size distributions of the average fluorescence intensity (AFI) remained in step with those of the WSOC concentrations and showed monomodal distributions in winter and bimodal distributions in summer, peaking at particle sizes between 0.26 and 0.44 μm (Fig. 2a, b)
Summary
The environmental, health, and climate effects of atmospheric aerosol particles have been reiterated for many years (Pósfai and Buseck, 2010; Burnett et al, 2018; Yan et al, 2020; Fan et al, 2020). Water-soluble organic compounds (WSOCs) comprise 10 % to 80 % of organic compounds in atmospheric aerosols (Qin et al, 2018; Almeida et al, 2020; Cai et al, 2020). WSOCs play significant roles in cloud formation, solar irradiation, and atmospheric chemistry (Asa-Awuku et al, 2009; Duarte et al, 2019). J. Qin et al.: Particle-size-dependent fluorescence properties of WSOCs aromatic nuclei and aliphatic chains (Decesari et al, 2001; Dasari et al, 2019), with functional groups or heteroatoms like hydroxyl, carboxyl, aldehyde, ketone, amino, and other nitrogen-containing groups (Duarte et al, 2007; Cai et al, 2020). Biomass burning and secondary transformation of organics are believed to be the main sources of WSOCs (Park et al, 2017; Xiang et al, 2017)
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