05 Oct 2023
 | 05 Oct 2023

Distribution, chemical and molecular composition of high and low-molecular-weight humic-like substances in ambient aerosols

Xingjun Fan, Ao Cheng, Xufang Yu, Tao Cao, Dan Chen, Wenchao Ji, Yongbing Cai, Fande Meng, Jianzhong Song, and Ping'an Peng

Abstract. Humic-like Substances (HULIS) encompass a continuum of molecular weight (MW) ranges, yet our understanding of how HULIS characteristics vary with MW is still limited and not well-established. In this study, a combination of ultrafiltration and solid-phase extraction protocols was employed to fractionate the high MW (HMW, >1 kDa) and low MW (LMW, < 1kDa) HULIS fractions from ambient aerosols collected during summer and winter at a rural site. Subsequently, comprehensive characterization by using total organic carbon, high-performance size exclusion chromatography (HPSEC), UV-vis and fluorescence spectroscopy, Fourier-transform infrared spectroscopy (FTIR), negative electrospray ionization high resolution mass spectrometry (ESI- HRMS) were conducted. The results revealed that HMW HULIS were dominated by larger-sized chromophores, substantially constituting a higher fraction of total organic carbon and UV absorption at 254 nm than LMW HULIS. While both HMW and LMW HULIS shared similar fluorophore types and functional groups, the former exhibited higher levels of humification and a greater presence of polar functional groups (e.g., -COOH, >C=O). HRMS analysis further unveiled that molecular formulas within HMW HULIS generally featured smaller sizes but higher degrees of unsaturation and aromaticity compared to those within LMW HULIS fractions. This observation suggests the possibility of small molecules assembling to form the HMW HULIS through intermolecular weak forces. Moreover, HMW HULIS contained a higher proportion of CHON but fewer CHO compounds than LMW HULIS. In both HMW and LMW HULIS, the unique molecular formulas were primarily characterized by lignin-like species, yet the former displayed a prevalence of N-enriched and highly aromatic species. Additionally, HMW HULIS contained more unique lipids-like compounds, while LMW HULIS exhibited a distinct presence of tannin-like compounds. These findings provide valuable insights into the distribution, optical properties, and molecular-level characteristics of HULIS in atmospheric aerosols, thereby advancing our understanding of their sources, composition, and environmental implications.

Xingjun Fan et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2029', Anonymous Referee #1, 05 Nov 2023
  • RC2: 'Comment on egusphere-2023-2029', Anonymous Referee #2, 11 Nov 2023

Xingjun Fan et al.


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Short summary
The molecular-level characteristics of High molecular weight (HMW) and Low MW (LMW) HULIS were comprehensively investigated. HMW HULIS had larger chromophores and larger molecular size than LMW HULIS. HMW HULIS exhibited higher aromaticity, and humification. ESI-HRMS revealed more aromatic molecules in HMW HULIS. Furthermore, HMW HULIS had more CHON compounds, while LMW HULIS had more CHO compounds. These findings enhance our understanding of HULIS sources and environmental implications.