06 Jul 2022
06 Jul 2022
Status: this preprint is currently under review for the journal ACP.

Underestimation of brown carbon absorption based on the methanol extraction method and its impacts on source analysis

Zhenqi Xu1, Wei Feng1, Yicheng Wang1, Haoran Ye1, Yuhang Wang2, Hong Liao1, and Mingjie Xie1 Zhenqi Xu et al.
  • 1Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, 219 Ningliu Road, Nanjing 210044, China
  • 2School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332, United States

Abstract. The methanol extraction method was widely applied to isolate organic carbon (OC) from ambient aerosols, followed by measurements of brown carbon (BrC) absorption. However, undissolved OC fractions will lead to underestimated BrC absorption. In this work, water, methanol (MeOH), MeOH/dichloromethane (MeOH/DCM, 1:1, v/v), MeOH/DCM (1:2, v/v), tetrahydrofuran (THF), and N,N-dimethylformamide (DMF) were tested for extraction efficiencies of ambient OC, and the light absorption of individual solvent extracts was determined. Among the five solvents and solvent mixtures, DMF dissolved the highest fractions of ambient OC (up to ~95 %), followed by MeOH and MeOH/DCM mixtures (< 90 %), and the DMF extracts had significant (p < 0.05) higher light absorption than other solvent extracts. This is because the OC fractions evaporating at higher temperatures (> 280 °C) are less soluble in MeOH (~80 %) than in DMF (~90 %) and contain stronger light-absorbing chromophores. Moreover, the light absorption of DMF and MeOH extracts of collocated aerosol samples in Nanjing showed distinct time series. Source apportionment results indicated that the MeOH insoluble OC mainly came from unburned fossil fuels and polymerization processes of aerosol organics. These results highlight the necessity of replacing MeOH with DMF for further investigations on structures and light absorption of low-volatile BrC.

Zhenqi Xu et al.

Status: open (until 17 Aug 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'Comment on acp-2022-460', Guofeng SHEN, 13 Jul 2022 reply
  • RC1: 'Comment on acp-2022-460', Guofeng SHEN, 13 Jul 2022 reply
  • CC2: 'Comment on acp-2022-460', Xiao He, 20 Jul 2022 reply
  • RC2: 'Comment on acp-2022-460', Xiao He, 20 Jul 2022 reply
  • RC3: 'Comment on acp-2022-460', Anonymous Referee #3, 29 Jul 2022 reply

Zhenqi Xu et al.


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Short summary
This study finds a new solvent (DMF) that can dissolve low-volatile OC more completely than currently used methods, which will benefit future investigations on the physicochemical properties of large organic molecules. The study results also shed light on potential sources for methanol-insoluble OC. These results suggest that methanol should be replaced in future solvent extraction-based investigations on the absorption, composition, sources, and formation pathways of low-volatile BrC.