4.8 Article

Connecting the Light Absorption of Atmospheric Organic Aerosols with Oxidation State and Polarity

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出版社

AMER CHEMICAL SOC
DOI: 10.1021/acs.est.2c02202

关键词

brown carbon; light-absorbing properties; water and methanol extracts; polarity; oxidation state; organic aerosols

资金

  1. National Natural Science Foundation of China [42175116, 41875167]
  2. National Key R&D Program of China [2019YFC0214703]
  3. China Postdoctoral Science Founda-tion [2021M692764]

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This study fractionated ambient organic aerosols (OA) from an urban environment according to polarity and found that high-polar compounds had a significantly higher oxygen to carbon ratio (O/C) compared to low-polar compounds. The presence of nitrogen-containing functional groups was found to enhance the absorptivity of low-polar compounds, while further oxidation of high-polar compounds likely led to bleaching and fragmentation of chromophores.
The light-absorbing organic aerosol (OA) constitutes an important fraction of absorbing components, counteracting major cooling effect of aerosols to climate. The mechanisms in linking the complex and changeable chemistry of OA with its absorbing properties remain to be elucidated. Here, by using solvent extraction, ambient OA from an urban environment was fractionated according to polarity, which was further nebulized and online characterized with compositions and absorbing properties. Water extracted high-polar compounds with a significantly higher oxygen to carbon ratio (O/C) than methanol extracts. A transition O/C of about 0.6 was found, below and above which the enhancement and reduction of OA absorptivity were observed with increasing O/C, occurring on the less polar and high polar compounds, respectively. In particular, the co-increase of nitrogen and oxygen elements suggests the important role of nitrogen-containing functional groups in enhancing the absorptivity of the less polar compounds (e.g., forming nitrogen-containing aromatics), while further oxidation (O/C > 0.6) on high-polar compounds likely led to fragmentation and bleaching chromophores. The results here may reconcile the previous observations about darkening or whitening chromophores of brown carbon, and the parametrization of O/C has the potential to link the changing chemistry of OA with its polarity and absorbing properties.

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