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Discussion papers
https://doi.org/10.5194/acp-2018-882
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2018-882
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Sep 2018

Research article | 03 Sep 2018

Review status
This discussion paper is a preprint. A revision of this manuscript was accepted for the journal Atmospheric Chemistry and Physics (ACP) and is expected to appear here in due course.

Dynamic changes of optical and chemical properties of tar ball aerosols by atmospheric photochemical aging

Chunlin Li1, Quanfu He1, Julian Schade2, Johannes Passig2,3, Ralf Zimmermann2,3, Alexander Laskin4, and Yinon Rudich1 Chunlin Li et al.
  • 1Department of Earth and Planetary Sciences, Weizmann Institute of Science, Rehovot 76100, Israel
  • 2Joint Mass Spectrometry Centre, University of Rostock, Dr.-Lorenz-Weg 2, 18059 Rostock, Germany
  • 3Joint Mass Spectrometry Centre, Cooperation Group "Comprehensive Molecular Analytics" (CMA), Helmholtz Zentrum München, Ingolstädter Landstrasse 1, 85764 Neuherberg, Germany
  • 4Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA

Abstract. Following wood pyrolysis, tar ball aerosols were generated from wood tar separated into polar and nonpolar phases. Chemical information of fresh tar balls was obtained from the high-resolution time-of-flight aerosol mass spectrometer (HiRes-ToF-AMS) and laser desorption/resonance enhanced multiphoton ionization mass spectrometry (LD-REMPI-MS), and their refractive index between 365 and 425nm were retrieved using a broadband cavity enhanced spectroscopy. Dynamic changes of the optical and chemical properties for the nonpolar tar ball aerosols in NOx-dependent photochemical process were investigated in an oxidation flow reactor (OFR). Distinct differences in the chemical composition of the polar and nonpolar tar aerosols were identified. Nonpolar tar aerosols contain predominantly high-molecular weight unsubstituted and alkyl-substituted polycylic aromatic hydrocarbons (PAHs), while polar tar aerosols consist of a high number of oxidized aromatic substances (e.g., methoxy-phenols, benzenediol) with higher O:C ratio and carbon oxidation state. Fresh tar aerosols have light absorption characteristics similar to atmospheric BrC with higher absorption efficiency towards the UV wavelengths. The average retrieved refractive index (RI) are 1.661+0.020i and 1.635+0.003i for the nonpolar and polar tar aerosols, respectively, with absorption Ångström exponent (AAE) between 5.7 and 7.8 in the wavelength region 365–425nm. The RI fits a volume mixing rule for internally mixed nonpolar/polar tar aerosols. The RI of the tar aerosols decreased with increasing wavelength under photochemical oxidation. Photolysis by UV light (254nm), without strong oxidants in the system, slightly decreased the RI and increased the oxidation state of the tar balls. Oxidation under varying OH exposure levels and in the absence of NOx diminished the absorption (bleaching), and increased the O:C ratio. The photobleaching of tar ball aerosols via photochemically induced OH-oxidation is mainly attributed to decomposition of chromophoric aromatics, nitrogen-containing organics, and high-molecular weight components. Photolysis of nitrous oxide (N2O) was used to simulate NOx-dependent photochemical aging of tar balls in the OFR. Under high NOx conditions, photochemical aging lead to the formation of organic-nitrates, increased oxidation degree and increased absorption for the tar ball aerosols. These observations suggest that secondary organic nitrate formation compensates the bleaching by photolysis and OH radical photooxidation to eventually regain some absorption of aged tar balls aerosols. The atmospheric implication and climatic effects from tar balls upon various oxidation processes are briefly discussed.

Chunlin Li et al.
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Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Interactive discussion
Status: closed
Status: closed
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement
Chunlin Li et al.
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