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Discussion papers
https://doi.org/10.5194/acp-2019-20
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-20
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 18 Feb 2019

Submitted as: research article | 18 Feb 2019

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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.

Secondary organic aerosol formation from OH-initiated oxidation of m-xylene: effects of relative humidity on yield and chemical composition

Qun Zhang1,2, Yongfu Xu1,2, and Long Jia1,2 Qun Zhang et al.
  • 1State Key Laboratory of Atmospheric Boundary Layer Physics and Atmospheric Chemistry, Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
  • 2Department of Atmospheric Chemistry and Environmental Sciences, College of Earth Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

Abstract. The effect of relative humidity (RH) on the secondary organic aerosol (SOA) formation from the photooxidation of m-xylene initiated by OH radicals in the absence of seed particles was investigated in a smog chamber. The SOA yields were determined based on the particle mass concentrations measured with a scanning mobility particle sizer (SMPS) and reacted m-xylene concentrations measured with a gas chromatograph-mass spectrometer (GC-MS). The SOA components were analyzed using Fourier transform infrared spectrometer (FTIR) and ultrahigh performance liquid chromatograph-electrospray ionization-high-resolution mass spectrometer (UPLC-ESI-HRMS). A significant discrepancy was observed in SOA mass concentration and yield variation with the RH conditions. The SOA yield is 13.8 % and 0.8 % at low RH (13.7 %) and high RH (79.1 %), respectively, with the difference being over an order of magnitude. The relative increase of C-O-C at high RH from the FTIR analysis of functional groups indicates that the oligomers from carbonyl compounds cannot well explain the suppression of SOA yield. Highly oxygenated molecules (HOMs) were observed to be suppressed in the HRMS spectra. The chemical mechanism for explaining the RH effects on SOA formation from m-xylene-OH system is proposed based on the analysis of both FTIR and HRMS measurements as well as Master Chemical Mechanism (MCM) simulations. The reduced SOA at high RH is mainly ascribed to the less formation of oligomers and the suppression of RO2 autoxidation. As a result, high RH can obstruct the oligomerization and autoxidation that contribute to the SOA formation.

Qun Zhang et al.
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Qun Zhang et al.
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Short summary
The significantly negative effects of relative humidity (RH) on secondary organic aerosol (SOA) formation from m-xylene under low NOx conditions were observed. The significant RH effects on chemical compositions of SOA are identified, in which highly oxygenated molecules (HOMs) are dominant. Our results show that the decrease of SOA formation under high RH conditions is mainly attributed to the suppression of the formation of oligomers and HOMs.
The significantly negative effects of relative humidity (RH) on secondary organic aerosol (SOA)...
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