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Volume 17, issue 18 | Copyright
Atmos. Chem. Phys., 17, 11605-11621, 2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 28 Sep 2017

Research article | 28 Sep 2017

Secondary organic aerosol from atmospheric photooxidation of indole

Julia Montoya-Aguilera1, Jeremy R. Horne2, Mallory L. Hinks1, Lauren T. Fleming1, Véronique Perraud1, Peng Lin3, Alexander Laskin3, Julia Laskin3, Donald Dabdub2, and Sergey A. Nizkorodov1 Julia Montoya-Aguilera et al.
  • 1Department of Chemistry, University of California, Irvine, CA 92697, USA
  • 2Department of Mechanical and Aerospace Engineering, University of California, Irvine, CA 92697, USA
  • 3Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA

Abstract. Indole is a heterocyclic compound emitted by various plant species under stressed conditions or during flowering events. The formation, optical properties, and chemical composition of secondary organic aerosol (SOA) formed by low-NOx photooxidation of indole were investigated. The SOA yield (1. 3 ± 0. 3) was estimated from measuring the particle mass concentration with a scanning mobility particle sizer (SMPS) and correcting it for wall loss effects. The high value of the SOA mass yield suggests that most oxidized indole products eventually end up in the particle phase. The SOA particles were collected on filters and analysed offline with UV–vis spectrophotometry to measure the mass absorption coefficient (MAC) of the bulk sample. The samples were visibly brown and had MAC values of  ∼ 2m2 g−1 at λ = 300nm and  ∼ 0. 5m2 g−1 at λ = 400nm, comparable to strongly absorbing brown carbon emitted from biomass burning. The chemical composition of SOA was examined with several mass spectrometry methods. Direct analysis in real-time mass spectrometry (DART-MS) and nanospray desorption electrospray high-resolution mass spectrometry (nano-DESI-HRMS) were both used to provide information about the overall distribution of SOA compounds. High-performance liquid chromatography, coupled to photodiode array spectrophotometry and high-resolution mass spectrometry (HPLC-PDA-HRMS), was used to identify chromophoric compounds that are responsible for the brown colour of SOA. Indole derivatives, such as tryptanthrin, indirubin, indigo dye, and indoxyl red, were found to contribute significantly to the visible absorption spectrum of indole SOA. The potential effect of indole SOA on air quality was explored with an airshed model, which found elevated concentrations of indole SOA during the afternoon hours contributing considerably to the total organic aerosol under selected scenarios. Because of its high MAC values, indole SOA can contribute to decreased visibility and poor air quality.

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Short summary
Various plant species emit a chemical compound called indole under stressed conditions or during flowering events. Our experiments show that indole can be oxidized in the atmosphere to produce a brownish haze containing well-known indole-derived dyes, such as indigo dye. An airshed model that includes indole chemistry shows that indole aerosol makes a significant contribution to the total aerosol burden and to visibility.
Various plant species emit a chemical compound called indole under stressed conditions or during...