10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Formation of Secondary Organic Aerosol from Photo-Oxidation of Benzene
SEBASTIAN H. SCHMITT, Thomas F. Mentel, Jürgen Wildt, Einhard Kleist, Iida Pullinen, Ying Liu, Baolin Wang, Defeng Zhao, Astrid Kiendler-Scharr, Forschungszentrum Jülich
Abstract Number: 733 Working Group: Aerosol Chemistry
Abstract Understanding the formation of secondary organic aerosol (SOA) is crucial for estimating its impact on the earth’s climate as well as on human health and the development of future mitigation and adaptation strategies. The potential of SOA formation from the oxidation of a certain volatile organic compound (VOC) is often expressed as the SOA mass yield being the ratio of formed SOA mass and consumed VOC. Typically, this is determined from single VOC oxidation experiments in atmosphere simulation chambers. Even though the SOA mass yield is an easy to calculate quantity, caution has to be taken to accurately determine it in a specific atmosphere simulation chamber.
Within this study the formation of SOA from the photo-oxidation of benzene was systematically studied in the Jülich Plant Atmosphere Chamber (JPAC) at the campus of Forschungszentrum Jülich GmbH. JPAC is a continuously stirred tank reactor made of borosilicate glass and it was operated with a typical mixing time of 2 minutes and a residence time of 45 minutes. The oxidizing conditions (OH concentrations, NOx concentrations) were systematically altered in order to study their respective influence on SOA mass formation. A combination of Scanning Mobility Particle Sizer (SMPS), Aerosol Mass Spectrometry (AMS) and Chemical Ionization Mass Spectrometry (NO3-CIMS) provided novel insights into the formation process of secondary organic aerosol.
The loss of oxidized SOA precursor molecules to the walls of the reaction chamber must be considered for retrieving the most accurate SOA mass yield. For this purpose an already existing correction function was extended, in order to take into account semi-volatile organic compounds with lifetimes longer than the residence time of the chamber. In order to accurately determine this correction function, ammonium sulfate seed aerosol was used to alter the fraction of SOA precursors lost to the chamber wall systematically. Here, the organic fraction of SOA was determined with the AMS while the NO3-CIMS provided measurements of the oxidized SOA precursors in the gas phase.
After correction of unseeded experiments for wall loss of SOA precursor by the modified correction function, the SOA mass yield of benzene was found to be 28±13 %. A similar yield (29±4 %) was obtained by using seed aerosol with the uncertainty being largely reduced in presence of seed aerosols. Within the accessible range of the steady state measurements, no dependence of the benzene SOA mass yield on OH could be detected. Also, NOx had no significant influence on the SOA mass yield from benzene photo-oxidation. However the formation of new particles (nucleation) was strongly suppressed by increasing NOx concentrations.
A comparison of the obtained SOA mass yield results with literature data shows a large variation, likely due to the sensitivity to the operational conditions of the different experiments.