Formation of Secondary Organic Aerosol from Photo-Oxidation of Benzene : a Chamber Study

Atmospheric aerosol plays a key role in the Earth’s climate system. Aerosol particles influence the Earth’s radiation budget because of their light scattering and absorbing properties (direct effect) and their ability to form cloud condensation nuclei (indirect effect). A large fraction of atmospheric aerosol is of organic origin, either directly emitted as solid or liquid particles (Primary Organic Aerosol; POA) or formed from volatile organic compounds (VOCs) by photo-oxidation (Secondary Organic Aerosol; SOA). SOA contributes up to 90 % to the total organic aerosol mass and participates in new particle formation (nucleation). Understanding the formation of SOA is crucial for estimating its impact on the climate as well as on human health and the development of future mitigation and adaptation strategies. Therefore, SOA is typically classified into anthropogenic SOA (ASOA) originating from anthropogenic precursors (e.g. aromatic hydrocarbons) and biogenic SOA (BSOA) formed by (photo-) oxidation of plant emissions (e.g. monoterpenes). The formation of both ASOA and BSOA is studied in atmosphere simulation chambers. The potential of a certain VOC to form SOA is expressed by the SOA mass yield. The SOA mass yield is defined as the ratio of the amount of SOA mass formed to the amount of VOC consumed. When transferring the SOA mass yields obtained in simulation chambers for both ASOA and BSOA to global atmospheric chemistry models, an underestimation of global SOA mass production is typically found , in particular for anthropogenically influenced regions. As a consequence, hypotheses have been developed to explain the so-called anthropogenic enhancement effect. One of these hypotheses is that the SOA mass yields determined for single compounds cannot be transferred to the real atmosphere because there typically many different compounds coexist. Therefore, interactions of NOx, SO2 and inorganic particles with single SOA forming substances have been studied extensively. However, only a few studies investigated the direct interaction of hydrocarbons from different sources during the atmospheric photo-oxidation. This still remains a great challenge because attributing SOA to different sources is not possible by current on-line measurement techniques. Within this thesis, a mass spectrometric method was developed allowing for on-line distinction between ASOA produced from photo-oxidation of benzene-d6 VOC and BSOA produced from photo-oxidation of tree emissions. Benzene-d6 was chosen as model substance since benzene is a largely emitted aromatic hydrocarbon of anthropogenic origin. A prerequisite for using benzene-d6 as a model substance was to show that photo-oxidation of benzene-d6 leads to similar oxidation products in the SOA as the photo-oxidation of benzene. This was proven by comparison of mass spectra of SOA produced from both VOCs. To generally understand the formation of ASOA from the photo-oxidation of benzene under varying oxidising conditions, experiments were performed in a continuously stirred tank reactor. The…