Semi-Volatile Organic Compounds: Behavior and Secondary Organic Aerosol Formation
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This thesis mainly described the development of studying the behavior and secondary organic aerosol formation from semi-volatile organic compounds (SVOCs). SVOCs comprises a significant fraction of the organic mass in particulate matter (PM), which has shown important impacts on human health and also influences on Earth’s climate. SVOCs are thought to play essential roles in the formation of SOA, chemical aging and mixing processes. Smog chambers have been extensively used to study SOA formation, chemical reaction and physical properties. The interaction between SVOC vapor with Teflon chamber wall and suspended particles is a key factor influencing organic aerosol formations and behaviors in chamber experiments. We observed that pinanediol (PD) showed a large chamber wall deposition and reached a steady concentration, only around 14% of mass left in the gas phase. But we did not observe the release of PD from the chamber walls during isothermal dilution of the chamber with fresh air at 22 oC, which indicated there was no PD released from the chamber walls during the SOA formation. This clearly shows the vapor loss of SVOC precursors need to be considered when studying their SOA formation. The average carbon oxidation states the SOA from PD were calculated as around -0.7, which were similar to the value observed in CLOUD. Our data are consistent with ~10% of the SOA with low volatility that could drive new particle formation. It is challenging to measure SVOC vapor concentrations and properties. A new approach is discussed in this thesis, studying SVOC vapors from measuring the particles. The SVOCs coated particles sustained the SVOCs in the gas phase at or near their saturation concentration. The mass loss of SVOCs from the suspended particles thus reflects SVOCs vapor wall loss. Our results show the vapor wall loss rate of SVOC is consistently proportional to the SVOC vapor concentrations. We observed PEG400 seeds can sorb semi-volatile α-pinene SOA vapors. This allows us to trap semi-volatile α-pinene SOA into PEG400 seeds and then analyze their compositions and properties through measuring particles. PEG400 is liquid, water-soluble, nearly non-volatile, good solvent for SOA and relative stable during the oxidation with OH radicals and ozone. It can also be easily separated from the SOA mass spectrum with the unique fragment C4H9O2+ at m/z=89. The results demonstrated that SOA prepared from α-pinene reacted with OH produced more semi-volatile SOA vapors comparing to α-pinene ozonolysis. More semi-volatile SOA vapors were observed in the gas phase with higher SOA loadings. With well-built particle measurement methods, we may get more knowledge on the SVOC vapors.