10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Molecular Markers and Thermal Decomposition of Biogenic Secondary Organic Aerosol: Insight from Lab and Field Observations by Thermal Desorption – Gas Chromatography – Mass Spectrometry
MICHAEL WALKER, Riley Martell, Audrey Dang, Raul Martinez, David Hagan, Thomas Berkemeier, Masayuki Takeuchi, Gamze Eris, Nga Lee Ng, Brent Williams, Washington University in St. Louis
Abstract Number: 1461 Working Group: Aerosol Chemistry
Abstract Secondary organic aerosol (SOA) constitutes a large fraction of global submicron aerosol. The composition of SOA is highly complex, consisting of thousands of different molecules that vary with both the source and atmospheric processing of the particles. Advances in mass spectrometry (MS) techniques have greatly increased our understanding of SOA composition. For molecular detection, most MS methods for SOA analysis either utilize chemical derivatization to alter SOA molecules to forms more amenable to gas chromatography (GC) separation, or rely on chemical ionization to select specific classes of molecules for detection. For both molecular-level analysis and bulk chemical analysis, MS techniques often rely on thermal desorption to introduce the collected aerosol. Several studies have demonstrated that thermal desorption techniques can thermally decompose samples such that the detected molecules are not necessarily the original SOA molecules. Stark et al. demonstrated that measured chemical formulas from chemical ionization mass spectrometry (CIMS) techniques cannot adequately predict the aerosol volatility (Stark et al., 2017). Additionally, during the initial thermal desorption step of a thermal desorption aerosol gas chromatograph (TAG), small, highly volatile fragments are produced from both organic and inorganic species (Williams et al., 2016). Measuring these fragment species enables the classification of decomposition products, which are not retained on the GC column.
Within the TAG family of instruments, the Volatility and Polarity Separator (VAPS) utilizes a thermal desorption step, followed by a modified two-dimensional GC technique coupled with a high-resolution time-of-flight mass spectrometer (Martinez et al., 2016). The VAPS inlet and newly incorporated miniature-GCs have several programmable temperature regions, which give unprecedented control to study the thermal decomposition of SOA.
Molecular markers and thermal decomposition products were recently studied through VAPS measurements as part of a larger study on the oxidation of biogenic volatile organic compounds (BVOCs) at the Georgia Tech Environmental Chamber. Specific molecular markers were determined for a range of SOA sources, but larger thermal decomposition fragments were most abundant from particulate organonitrate compounds that result from the NO3 oxidation of BVOCs, which have typically been identified with CIMS or LC-MS techniques (Ng et al. 2017). Despite the loss of direct information on the original SOA molecules, decomposition serves as a “thermal derivatization” process, in which the resulting distinct molecular fragments are more amenable to detection and can serve as decomposition tracers. In combination with additional oxidation experiments using a Potential Aerosol Mass (PAM) oxidation flowtube reactor, the identification of these decomposition tracers has led to new insights into field measurements by the VAPS and TAG systems from field measurements during the Southern Oxidant and Aerosol Study (SOAS), the St. Louis Air Quality Regional Study (SLAQRS), and a more-recent campaign at the Jefferson Street site in Atlanta.
[1] Martinez, R. E. et al. Development of a volatility and polarity separator (VAPS) for volatility- and polarity-resolved organic aerosol measurement. Aerosol Science and Technology 50, 255–271 (2016). [2] Ng, N. L. et al. Nitrate radicals and biogenic volatile organic compounds: oxidation, mechanisms, and organic aerosol. Atmospheric Chemistry and Physics 17, 2103–2162 (2017). [3] Stark, H. et al. Impact of Thermal Decomposition on Thermal Desorption Instruments: Advantage of Thermogram Analysis for Quantifying Volatility Distributions of Organic Species. Environmental Science & Technology 51, 8491–8500 (2017). [4] Williams, B. J. et al. Organic and inorganic decomposition products from the thermal desorption of atmospheric particles. Atmospheric Measurement Techniques 9, 1569–1586 (2016).