SOA formation from the atmospheric oxidation of 2-methyl-3-buten-2-ol and its implications for PM_2.5

Abstract. The formation of secondary organic aerosol (SOA) generated by irradiating 2-methyl-3-buten-2-ol (MBO) in the presence and/or absence of NO_x, H₂O₂, and/or SO₂ was examined. Experiments were conducted in smog chambers operated in either dynamic or static mode. A filter/denuder sampling system was used for simultaneously collecting gas- and particle-phase products. The structural characterization of gas and particulate products was investigated using BSTFA, BSTFA + PFBHA, and DNPH derivatization techniques followed by GC-MS and liquid chromatography analysis. This analysis showed the occurrence of more than 68 oxygenated organic compounds in the gas and particle phases, 28 of which were tentatively identified. The major components observed include 2,3-dihydroxyisopentanol (DHIP), 2-hydroxy-2-oxoisopentanol, 2,3-dihydroxy-3-methylbutanal, 2,3-dihydroxy-2-methylsuccinic acid, 2-hydroxy-2-methylpropanedioic acid, acetone, glyoxal, methylglyoxal, glycolaldehyde, and formaldehyde. Most of these oxygenated compounds were detected for the first time in this study.

While measurements of the gas-phase photooxidation products have been made, the focus of this work has been an examination of the particle phase. SOA from some experiments was analyzed for the organic mass to organic carbon ratio (OM/OC), the effective enthalpy of vaporization (ΔH_vap^eff), and the aerosol yield. Additionally, aerosol size, volume, and number concentrations were measured by a Scanning Mobility Particle Sizer coupled to a Condensation Particle Counter system. The OM/OC ratio was 2.1 in the MBO/H₂O₂ system. The ΔH_vap^eff was 41 kJ mol⁻¹, a value similar to that of isoprene SOA. The laboratory SOA yield measured in this study was 0.7% in MBO/H₂O₂ for an aerosol mass of 33 μg m⁻³. Secondary organic aerosol was found to be negligible under conditions with oxides of nitrogen (NO_x) present. Time profiles and proposed reaction schemes are provided for selected compounds.

The contribution of SOA products from MBO oxidation to ambient PM_2.5 was investigated by analyzing a series of ambient PM_2.5 samples collected in several places around the United States. In addition to the occurrence of several organic compounds in both field and laboratory samples, DHIP was found to originate only from the oxidation of MBO, and therefore this compound could potentially serve as a tracer for MBO SOA. Initial attempts have been made to quantify the concentrations of DHIP and other compounds based on surrogate compound calibrations. The average concentrations of DHIP in ambient PM_2.5 samples from Duke Forest in North Carolina ranged from zero during cold seasons to approximately 1 ng m⁻³ during warm seasons. This appears to be the first time that DHIP has been detected in ambient PM_2.5 samples. The occurrence of several other compounds in both laboratory and field samples suggests that SOA originating from MBO can contribute under selected ambient conditions to the ambient aerosol mainly in areas where MBO emissions are high.