The formation of secondary organic aerosol from the isoprene + OH reaction in the absence of NOx
1National Exposure Research Laboratory, US Environmental Protection Agency, Research Triangle Park, NC 27711, USA
2Alion Science and Technology, Box 12313, Research Triangle Park, NC 27709, USA
Abstract. The reaction of isoprene (C5H8) with hydroxyl radicals has been studied in the absence of nitrogen oxides (NOx) to determine physical and chemical characteristics of the secondary organic aerosol formed. Experiments were conducted using a smog chamber operated in a steady-state mode permitting measurements of moderately low aerosol levels. GC-MS analysis was conducted to measure methyl butenediols in the gas phase and polyols in the aerosol phase. Analyses were made to obtain several bulk aerosol parameters from the reaction including values for the organic mass to organic carbon ratio, the effective enthalpy of vaporization (ΔHvapeff), the organic peroxide fraction, and the aerosol yield.
The gas phase analysis showed the presence of methacrolein, methyl vinyl ketone, and four isomers of the methyl butenediols. These gas-phase compounds may serve as precursors for one or more of several compounds detected in the aerosol phase including 2-methylglyceric acid, three 2-methyl alkenetriols, and two 2-methyl tetrols. In contrast to most previous studies, the 2-methyl tetrols (and the 2-methyl alkenetriols) were found to form in the absence of acidic sulfate aerosol. A re-evaluation of field samples in Research Triangle Park, North Carolina, during the summers of 2000 and 2001 indicated in the presence of the methyl butenediols from gas-phase denuder samples taken at the time.
SOA/SOC (i.e., OM/OC) was found to average 1.9 in the absence of NOx. The effective enthalpy of vaporization was measured as 38.6 kJ mol−1, consistent with values that have previously been used in modeling studies. The yields in this work (using an independent technique than that previous) are lower than those of Kroll et al. (2006) for similar aerosol masses. SOC yields reported in this work range from 0.5–1.4% for carbon masses between 17 and 49 μgC m−3.