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, North Carolina 27711, USA
2Alion Science and Technology, Box 12313, Research Triangle Park, NC 27709, USA
Received: 25 February 2009 – Published in Atmos. Chem. Phys. Discuss.: 20 April 2009 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), organic peroxide fraction, and the aerosol yield.
Revised: 28 July 2009 – Accepted: 17 August 2009 – Published: 10 September 2009
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. However, reaction
conditions did not favor the production of HO2 radicals, thus allowing
RO2+RO2 reactions to proceed more readily than if higher
HO2 levels had been generated.
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 used previously in modeling studies. The yields in
this work (using an independent technique than used previously) 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.
Citation: Kleindienst, T. E., Lewandowski, M., Offenberg, J. H., Jaoui, M., and Edney, E. O.: The formation of secondary organic aerosol from the isoprene + OH reaction in the absence of NOx, Atmos. Chem. Phys., 9, 6541-6558, doi:10.5194/acp-9-6541-2009, 2009.