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OZONE-ISOPRENE REACTION: RE-EXAMINATION OF THE FORMATION OF SECONDARY ORGANIC AEROSOL
Citation:
KLEINDIENST, T. E., M. LEWANDOWSKI, J. H. OFFENBERG, M. JAOUI, AND E. O. EDNEY. OZONE-ISOPRENE REACTION: RE-EXAMINATION OF THE FORMATION OF SECONDARY ORGANIC AEROSOL. GEOPHYSICAL RESEARCH LETTERS. American Geophysical Union, Washington, DC, 34(1):01/05/07, (2007).
Impact/Purpose:
PM Chemistry models for predicting chemical compositions of PM2.5 are needed to assess PM2.5 control strategies. Because of the chemical complexity of ambient PM2.5, models must be developed to predict ambient concentrations of water, inorganics, and organics in the aerosols. Under this research task, the current treatments of secondary organic aerosol formation and the chemistry of N2O5 in CMAQ will be improved by HEASD and integrated into CMAQ by AMD. CMAQ will be used by AMD to predict ambient concentrations and compositions of PM2.5 for OAQPS.
Description:
The reaction of ozone and isoprene has been studied to examine physical and chemical characteristics of the secondary organic aerosol formed. Using a scanning mobility particle sizer, the volume distribution of the aerosol was found in the range 0.05 - 0.2 µm. The aerosol yield was estimated to be 0.01, a value which is a factor of 5 - 10 higher than previous reports. The aerosol formation is complicated by the presence of minor impurities in the isoprene and the fact that OH-radicals produced in the ozonolysis can react with isoprene to produce organic aerosol. Without an OH-radical scavenger present, up to 50% of the observed aerosol comes from the OH channel. A GC-MS analysis of the products of the composite aerosol showed that two methyl tetrols and 2-methylglyceric acid are formed which can be attributed to the OH reaction channel. A measurement of the effective enthalpy of vaporization using a volatility differential mobility analyzer found the aerosol to have ΔHeff of -42 kJ mol-1, a value at the upper end of the range of organic aerosols previously studied. Even with the increased yield found in this study, the ozonolysis reaction probably remains a minor contributor to secondary organic aerosol in PM2.5 from the atmospheric oxidation of isoprene.