INTRODUCTION Fine particle composition is of great interest to those studying health effects, global climate change, and cloud formation. While up to 70% of the mass of these particles can be organic in nature, questions remain about the molecular composition particularly with regard to secondary organic aerosol (SOA). Studies of the SOA formed following the oxidation of monoterpenes, such as alpha-pinene, show that the original oxidation products quickly oligomerize and that these oligomers seem make up the bulk of the SOA mass. However, uncertainty remains about the identity of these oligomers, how they form and precisely what fraction of the SOA mass they represent. Figure 1: (Top) A few of the possible products formed after the ozonolysis of alpha-pinene. (Bottom) After 45 minutes of reaction thousands of compounds are formed with the highest intensity grouped around mass ranges corresponding to oligomers. O3O3 O3O3 SCI 2 SCI 1 SCI 2 Intermedia tes MonomersDimers OVERVIEW Evaporative light scattering detector (ELSD) to determine SOA collection efficiency. Evaporative light scattering detector (ELSD) to determine SOA collection efficiency. ELSD and ESI-MS to determine the fraction of SOA that is oligomeric. ELSD and ESI-MS to determine the fraction of SOA that is oligomeric. High performance mass spectrometry to characterize the pathways of oligomer formation High performance mass spectrometry to characterize the pathways of oligomer formation. EXPERIMENTAL Allow alpha-pinene and ozone to react for 45 min. Collect SOA on filter and divide into halves or quarters. Extract each filter section with a different solvent. Centrifuge under vacuum to near dryness, reconstitute with acetonitrile. LC-ELSD to determine extraction efficiency and mass distribution. ESI-FTICR with IRMPD to determine composition Monom ers Dimers Trimers Tetrame rs Pentam ers ACKNOWLEDGEMENTS NSF Grant #CHE Jeff Spraggins, Univ. of Delaware John Dykins, Univ. of Delaware Measurements of SOA Mass OCEC / SMPS 2.33+/ (no Dilution) (6X Dilution) OCEC / SMPS 0.98+/ (6X Dilution) Filter / SMPS 1.48+/ (no Dilution) (6X Dilution) Filter / SMPS TBD (6X Dilution) Isolate an oligomer in the ICR cell, fragment with IRMPD, and determine monomer content by neutral loss Frequency of monomer detection correlates with molar yield but not vapor pressure Standard addition curves were prepared for pinic and pinonic acid, two known monomers that are commercially available Response factors for both compounds were then applied to each peak in the monomer region for a high end estimate of monomer content (and a low end estimate of oligomers) SolventAvg. % OligomerStdev ACN55%7% 50% ACN58%5% Water51%1% OLIGOMER YIELD OLIGOMER COMPOSITION Van Krevelen plot summarizes molecular formulas of detected oligomers Within experimental reproducibility, the same oligomers are extracted from the filter regardless of solvent Particulate mass is significantly higher when measured by OCEC than SMPS (semivolatile mass is lost via dilution inside the SMPS) Diluting the OCEC sample flow to match dilution inside the SMPS gives similar particulate mass measurements Particulate mass extracted from the filter is greater than that measured by SMPS owing to loss of semivolatile mass inside the SMPS High volatility High yield Low volatility Medium yield High volatility Low yield Low volatility Low reactivity ACN vs MeOH Wt. Match O:C Ratio H:C Ratio DBEs MeOH vs Water Wt. Match O:C Ratio H:C Ratio DBEs ACN88% MeOH83% MeOH84% H2O85% ACN vs Water Wt. Match O:C Ratio H:C Ratio DBEs ACN vs ACN Wt. Match O:C Ratio H:C Ratio DBEs ACN79% Rep #180% H2O83% Rep #288% IRMPD OF OLIGOMERS