| Abstract
| - Organic aerosols are a major fraction, often more than50%, of the total atmospheric aerosol mass. The chemicalcomposition of the total organic aerosol mass is poorlyunderstood, although hundreds of compounds have beenidentified in the literature. High molecular weight compounds have recently gained much attention because thisclass of compounds potentially represents a major fractionof the unexplained organic aerosol mass. Here we analyzesecondary organic aerosols, generated in a smog chamberfrom α-pinene ozonolysis with ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry(FTICR-MS). About 450 compounds are detected in themass range of m/z 200−700. The mass spectrum isclearly divided into a low molecular weight range (monomer) and a high molecular weight range, where dimersand trimers are distinguishable. Using the Kendrick massanalysis, the elemental composition of about 60% of allpeaks could be determined throughout the whole massrange. Most compounds have high O:C ratios between 0.4and 0.6. Small compounds (i.e., monomers) have a highermaximum O:C ratio than dimers and trimers, suggestingthat condensation reactions with, for example, the loss ofwater are important in the oligomer formation process. Aprogram developed in-house was used to determine exactmass differences between peaks in the monomer, dimer,and trimer mass range to identify potential monomerbuilding blocks, which form the co-oligomers observedin the mass spectrum. A majority of the peaks measuredin the low mass region of the spectrum (m/z< 300) isalso found in the calculated results. For the first time theelemental composition of the majority of peaks over a widemass range was determined using advanced data analysismethods for the analysis of ultra-high-resolution MS data.Possible oligomer formation mechanisms in secondaryorganic aerosols were investigated.
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