Characterizing the secondary organic aerosol products of ozone and α-pinene using ultrahigh-resolution FT-ICR mass spectrometry
Three samples of secondary organic aerosol (SOA) were generated by reacting a-pinene and ozone in the presence of variable concentrations of hydroxyl radical scavenging cyclohexane and were characterized by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT ICR MS). The reactions were performed in the presence of different concentrations of hydroxyl radical scavenger. This provided an opportunity to examine the molecular level differences of SOA. More than 900 chemical formulas for negative ions were identified over the mass range of 100 to 820 u. The experimental reproducibility of the SOA composition and the technical reproducibility of the mass spectra were evaluated. Similar chemical formulas with similar relative abundances were observed in all three experiments. A few exceptions were particular high relative abundance signals such as m/z 357, 367 and 539, whose production efficiency increased in the presence of cyclohexane, and m/z 185, 199, 215, 231 and 261, whose production efficiency decreased in the presence of cyclohexane. In general, the composition of a-pinene SOA was only slightly influenced by the concentration of the hydroxyl radical scavenger, cyclohexane. The negative ion spectra of the SOA contained four groups of peaks over the following mass ranges: 150 < n < 300, 300 < n < 475, 475 < n < 600, 600 < n < 850. As the molecular weight increased, a variety of changes occurred. The number of individual compounds within one nominal mass increased. The range of oxygen to carbon and hydrogen ratios decreased from group I to IV. Likewise, the mean values of oxygen to carbon decreased from 0.55 to 0.42. The mean value of hydrogen to carbon, approximately 1.5, did not change with respect to molecular weight, although the range of values did decrease. The chemical formulas of groups I and II with the highest relative abundances contained 5-7 and 7-10 oxygen atoms and double bond equivalents (DBE) of 3-4 and 5-7, respectively. The chemical formulas of groups III and IV with the highest relative abundances contained 10-13 and 13-16 oxygen atoms and DBE values of 7-9 and 9-11, respectively. Several SOA accretion mechanisms cause increases of DBE of 2 or 3 and alter the O:C and H:C ratios in different ways. Observations of the oxygen content and the DBE of the SOA products suggest they resulted from a complex mixture of accretions, such as reactions of neutral molecules with hydroperoxy or criegee radicals, hemi-acetal reactions, aldol condensations or esterification reactions. To provide insight into the formation mechanisms, the molecular structures of selected group II compounds (300 < n < 475) were investigated using ultra-high resolution MS2.
Fall Meeting 2011
Putman, Annie L.; Offenberg, John H.; Fisseha, Rebeka; Kundu, Shuvashish; Rahn, Thom A.; and Mazzoleni, Lynn, "Characterizing the secondary organic aerosol products of ozone and α-pinene using ultrahigh-resolution FT-ICR mass spectrometry" (2011). Department of Chemistry Publications. 49.