Abstract
Abstract. The detailed molecular composition of laboratory generated limonene ozonolysis secondary organic aerosol (SOA) was studied using ultrahigh-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. Approximately 1200 molecular formulas were identified in the SOA over the mass range of 140 to 850 Da. Four characteristic groups of high relative abundance species were observed; they indicate an array of accretion products that retain a large fraction of the limonene skeleton. The identified molecular formulas of each of the groups are related to one another by CH2, O and CH2O homologous series. The CH2 and O homologous series of the low molecular weight (MW) SOA (m/z < 300) are explained with a combination of functionalization and fragmentation of radical intermediates and reactive uptake of gas-phase carbonyls. They include isomerization and elimination reactions of Criegee radicals, reactions between alkyl peroxy radicals, and scission of alkoxy radicals resulting from the Criegee radicals. The presence of compounds with 10–15 carbon atoms in the first group (e.g. C11H18O6) provides evidence for SOA formation by the reactive uptake of gas-phase carbonyls during limonene ozonolysis. The high MW compounds (m/z > 300) were found to constitute a significant number fraction of the identified SOA components. The formation of high MW compounds was evaluated by molecular formula trends, fragmentation analysis of select high MW compounds and a comprehensive reaction matrix including the identified low MW SOA, hydroperoxides and Criegee radicals as building blocks. Although the formation of high MW SOA may occur via a variety of radical and non-radical reaction channels, the combined approach indicates a greater importance of the non-condensation reactions over aldol and ester condensation reaction channels. Among these hemi-acetal reactions appear to be most dominant followed by hydroperoxide and Criegee reaction channels.
Highlights
Organic aerosols constitute up to 90 % of the aerosol mass fraction and are known to have adverse impacts on visibility, climate, public health and biogeochemistry (Kanakidou et al, 2005; Salma et al, 2008)
1200 monoisotopic molecular formulas were identified in limonene ozonolysis secondary organic aerosols (SOA) samples
The limonene ozonolysis SOA mass spectra has four prominent clusters of high relative abundance ions. This type of clustering of high relative abundance ions was previously observed in the soft ionization mass spectra of SOA from limonene ozonolysis and other terpene ozonolysis (Reinhardt et al, 2007; Tolocka et al, 2006; Walser et al, 2008)
Summary
Organic aerosols constitute up to 90 % of the aerosol mass fraction and are known to have adverse impacts on visibility, climate, public health and biogeochemistry (Kanakidou et al, 2005; Salma et al, 2008). In another study using high resolution mass spectrometry, Walser et al (2008) observed a higher number of low MW components in SOA from limonene ozonolysis They described the formation of additional compounds with alkyl peroxy and alkoxy radical reactions. To evaluate the reaction channels, Heaton et al (2007) created a reaction matrix of building units to explain the observed high MW compound formation in monoterpene (β-pinene, carene, limonene and sabinene) ozonolysis SOA. This approach resulted in individual high MW compounds formed by different reaction channels.
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