Abstract
Abstract. Long-chain alkanes, which can be categorized as intermediate volatility organic compounds, are an important source of secondary organic aerosol (SOA). Mechanisms for the gas-phase OH-initiated oxidation of long-chain alkanes have been well documented; particle-phase chemistry, however, has received less attention. The δ-hydroxycarbonyl, which is generated from the isomerization of alkoxy radicals, can undergo heterogeneous cyclization and dehydration to form substituted dihydrofuran. Due to the presence of C=C bonds, the substituted dihydrofuran is predicted to be highly reactive with OH, and even more so with O3 and NO3, thereby opening a reaction pathway that is not usually accessible to alkanes. This work focuses on the role of substituted dihydrofuran formation and its subsequent reaction with OH, and more importantly ozone, in SOA formation from the photooxidation of long-chain alkanes. Experiments were carried out in the Caltech Environmental Chamber using dodecane as a representative alkane to investigate the difference in aerosol composition generated from "OH-oxidation-dominating" vs. "ozonolysis-dominating" environments. A detailed mechanism incorporating the specific gas-phase photochemistry, together with the heterogeneous formation of substituted dihydrofuran and its subsequent gas-phase OH/O3 oxidation, is used to evaluate the importance of this reaction channel in dodecane SOA formation. We conclude that (1) the formation of δ-hydroxycarbonyl and its subsequent heterogeneous conversion to substituted dihydrofuran is significant in the presence of NOx; (2) the ozonolysis of substituted dihydrofuran dominates over the OH-initiated oxidation under conditions prevalent in urban and rural air; and (3) a spectrum of highly oxygenated products with carboxylic acid, ester, and ether functional groups are produced from the substituted dihydrofuran chemistry, thereby affecting the average oxidation state of the SOA.
Highlights
Alkanes are important constituents of gasoline and vehicle emissions (Hoekman, 1992; Zielinska et al, 1996; Kirchstetter et al, 1999; Gentner et al, 2012; Jathar et al, 2013), accounting for ∼50 % of volatile organic compounds (VOCs) in the urban atmosphere (Fraser et al, 1997; Schauer et al, 1999 and 2002)
The unresolved complex mixture (UCM) of organics, which is potentially a significant source of secondary organic aerosol (SOA) formation in the atmosphere, has recently been shown to comprise many long-chain alkanes (Isaacman et al, 2012), which are expected to contribute to SOA formation (Robinson et al, 2007)
The changes in slopes along with changes in RH values are consistent for both ions. This indicates that the formation of the carboxylic acid functional group detected in particles is associated with the water vapor concentration in the gas phase, consistent with the reaction of the stabilized Criegee intermediates with water in the substituted dihydrofuran oxidation system
Summary
Alkanes are important constituents of gasoline and vehicle emissions (Hoekman, 1992; Zielinska et al, 1996; Kirchstetter et al, 1999; Gentner et al, 2012; Jathar et al, 2013), accounting for ∼50 % of volatile organic compounds (VOCs) in the urban atmosphere (Fraser et al, 1997; Schauer et al, 1999 and 2002). The unresolved complex mixture (UCM) of organics, which is potentially a significant source of secondary organic aerosol (SOA) formation in the atmosphere, has recently been shown to comprise many long-chain alkanes (Isaacman et al, 2012), which are expected to contribute to SOA formation (Robinson et al, 2007). Particle-phase products from OH oxidation of alkanes contain a number of functional groups: organonitrate (–ONO2), hydroxyl (–OH), carbonyl (–C=O), ester (–C(O)O–), and hydroxyperoxide (–OOH). Gas-phase products that are unique to the substituted dihydrofuran chemistry are identified. The presence of water vapor can, in prin- substituted dihydrofuran chemistry and simulate the effect of methylfurancibpalcek, setrove5t-ohcyodnrovxeyrt-2th-epe4n,5ta-ndoihnyed, role-a2d-minegthtyolfuarnanebqauciklibtorium this reaction channel on SOA yield from the photooxidation e two specie5s-hwyidtrhoinxys-e2v-epreanltahnoounrse,(Mleadrtiinngetoala.n, 2e0q0u2i;libBraiukmer ebtetawl.e, e2n005; of dodecane. 005; Reisen etht easl.e, 2tw00o5s)p: ecies within several hours (Martin et al, 2002; Baker et al, 2005; Holt et al, 2005; Reisen et al, 2005):
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