Article pubs.acs.org/est
Secondary Organic Aerosol Formation from Intermediate-Volatility Organic Compounds: Cyclic, Linear, and Branched Alkanes Daniel S. Tkacik, Albert A. Presto, Neil M. Donahue, and Allen L. Robinson* Center for Atmospheric Particle Studies, Carnegie Mellon University, Pittsburgh, PA, U.S.A. S Supporting Information *
ABSTRACT: Intermediate volatility organic compounds (IVOCs) are an important class of secondary organic aerosol (SOA) precursors that have not been traditionally included in chemical transport models. A challenge is that the vast majority of IVOCs cannot be speciated using traditional gas chromatographybased techniques; instead they are classified as an unresolved complex mixture (UCM) that is presumably made up of a complex mixture of branched and cyclic alkanes. To better understand SOA formation from IVOCs, a series of smog chamber experiments was conducted with different alkanes, including cyclic, branched, and linear compounds. The experiments focused on freshly formed SOA from hydroxyl (OH) radical-initiated reactions under high-NOx conditions at typical atmospheric organic aerosol concentrations (COA). SOA yields from cyclic alkanes were comparable to yields from linear alkanes three to four carbons larger in size. For alkanes with equivalent carbon numbers, branched alkanes had the lowest SOA mass yields, ranging between 0.05 and 0.08 at a COA of 15 μg m−3. The SOA yield of branched alkanes also depends on the methyl branch position on the carbon backbone. High-resolution aerosol mass spectrometer data indicate that the SOA oxygen-to-carbon ratios were largely controlled by the carbon number of the precursor compound. Depending on the precursor size, the mass spectrum of SOA produced from IVOCs is similar to the semivolatile-oxygenated and hydrocarbon-like organic aerosol factors derived from ambient data. Using the new yield data, we estimated SOA formation potential from diesel exhaust and predict the contribution from UCM vapors to be nearly four times larger than the contribution from single-ring aromatics and comparable to that of polycyclic aromatic hydrocarbons after several hours of oxidation at typical atmospheric conditions. Therefore, SOA from IVOCs may be an important contributor to urban OA and should be included in SOA models; the yield data presented in this study are suitable for such use.
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INTRODUCTION Secondary organic aerosols (SOA) are formed from the condensable products of oxidation reactions of organic vapors. Many atmospheric models systematically underpredict ambient SOA concentrations.1−5 There are also differences between the composition and volatility of laboratory and ambient SOA.6 For example, laboratory-generated SOA is generally less oxygenated than ambient SOA.7−10 These discrepancies could arise for a number of reasons, such as a failure to measure laboratoryproduced SOA under conditions representative of the atmosphere, the possibility of models missing atmospheric processes or sources of ambient OA,1,4 or a combination of both. Recent research has proposed that low-volatility organic vapors may be important precursors of SOA, but these vapors are often not included in models.11,12 Intermediate-volatility organic compounds (IVOCs) are an important class of lowvolatility organics; they have saturation vapor pressures (C*) between 103−106 μg m−3 (roughly equivalent to alkanes with 10−20 carbons). IVOCs exist exclusively as vapors under ambient conditions. IVOCs are abundant in gasoline- and diesel-powered vehicle exhaust,13,14 but little is known about their molecular composition. The vast majority of IVOC mass © 2012 American Chemical Society
cannot be speciated using one-dimensional gas chromatography (GC); instead, it is reported as an unresolved complex mixture (UCM).13,14 UCM likely consists of hundreds to thousands of branched alkane isomers that cannot be separated using traditional GC-based techniques. The number of alkane isomers increases exponentially with carbon number;15 even a 12-carbon alkane has more than 100 possible constitutional isomers. To investigate the effects of alkane structure on SOA formation, Lim and Ziemann16,17 measured the SOA mass yields from linear, branched, and cyclic alkanes under high-NOx conditions. They observed that for a given carbon number, SOA mass yields from alkanes followed the order of cyclic > linear > branched and attributed this trend to the likelihood of the intermediate alkoxy radicals either decomposing or isomerizing. They concluded that molecular structure is the primary driver in determining the relative SOA yields of alkanes, while the volatility of the initial precursor holds Received: Revised: Accepted: Published: 8773
March 21, 2012 June 29, 2012 July 23, 2012 July 23, 2012 dx.doi.org/10.1021/es301112c | Environ. Sci. Technol. 2012, 46, 8773−8781
Environmental Science & Technology
Article
Table 1. Alkanes Used in This Study alkane name cyclooctane cyclodecane cyclopentadecane pentylcyclohexane decylcyclohexane pristane 7-methyltridecane 2-methylundecane n-dodecane a b
alkane formula
C*a (μg m−3)
structure type
SOA yield (COA = 15 μg m−3)
C8H16 C10H18 C15H28 C11H22 C16H32 C19H40 C14H30 C12H26 C12H26
× × × × × × × × ×
cyclic cyclic cyclic branched-cyclic branched-cyclic branched branched branched linear
0.11 0.25
9.2 4.3 1.2 3.2 1.4 1.3 5.7 2.8 1
7
10 106 104 106 104 104 105 106 106
0.05
0.08 0.05 0.10b
Calculated at 298K using ACD/Laboratories’ ACD/PhysChem Suite, http://www.acdlabs.com/products/pc_admet/physchem/physchemsuite/. From Presto et al., 2010.11
The experimental setup and analysis techniques used here were similar to those used in previous SOA smog chamber studies conducted at Carnegie Mellon University.11,21,22 Briefly, experiments were performed in a 10 m3 temperature-controlled Teflon environmental chamber. Prior to each experiment, the chamber was continuously flushed for >12 h with HEPA and activated-carbon filtered air at 40 °C to reduce residual particle and vapor concentrations. Prior to an experiment, particle number concentrations were
