Article pubs.acs.org/est
Intermediate-Volatility Organic Compounds: A Large Source of Secondary Organic Aerosol Yunliang Zhao,† Christopher J. Hennigan,†,⊥ Andrew A. May,†,∥ Daniel S. Tkacik,† Joost A. de Gouw,‡ Jessica B. Gilman,‡ William C. Kuster,‡ Agnes Borbon,‡,§ and Allen L. Robinson*,† †
Center for Atmospheric Particle Studies, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Chemical Science Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, United States § Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS UMR7583, Université Paris Est Créteil, Université Paris Diderot, IPSL, Créteil 94010, France S Supporting Information *
ABSTRACT: Secondary organic aerosol (SOA) is a major component of atmospheric fine particle mass. Intermediate-volatility organic compounds (IVOCs) have been proposed to be an important source of SOA. We present a comprehensive analysis of atmospheric IVOC concentrations and their SOA production using measurements made in Pasadena, California during the California at the Nexus of Air Quality and Climate Change (CalNex) study. The campaign-average concentration of primary IVOCs was 6.3 ± 1.9 μg m−3 (average ± standard deviation), which is comparable to the concentration of organic aerosol but only 7.4 ± 1.2% of the concentration of speciated volatile organic compounds. Only 8.6 ± 2.2% of the mass of the primary IVOCs was speciated. Almost no weekend/weekday variation in the ambient concentration of both speciated and total primary IVOCs was observed, suggesting that petroleum-related sources other than onroad diesel vehicles contribute substantially to the IVOC emissions. Primary IVOCs are estimated to produce about 30% of newly formed SOA in the afternoon during CalNex, about 5 times that from single-ring aromatics. The importance of IVOCs in SOA formation is expected to be similar in many urban environments.
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INTRODUCTION Atmospheric fine particulate matter has important impacts on climate and human health. Secondary organic aerosol (SOA) contributes one-third or more of fine particle mass in many regions of the atmosphere.1 SOA is formed from gas-phase oxidation, heterogeneous reactions on aerosol surfaces, and multiphase chemistry of gas-phase organic compounds.2 SOA chemistry is complex and the contribution of these different pathways is not well understood. Robinson et al.3 proposed that intermediate-volatility organic compounds (IVOCs) were an important class of SOA precursors. IVOCs are compounds with effective saturation concentrations (C*) between 103 and 106 μg/m3; this roughly corresponds to the volatility range of C12−C22 n-alkanes. Laboratory experiments indicate that IVOCs can form SOA with high yields,4−6 but their atmospheric abundance and contribution to ambient SOA are poorly constrained. Speciated IVOCs, such as polycyclic aromatic hydrocarbons, have been routinely measured in the atmosphere. 7−10 Atmospheric concentrations of speciated IVOCs are generally lower than VOCs9,10 and speciated IVOCs appear to modestly contribute to SOA formation.11 However, the dominant fraction of atmospheric IVOCs is not resolved by traditional gas-chromatographic analysis. Instead it appears as an unresolved complex mixture (UCM) of coeluting compounds9 © 2014 American Chemical Society
because of the exponential increase in the number of constitutional isomers with increasing carbon number.12 Quantitative measurements of total atmospheric IVOCs (speciated + UCM) are needed to better constrain their major sources and their contribution to SOA formation. In this study, IVOCs were measured in Pasadena, CA during the California at the Nexus of Air Quality and Climate Change (CalNex) study. Concentrations of both speciated and unspeciated IVOCs were quantified and weekday and weekend diurnal patterns were characterized to investigate source contributions to atmospheric IVOCs. The IVOC data were then implemented into a box model to assess their contribution to ambient SOA formation.
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MATERIALS AND METHODS
Field Sampling and Chemical Analysis. Ambient sampling was conducted in Pasadena, CA during CalNex from May 17th to June 11th, 2010. IVOCs were collected by sampling ambient air through Tenax TA filled glass tubes (Gerstel 6 mm OD, 4.5 mm ID glass tube filled with ∼290 mg
Received: Revised: Accepted: Published: 13743
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of Tenax TA) at a flow rate of 0.5 L/min. Six sets of composite samples were collected to characterize average weekday and weekend diurnal profiles. Each set of samples consisted of 10 Tenax tubes: 8 tubes were used to collect ambient samples over 3 h intervals evenly distributed throughout the day (midnight to 3 AM, 3 AM to 6 AM, etc.); the ninth tube was exposed to ambient air but no active air flow for the entire sampling period to serve as a field blank; the final tube served as a handling blank. For a weekday composite sample, one set of tubes was used over the work week (Monday−Friday). For a weekend composite sample, the same strategy was employed over two weekends (two Saturdays and two Sundays) in order to increase sampled mass; these sets of tubes were stored in a freezer at −18 °C between weekends. In total, four sets of composite weekday and two sets of composite weekend samples were collected. This sampling strategy was employed to ensure the collection of sufficient IVOC mass and to capture repeatable atmospheric processes and variations in source emissions. The Tenax samples were analyzed by a gas chromatography (GC)/mass spectrometry (MS) system (Agilent, 6890 GC/ 5975 MS) equipped with a thermal desorption sample extraction and injection system (Gerstel, Baltimore, MD) and a capillary column (Agilent HP-5MS, 30 m × 0.25 mm). The recovery of IVOCs during analysis was tracked using deuterated standards that were spiked into each Tenax tube prior to the thermal desorption. Twenty individual IVOC compounds were quantified using authentic standards. The speciated IVOCs had an average ambient-to-blank ratio greater than 6.0, except for ndecylcyclohexane, with a ratio of 5.2. Individual VOCs were measured using an in situ GC/MS.13,14 See the Supporting Information for a detailed description of instrumentation and analytical methods used in this study. Quantification of IVOCs. Figure 1a shows chromatograms of a typical ambient Tenax sample collected during CalNex. Although there are many individual peaks in the GC/MS data, the vast majority of the total ion current (TIC) appears as a
broad hump of coeluting hydrocarbons and oxygenated compounds that cannot be speciated. These unspeciated compounds are commonly referred to as the UCM. To calculate the total mass of IVOCs (UCM + speciated IVOCs), the TIC of each IVOC sample was binned on the basis of the retention time of n-alkanes (Supporting Information). The n-alkanes were used as the metric to bin IVOCs because their saturation concentrations vary systematically with carbon number15,16 and they were detected across the entire IVOC range. Each bin is defined by the carbon number of one n-alkane (Figure 1a). For example, the bin with heptadecane (C 17 ) is defined as B 17 . Presto et al. 15 demonstrates that there is a near linear relationship between retention time and saturation concentration (C*) for hydrocarbons measured with the same analytical protocol. Therefore, binning the chromatogram by retention time provides the volatility distribution of compounds that are both collected on the Tenax sorbent and detected in the GC/MS. This method does not provide a carbon number distribution; for example, in addition to heptadecane, other compounds that elute in the B17 bin include pristane (C19) and 7-methylheptadecane (C18). The IVOC mass of each bin (MIVOCs,Bn) is calculated using the approach of Fraser et al.:9 MIVOCs,Bn =
TA TIC,Bn RFn‐alkane,Cn
=
TA m / z57,Bn RFn‐alkane,Cn
×
1 fm / z57,TIC B
n
(1)
where TATIC,Bn is the abundance of the TIC in the Bn bin; RFn‑alkane,Cn is the response factor of the Cn n-alkane; TAm/z57,Bn is the abundance (integrated area) of the m/z 57 fragment (m/ z 57) in the Bn bin; f m/z57,TIC Bn is the fraction of the m/z 57 in the TIC of the Bn bin of ambient IVOCs. Finally, the mass of the UCM in each bin is determined by subtracting off the contribution of speciated IVOCs from the IVOC mass of each bin. By using the abundance of the TIC (TATIC,Bn) to quantify the mass of IVOCs, this method likely provides a conservative estimate of the true atmospheric concentration of IVOCs. Atmospheric IVOCs are a complex mixture of hydrocarbons and oxygenated compounds that are both emitted by sources and formed in the atmosphere from photochemical reactions. First, oxygenated compounds only partially elute from the GC column in their underivatized form;17 our estimate does not account for any uneluted organics. Second, oxygenated compounds typically have lower response factors than hydrocarbons,18 but we convert the entire IVOC TIC signal to mass concentrations using hydrocarbon response factors (eq 1). Both of these factors will lead to an underestimate of the IVOC mass. IVOCs directly emitted into the atmosphere (primary IVOCs) have been proposed as an important class of SOA precursors.3 In order to examine the contribution of primary IVOCs to SOA formation, the ambient IVOCs first need to be separated into primary and secondary components. The speciated IVOCs are all primary.19 The mass of the primary IVOC UCM in each bin is determined by the difference between the mass of primary IVOCs and the mass of speciated IVOCs in the same bin. The mass of primary IVOCs is estimated using the signal of m/z 57 measured in atmospheric IVOCs and the fraction of m/ z 57 in the TIC (f m/z57,TIC Bn) measured in mobile source
Figure 1. (a) Chromatograms from a typical CalNex Tenax sorbent sample; (b) average concentrations of quantified primary IVOCs in each bin during the field campaign. 13744
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emissions because this is likely a dominant source of primary IVOCs in the Los Angeles area. During the CalNex study, there was little impact from biomass burning.20,21 Cooking emissions also do not appear to be a major source of IVOCs.20,22 In addition, Chan et al.21 reported that hydrocarbons in the carbon number range of C21 to C24 (at the top end of the IVOC range) were from mobile sources during CalNex. The m/z 57 was chosen as the quantification ion because it was used as the quantification ion for n-alkanes and detected across the entire IVOC retention time. Figure 1b shows the application of this method to the chromatogram in Figure 1a. To estimate the fraction of m/z 57 in primary IVOCs, we measured the fraction of m/z 57 in the TIC derived from GC/ MS analysis of Tenax samples collected in a highway tunnel in Pittsburgh, Pennsylvania and during chassis dynamometer tests of individual on- and off-road engines (Supporting Information). In the tunnel samples (n = 8), m/z 57 contributed 4.0% ± 0.3% of the TIC. The average fraction of m/z 57 in primary emissions from both on- and off-road engines ranged from 2.3% to 5%. Using a weighted average approach based on the fuel consumption in the South Coast Air Basin, which encompasses the CalNex sampling site, the overall fraction of m/z 57 derived from dynamometer source tests was 3.3%. The value is somewhat lower than the tunnel data because of the influence of small off-road gasoline sources. Because the highway tunnel measurements characterized a much larger vehicle fleet with a similar gasoline-diesel split as the Los Angeles area, the fraction of m/z 57 of primary IVOCs in our study was derived from the highway tunnel data.
Figure 2. Weekday and weekend distributions of speciated IVOCs and primary IVOC UCM bins. (a) Concentrations of speciated and primary IVOC UCM bins. (b) Weekend-to-weekday ratio for average concentrations of speciated and primary IVOC UCM. The weekendto-weekday ratios of EC and NOx plotted in panel b are from Bahreini et al.26 Speciated IVOCs are grouped into linear alkanes, branched alkanes, alkylcyclohexanes and aromatics. The “Total” category on the x-axis refers to the sum of speciated IVOCs and the sum of primary IVOC UCM bins. In the box plots, the center line of each box is the median of the data, the top and bottom of the box are 75th and 25th percentiles and top and bottom whiskers are 90th and 10th percentiles.
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RESULTS AND DISCUSSIONS Ambient Concentrations of Primary IVOCs. Ambient IVOCs are a complex mixture of organics contributed by both primary emissions and photochemical oxidation of gas-phase organics. Based on the fraction of m/z 57 measured in a highway tunnel, 60 ± 17% of the IVOC signals measured during CalNex were primary with the balance being contributed by oxygenated compounds formed through photochemical reactions. Dynamometer tests of individual on- and off-road engines suggest that the primary IVOC contribution was even higher during CalNex(∼70%), indicating that the use of the tunnel samples does not likely overstate the contribution of primary IVOCs to SOA. In addition, the average concentration of primary IVOCs calculated using the fraction of m/z 57 measured in a highway tunnel is consistent with the one calculated using the fraction of n-alkanes in primary IVOCs measured in the same highway tunnel. The difference between two campaign-average concentrations of primary IVOCs is less than 10% with the greater one being calculated using the fraction of n-alkanes in primary IVOCs. The campaign-average concentration of primary IVOCs (the sum of primary IVOC UCM and speciated IVOCs) was 6.3 ± 1.9 μg/m3 (average ± one standard deviation), with only 8.6 ± 2.2% of them being speciated (Figure 2). The average concentration of primary IVOCs is similar to that of organic PM1, 7.0 μg/m3,20 and single-ring aromatics (C6−C9), 6.6 ± 0.2 μg/m3, but is only 7.4 ± 1.2% of the total speciated VOCs (Figure 4). Figure 2 presents concentration data for the speciated IVOCs, including n-alkanes, branched alkanes (b-alkanes), alkylcyclohexanes and aromatic hydrocarbons. Campaignaverage concentrations of speciated IVOCs ranged from 2.0 to 81.0 ng/m3, which are less than most primary speciated
VOCs.23 Concentrations of most speciated IVOCs during CalNex are also a factor of 4 lower than those in 1993,9,10 highlighting the substantial reduction in organic emissions that has occurred over the last two decades.24 Figure 3 shows primary IVOCs and VOCs have similar average weekday and weekend diurnal patterns. There is an afternoon peak associated with the transport of morning emissions from downtown Los Angeles to the sampling site and a nighttime peak associated with trapping of fresh emissions in a shallow boundary layer.14 The average mass concentration of primary IVOCs decreases monotonically with decreasing volatility (Figures 1 and 2a), in qualitative agreement with prior studies.9 The total mass concentration of primary IVOCs in Pasadena is consistent with the predictions based on the volatility distribution of Robinson et al.3 and results from factor analysis of ambient aerosol mass spectrometer data.20 However, the mass distribution of measured primary IVOCs in each volatility bin differs somewhat from the prediction (Figure S1, Supporting Information). Weekday/Weekend Patterns and Sources of IVOCs. Figure 2 compares weekday and weekend concentrations of both speciated IVOCs and the primary IVOC UCM. The average weekday-to-weekend ratio for speciated IVOCs ranges from 0.9 to 1.3 and 0.9 to 1.1 for the UCM bins, except for the B21 bin (Figure 2b). The lack of a weekday/weekend pattern provides insight into the possible source(s) of IVOCs. Gentner et al.25 proposed that on-road diesel vehicles were a major source of IVOC emissions in the Los Angeles area. In California’s South Coast Air Basin, which encompasses the 13745
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Pristane, phytane and primary IVOC UCM have similar average concentrations between weekdays and weekends, suggesting that the major IVOC source(s) are likely petroleum-related.9,28 Off-road mobile sources could be one of the candidates for the high contribution to weekend IVOC emissions. The activity of off-road mobile sources such as residential lawn, garden and recreational equipment, is greater on weekends than on weekdays;29 however, more work is needed to characterize their emissions. Estimation of SOA Production. In this section, we estimate the SOA production from measured IVOC and SVOC data. The analysis focuses on the early afternoon time period (12:00−15:00) when the peak CalNex SOA concentrations were observed.20 The next section compares the predicted SOA production to SOA concentration data of Hayes et al.20 The SOA formation from the oxidation of gas-phase organic compounds can be estimated by5,30 ⎛ 1 − e−kOH,i[OH]Δt ⎞ ΔMSOA, i = [HCi] × ⎜ −k [OH]Δt ⎟ × Yi ⎝ e OH,i ⎠
(2)
Where [HCi] is the measured concentration of SOA precursor i; Yi is its SOA yield and kOH,i is its reaction rate constant with OH radicals (cm3 molecule−1 s−1); [OH] is the OH radical concentration; Δt is the OH radical exposure time. The OH exposure, [OH]Δt (molecules cm−3 s), is estimated by the ratio of 1,2,4-trimethylbenzene to benzene.14,20,31 To estimate the SOA formation from speciated compounds (VOCs and IVOCs), the OH reaction rate constant of each compound is taken from Atkinson and Arey32 or estimated on the basis of a structure−reactivity relationship (Tables S5 and S6, Supporting Information).33 SOA mass yields for speciated compounds are taken from published parametrizations of smog chamber results under high-NOx conditions (Hayes et al.20 showed that the high-NOx channel was dominant, with mean branching ratios over 90% during CalNex). These yields (Yi) are estimated at an organic aerosol (OA) concentration of 12 μg m−3, which corresponds to the average OA concentration during the 12:00−15:00 time period.20 SOA formed by primary IVOC UCM is also estimated by eq 2 using SOA yields of surrogate compounds (n-alkanes) assigned to each IVOC UCM bin. These surrogate compounds represent conservative estimates of the SOA yields; they are selected on the basis of the estimated distribution of known balkanes and cyclic compounds in each bin and published highNOx smog chamber data for speciated n-alkanes, b-alkanes and cyclic compounds of similar volatility (Table S7, Supporting Information). Primary IVOC UCM is thought to be dominated by a complex mixture of branched and cyclic hydrocarbons.9,21,25 For species with the same carbon number, branched hydrocarbons generally form less SOA than cyclic hydrocarbons and react at different rates than cyclic alkanes.4,21,34 Therefore, we classified the primary IVOC UCM into two lumped groups, unspeciated b-alkanes and the residual primary IVOC UCM (Figure 1), to provide better observational constraints on OH reaction rate constants and SOA yields of primary IVOC UCM. The unspeciated b-alkane component is calculated by replacing the TAm/z57,Bn and f m/z57,TIC Bn in eq 1 with the abundance of m/z 57 produced by b-alkanes and the average fraction of m/z 57 in the TIC of b-alkanes, respectively. The abundance of m/z 57 produced by b-alkanes is calculated by assuming that all of m/z 57 signal is only produced by n- and b-
Figure 3. Average diurnal patterns of primary IVOCs and single-ring aromatics on weekdays and weekends. The concentrations in each plot are normalized to the campaign average value of each compound. Single-ring aromatics are the sum of benzene, toluene, o-, p-, m-xylene, 1-ethyl-2-methylbenzene, 1-ethyl-3-methylbenzene, 1-ethyl-4-methylbenzene, 1,3,5-trimethybenzene, 1,2,4-trimethylbenezen and 1,2,3trimethylbenzene.
CalNex site, on-road diesel vehicle activity during weekends is less than half that on weekdays.20,26 This pattern drives large weekday/weekend changes in certain pollutant concentrations; for example, Figure 2 shows that there was roughly a factor of 2 difference in weekday/weekend NOx and elemental carbon (EC) concentrations during CalNex.26 The difference in the weekday/weekend pattern of IVOCs compared to NOx and EC strongly suggests that a source other than on-road diesel vehicles dominates the emissions of IVOCs. Or, if on-road diesel vehicle emissions contribute significantly to weekday IVOC concentrations; then an unidentified weekend source of IVOCs compensates for the reduction in the on-road diesel activity. The diurnal pattern of IVOC concentrations is similar to single-ring aromatics (Figure 3), which are dominated by onroad gasoline vehicle emissions.25,26 However, the emission factors of n-alkanes from gasoline vehicles show a steeper decrease in the carbon range of C12 to C1525,27 than our observations. Therefore, the similarity in IVOC and single ring aromatic diurnal patterns may be due to mixing during transport to the Pasadena area from other parts of the air basin. 13746
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alkanes and then subtracting off the abundance of m/z 57 produced by n-alkanes (all of which were individually quantified). The average fraction of m/z 57 in the TIC of balkanes (18% ± 6%) is calculated from the mass spectra of 11 b-alkanes (Tables S1 and S2, Supporting Information). The residual primary IVOC UCM (primary IVOC UCM minus estimated unspeciated b-alkane component) is likely composed of both cyclic alkanes and alkylated aromatics,25 but studies focusing on vehicle emissions suggest that UCM hydrocarbons are dominated by cyclic alkanes.18,19 We therefore classify all of the residual primary IVOC UCM as unspeciated cyclic alkanes. The estimated campaign average concentration of unspeciated b-alkanes is 1.1 ± 0.4 μg/m3; the residual primary IVOC UCM (4.7 ± 1.4 μg/m3) is therefore classified as unspeciated cyclic alkanes. This approach likely overestimates the abundance of b-alkanes because other compounds in addition to n- and b-alkanes also produce m/z 57 in the MS. This, in turn, leads to the underestimation of SOA production from primary IVOC UCM because SOA yields of b-alkanes are lower than cyclic alkanes and n-alkanes with the same carbon number.4,34 Following the separation of unspeciated b-alkanes and cyclic alkanes, surrogate compounds (n-alkanes) are assigned for unspeciated b-alkanes and cyclic components in each UCM bin to estimate the SOA yields and OH reaction rate constants (Table S7, Supporting Information). The SOA yield of the unspeciated b-alkanes in the Bn bin is represented using the nalkane whose carbon number is equal to bin number minus two (e.g., b-alkanes in the B 17 bin are represented using pentadecane). This shift roughly corresponds to a mean balkane structure with four methyl branches and accounts for the effects of branching on the elution time and SOA yield (Supporting Information). This is likely a conservative estimate (lower bound for SOA production) because the analysis of diesel fuel indicates that the maximum (not average) branching is four methyl groups in the carbon number range of 16−21.35 By accounting for the effects of branching on the OH reaction rate constant and retention time,21,33,35 the OH reaction rate constant of unspeciated b-alkanes in the Bn bin is represented by the n-alkane with the carbon number equal to the bin number (e.g., unspeciated b-alkanes in the B17 bin are represented using heptadecane). Both the SOA yield and OH reaction rate constant of unspeciated cyclic components are represented by the n-alkane with the same carbon number as the bin number (Supporting Information). With 60%, on average, of the ambient IVOC signal classified as primary IVOCs based on the fraction of m/z 57, the remaining 40% of the IVOC signal is attributed to oxygenated compounds (grouped as oxygenated IVOC UCM), which represent another class of SOA precursors. These oxygenated IVOCs are likely produced from gas-phase oxidation of organic compounds because they cannot be accounted for by the fraction of m/z 57 derived from highway tunnel sampling (which is overwhelmingly primary). To estimate SOA production from the oxygenated IVOC UCM, we used the average concentration of the oxygenated IVOC UCM during the 6:00−9:00 sample. Although the concentration of the oxygenated IVOC UCM increased during the day, this approach avoids double counting the contribution of sameday hydrocarbon oxidation. The overall SOA yield of the oxygenated IVOC UCM is represented using an n-alkane whose carbon number is equal to the bin number minus three (e.g., oxygenated components in the B17 bin are represented
using tetradecane). However, the OH reaction rate is represented by the n-alkane with the carbon number equal to the bin number. Again these likely are conservative assumptions that will lead to a lower estimate of SOA production (Supporting Information). Primary semivolatile organic compounds (SVOCs) are another important class of SOA precursors.3 The concentration of primary SVOC vapors is estimated to be 0.6 μg m−3 based on the average concentration of the hydrocarbon-like OA factor during the 12:00−15:00 period20 and the volatility distribution from Robinson et al.3 The SOA production from primary SVOCs is estimated assuming that their overall SOA yield is same as the C17 n-alkane and OH reaction rate constant is same as the C21 n-alkane. Given the relatively low concentration of SVOCs compared to both IVOCs and VOCs, these assumptions have relatively little effect on the total predicted SOA. All of the SOA yield estimates used here are based on laboratory smog chamber data measured under high-NOx conditions. These data have been corrected for particle losses to the chamber walls, but not vapor losses. A recent study suggests that high-NOx SOA mass yields used here may be underestimated by a factor of ∼1.25 due to vapor loses (see Table 1 in Zhang et al.36). (Zhang et al.36 estimated that there were much larger biases in SOA yield data from experiments with slow chemistry; all of the yields are from experiments that used HONO as the OH radical source, which leads to more rapid chemistry and lower vapor losses.) Given the uncertainty in vapor losses, we have not corrected yield data used here for them. This means that our calculations likely underestimate the total SOA production. However, correcting for vapor losses will likely not significantly alter the relative contributions of different classes of precursors (e.g., single ring aromatics versus primary IVOCs) to SOA. Vapor losses occur in experiments with all SOA precursors (VOCs and IVOCs); they are not restricted to the relatively few species considered in Zhang et al.36 These losses depend on both the properties of the precursor (vapor pressures) and the relative condensation rate between the particles and the walls (which depends on experimental conditions such as reaction rate, seed particle surface area, turbulence and the accommodation coefficient). In fact, vapor losses may be more significant for IVOCs than for VOCs;37 therefore, if anything, not correcting for vapor losses may result in a larger underestimate for the IVOC yields relative to VOC yields.
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DISCUSSIONS AND ATMOSPHERIC IMPLICATIONS Figure 4 compares the estimated SOA production from VOCs and IVOCs to the semivolatile oxygenated organic aerosol (SVOOA) factor derived from factor analysis of Aerodyne aerosol mass spectrometer data collected during CalNex. The SV-OOA factor is commonly associated with fresh SOA formed in urban environments.20 During CalNex, the concentration of SV-OOA peaked during the early afternoon (12:00−15:00).20 Figure 4 shows that VOCs, IVOCs and SVOCs together are predicted to produce 2.5 μg m−3 of SOA during the 12:00− 15:00 period. This corresponds to 50% of the SV-OOA factor. Only 12% of the predicted SOA is from VOCs (mainly single ring aromatics); 12% is from speciated IVOCs; 45% is from primary IVOC UCM; 19% is from oxygenated IVOC UCM; 12% is from unspeciated primary SVOC. The estimated SOA from primary IVOCs (UCM + speciated compounds) is approximately 5 times that produced from 13747
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precursors, especially IVOCs, into the SOA model in addition to VOCs substantially improves model predictions, helping to move the analysis much closer toward a mass closure. We expect that IVOCs are an important class of SOA precursors in most urban environments. However, the sources of these IVOCs are not well understood. Our results indicate that the petroleum sources other than on-road vehicles likely contribute substantially to primary IVOCs. Given their importance as SOA precursors demonstrated by this study, there is a need for further identification and quantification of IVOC emissions from a range of sources.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 4. Average concentrations of measured and estimated gasphase organics, predicted SOA mass and SV-OOA factor derived by Hayes et al.20 in Pasadena, CA during CalNex.
Detailed description sampling and quantification of IVOCs, estimation of SOA production from both VOCs and IVOCs and data used in the discussions. This material is available free of charge via the Internet at http://pubs.acs.org.
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single-ring aromatics, which is the dominant class of anthropogenic SOA precursors in many models.16 Therefore, primary IVOCs, less than 10% of the mass of VOCs, were a major class of SOA precursors during CalNex. Furthermore, unspeciated precursors (mainly IVOCs, but also SVOCs) contribute the majority of the predicted SOA mass, consistent with recent analyses of smog chamber experiments performed with dilute vehicle exhaust.38 The overall SOA yield of the primary IVOCs are 29%, which is comparable to estimates (10−40%) from Jathar et al.38 for unspeciated organic emissions from combustion sources. Given their unknown composition, the SOA yields of the primary IVOC UCM are uncertain. A lower bound estimate can be made by treating the IVOC UCM entirely as b-alkanes. Under this assumption, the SOA formed from primary IVOCs is 1.0 ± 0.7 μg/m3 during the 12:00−15:00 period with an overall SOA yield of 21% versus 1.4 ± 0.9 μg/m3 μg m−3 with an overall SOA yield of 29% for the best estimate. Even under this lower limit, the amount of SOA from primary IVOC UCM is substantial and dominates the SOA production over speciated IVOCs and VOCs, contributing about 60% of the predicted SOA from primary IVOCs and VOCs. This underscores the need to account for organic compounds of intermediate volatility that cannot be speciated with traditional analysis methods (e.g., 1-D GC). As discussed previously, the predictions shown in Figure 4 have not been corrected for vapor losses. A best correction using the bias estimates listed in Table 1 of Zhang et al.36 increases the predicted SOA production from VOCs from 0.3 to 0.35 μg m−3, which corresponds to 14% of the predicted SOA mass. Even after correcting for vapor losses, primary IVOCs are still predicted to contribute 56% of the predicted SOA (versus 57% for the uncorrected estimate shown in Figure 4). Therefore, the conclusion that primary IVOCs are a dominant class of SOA precursors is not altered by biases in yield estimates associated vapor losses to smog chamber walls. The amount of predicted SOA shown in Figure 4 is conservative. First, we have not corrected the SOA yield data for vapor losses. Second, the SOA yields assigned to each volatility bin are conservative (low). Third, the concentration of oxygenated IVOCs is likely underestimated. Accounting for all these factors could easily close the mass balance between the measured and predicted SOA in Figure 4. The comparison shown in Figure 4 demonstrates that including more quantified and observationally constrained
AUTHOR INFORMATION
Corresponding Author
*A. L. Robinson. E-mail: [email protected]. Phone: 4122683657. Present Addresses ⊥
C. J. Hennigan: Department of Civil, Environmental, and Geodetic Engineering, University of Maryland, Baltimore, Maryland 21250. ∥ A. A. May: Department of Civil, Environmental, and Geodetic Engineering, The Ohio State University, Columbus, Ohio 43210. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Carnegie Mellon University was funded by the U.S. Environmental Protection Agency National Center for Environmental Research through the STAR program (Project RD834554). The views, opinions, and/or findings contained in this paper are those of the authors and should not be construed as an official position of the funding agencies.
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REFERENCES
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(18) Worton, D. R.; Isaacman, G.; Gentner, D. R.; Dallmann, T. R.; Chan, A. W. H.; Ruehl, C. R.; Kirchstetter, T. W.; Wilson, K. R.; Harley, R. A.; Goldstein, A. H. Lubricating oil dominates primary organic aerosol emissions from motor vehicles. Environ. Sci. Technol. 2014, 48 (7), 3698−3706. (19) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of emissions from air pollution sources. 2. C-1 through C-30 organic compounds from medium duty diesel trucks. Environ. Sci. Technol. 1999, 33 (10), 1578−1587. (20) Hayes, P. L.; Ortega, A. M.; Cubison, M. J.; Froyd, K. D.; Zhao, Y.; Cliff, S. S.; Hu, W. W.; Toohey, D. W.; Flynn, J. H.; Lefer, B. L.; Grossberg, N.; Alvarez, S.; Rappenglueck, B.; Taylor, J. W.; Allan, J. D.; Holloway, J. S.; Gilman, J. B.; Kuster, W. C.; De Gouw, J. A.; Massoli, P.; Zhang, X.; Liu, J.; Weber, R. J.; Corrigan, A. L.; Russell, L. M.; Isaacman, G.; Worton, D. R.; Kreisberg, N. M.; Goldstein, A. H.; Thalman, R.; Waxman, E. M.; Volkamer, R.; Lin, Y. H.; Surratt, J. D.; Kleindienst, T. E.; Offenberg, J. H.; Dusanter, S.; Griffith, S.; Stevens, P. S.; Brioude, J.; Angevine, W. M.; Jimenez, J. L. Organic aerosol composition and sources in Pasadena, California, during the 2010 CalNex campaign. J. Geophys. Res. 2013, 118 (16), 9233−9257. (21) Chan, A. W. H.; Isaacman, G.; Wilson, K. R.; Worton, D. R.; Ruehl, C. R.; Nah, T.; Gentner, D. R.; Dallmann, T. R.; Kirchstetter, T. W.; Harley, R. A.; Gilman, J. B.; Kuster, W. C.; deGouw, J. A.; Offenberg, J. H.; Kleindienst, T. E.; Lin, Y. H.; Rubitschun, C. L.; Surratt, J. D.; Hayes, P. L.; Jimenez, J. L.; Goldstein, A. H. Detailed chemical characterization of unresolved complex mixtures in atmospheric organics: Insights into emission sources, atmospheric processing, and secondary organic aerosol formation. J. Geophys. Res. 2013, 118 (12), 6783−6796. (22) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of emissions from air pollution sources. 1. C-1 through C-29 organic compounds from meat charbroiling. Environ. Sci. Technol. 1999, 33 (10), 1566−1577. (23) Warneke, C.; de Gouw, J. A.; Edwards, P. M.; Holloway, J. S.; Gilman, J. B.; Kuster, W. C.; Graus, M.; Atlas, E.; Blake, D.; Gentner, D. R.; Goldstein, A. H.; Harley, R. A.; Alvarez, S.; Rappenglueck, B.; Trainer, M.; Parrish, D. D. Photochemical aging of volatile organic compounds in the Los Angeles Basin: Weekday-weekend effect. J. Geophys. Res. 2013, 118 (10), 5018−5028. (24) Warneke, C.; de Gouw, J. A.; Holloway, J. S.; Peischl, J.; Ryerson, T. B.; Atlas, E.; Blake, D.; Trainer, M.; Parrish, D. D., Multiyear trends in volatile organic compounds in Los Angeles, California: Five decades of decreasing emissions. J. Geophys. Res. 2012, 117. (25) Gentner, D. R.; Isaacman, G.; Worton, D. R.; Chan, A. W. H.; Dallmann, T. R.; Davis, L.; Liu, S.; Day, D. A.; Russell, L. M.; Wilson, K. R.; Weber, R.; Guha, A.; Harley, R. A.; Goldstein, A. H. Elucidating secondary organic aerosol from diesel and gasoline vehicles through detailed characterization of organic carbon emissions. Proc. Natl. Acad. Sci. U. S. A. 2012, 109 (45), 18318−18323. (26) Bahreini, R.; Middlebrook, A. M.; de Gouw, J. A.; Warneke, C.; Trainer, M.; Brock, C. A.; Stark, H.; Brown, S. S.; Dube, W. P.; Gilman, J. B.; Hall, K.; Holloway, J. S.; Kuster, W. C.; Perring, A. E.; Prevot, A. S. H.; Schwarz, J. P.; Spackman, J. R.; Szidat, S.; Wagner, N. L.; Weber, R. J.; Zotter, P.; Parrish, D. D. Gasoline emissions dominate over diesel in formation of secondary organic aerosol mass. Geophys. Res. Lett. 2012, 39, L06805. (27) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of emissions from air pollution sources. 5. C-1-C-32 organic compounds from gasoline-powered motor vehicles. Environ. Sci. Technol. 2002, 36 (6), 1169−1180. (28) Simoneit, B. R. T.; Mazurek, M. A. Organic-matter of the troposphere .2. Natural background of biogenic lipid matter in aerosols over the rural Western United-States. Atmos. Environ. 1982, 16 (9), 2139−2159. (29) Janssen, G.; Carey, P. Weekday and weekend day temporal allocation of activity in the nonroad model; U.S. Environmental Protection Agency: Washington, DC, 1999. Available at www.epa.gov/ otaq/models/nonrdmdl/nr-015.pdf (accessed January 2014). 13749
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Intermediate-Volatility Organic Compounds: A Large Source of Secondary Organic Aerosol Yunliang Zhao,† Christopher J. Hennigan,†,⊥ Andrew A. May,†,∥ Daniel S. Tkacik,† Joost A. de Gouw,‡ Jessica B. Gilman,‡ William C. Kuster,‡ Agnes Borbon,‡,§ and Allen L. Robinson*,† †
Center for Atmospheric Particle Studies, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, Pennsylvania 15213, United States ‡ Chemical Science Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, Colorado 80305, United States § Laboratoire Interuniversitaire des Systèmes Atmosphériques, CNRS UMR7583, Université Paris Est Créteil, Université Paris Diderot, IPSL, Créteil 94010, France S Supporting Information *
ABSTRACT: Secondary organic aerosol (SOA) is a major component of atmospheric fine particle mass. Intermediate-volatility organic compounds (IVOCs) have been proposed to be an important source of SOA. We present a comprehensive analysis of atmospheric IVOC concentrations and their SOA production using measurements made in Pasadena, California during the California at the Nexus of Air Quality and Climate Change (CalNex) study. The campaign-average concentration of primary IVOCs was 6.3 ± 1.9 μg m−3 (average ± standard deviation), which is comparable to the concentration of organic aerosol but only 7.4 ± 1.2% of the concentration of speciated volatile organic compounds. Only 8.6 ± 2.2% of the mass of the primary IVOCs was speciated. Almost no weekend/weekday variation in the ambient concentration of both speciated and total primary IVOCs was observed, suggesting that petroleum-related sources other than onroad diesel vehicles contribute substantially to the IVOC emissions. Primary IVOCs are estimated to produce about 30% of newly formed SOA in the afternoon during CalNex, about 5 times that from single-ring aromatics. The importance of IVOCs in SOA formation is expected to be similar in many urban environments.
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INTRODUCTION Atmospheric fine particulate matter has important impacts on climate and human health. Secondary organic aerosol (SOA) contributes one-third or more of fine particle mass in many regions of the atmosphere.1 SOA is formed from gas-phase oxidation, heterogeneous reactions on aerosol surfaces, and multiphase chemistry of gas-phase organic compounds.2 SOA chemistry is complex and the contribution of these different pathways is not well understood. Robinson et al.3 proposed that intermediate-volatility organic compounds (IVOCs) were an important class of SOA precursors. IVOCs are compounds with effective saturation concentrations (C*) between 103 and 106 μg/m3; this roughly corresponds to the volatility range of C12−C22 n-alkanes. Laboratory experiments indicate that IVOCs can form SOA with high yields,4−6 but their atmospheric abundance and contribution to ambient SOA are poorly constrained. Speciated IVOCs, such as polycyclic aromatic hydrocarbons, have been routinely measured in the atmosphere. 7−10 Atmospheric concentrations of speciated IVOCs are generally lower than VOCs9,10 and speciated IVOCs appear to modestly contribute to SOA formation.11 However, the dominant fraction of atmospheric IVOCs is not resolved by traditional gas-chromatographic analysis. Instead it appears as an unresolved complex mixture (UCM) of coeluting compounds9 © 2014 American Chemical Society
because of the exponential increase in the number of constitutional isomers with increasing carbon number.12 Quantitative measurements of total atmospheric IVOCs (speciated + UCM) are needed to better constrain their major sources and their contribution to SOA formation. In this study, IVOCs were measured in Pasadena, CA during the California at the Nexus of Air Quality and Climate Change (CalNex) study. Concentrations of both speciated and unspeciated IVOCs were quantified and weekday and weekend diurnal patterns were characterized to investigate source contributions to atmospheric IVOCs. The IVOC data were then implemented into a box model to assess their contribution to ambient SOA formation.
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MATERIALS AND METHODS
Field Sampling and Chemical Analysis. Ambient sampling was conducted in Pasadena, CA during CalNex from May 17th to June 11th, 2010. IVOCs were collected by sampling ambient air through Tenax TA filled glass tubes (Gerstel 6 mm OD, 4.5 mm ID glass tube filled with ∼290 mg
Received: Revised: Accepted: Published: 13743
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of Tenax TA) at a flow rate of 0.5 L/min. Six sets of composite samples were collected to characterize average weekday and weekend diurnal profiles. Each set of samples consisted of 10 Tenax tubes: 8 tubes were used to collect ambient samples over 3 h intervals evenly distributed throughout the day (midnight to 3 AM, 3 AM to 6 AM, etc.); the ninth tube was exposed to ambient air but no active air flow for the entire sampling period to serve as a field blank; the final tube served as a handling blank. For a weekday composite sample, one set of tubes was used over the work week (Monday−Friday). For a weekend composite sample, the same strategy was employed over two weekends (two Saturdays and two Sundays) in order to increase sampled mass; these sets of tubes were stored in a freezer at −18 °C between weekends. In total, four sets of composite weekday and two sets of composite weekend samples were collected. This sampling strategy was employed to ensure the collection of sufficient IVOC mass and to capture repeatable atmospheric processes and variations in source emissions. The Tenax samples were analyzed by a gas chromatography (GC)/mass spectrometry (MS) system (Agilent, 6890 GC/ 5975 MS) equipped with a thermal desorption sample extraction and injection system (Gerstel, Baltimore, MD) and a capillary column (Agilent HP-5MS, 30 m × 0.25 mm). The recovery of IVOCs during analysis was tracked using deuterated standards that were spiked into each Tenax tube prior to the thermal desorption. Twenty individual IVOC compounds were quantified using authentic standards. The speciated IVOCs had an average ambient-to-blank ratio greater than 6.0, except for ndecylcyclohexane, with a ratio of 5.2. Individual VOCs were measured using an in situ GC/MS.13,14 See the Supporting Information for a detailed description of instrumentation and analytical methods used in this study. Quantification of IVOCs. Figure 1a shows chromatograms of a typical ambient Tenax sample collected during CalNex. Although there are many individual peaks in the GC/MS data, the vast majority of the total ion current (TIC) appears as a
broad hump of coeluting hydrocarbons and oxygenated compounds that cannot be speciated. These unspeciated compounds are commonly referred to as the UCM. To calculate the total mass of IVOCs (UCM + speciated IVOCs), the TIC of each IVOC sample was binned on the basis of the retention time of n-alkanes (Supporting Information). The n-alkanes were used as the metric to bin IVOCs because their saturation concentrations vary systematically with carbon number15,16 and they were detected across the entire IVOC range. Each bin is defined by the carbon number of one n-alkane (Figure 1a). For example, the bin with heptadecane (C 17 ) is defined as B 17 . Presto et al. 15 demonstrates that there is a near linear relationship between retention time and saturation concentration (C*) for hydrocarbons measured with the same analytical protocol. Therefore, binning the chromatogram by retention time provides the volatility distribution of compounds that are both collected on the Tenax sorbent and detected in the GC/MS. This method does not provide a carbon number distribution; for example, in addition to heptadecane, other compounds that elute in the B17 bin include pristane (C19) and 7-methylheptadecane (C18). The IVOC mass of each bin (MIVOCs,Bn) is calculated using the approach of Fraser et al.:9 MIVOCs,Bn =
TA TIC,Bn RFn‐alkane,Cn
=
TA m / z57,Bn RFn‐alkane,Cn
×
1 fm / z57,TIC B
n
(1)
where TATIC,Bn is the abundance of the TIC in the Bn bin; RFn‑alkane,Cn is the response factor of the Cn n-alkane; TAm/z57,Bn is the abundance (integrated area) of the m/z 57 fragment (m/ z 57) in the Bn bin; f m/z57,TIC Bn is the fraction of the m/z 57 in the TIC of the Bn bin of ambient IVOCs. Finally, the mass of the UCM in each bin is determined by subtracting off the contribution of speciated IVOCs from the IVOC mass of each bin. By using the abundance of the TIC (TATIC,Bn) to quantify the mass of IVOCs, this method likely provides a conservative estimate of the true atmospheric concentration of IVOCs. Atmospheric IVOCs are a complex mixture of hydrocarbons and oxygenated compounds that are both emitted by sources and formed in the atmosphere from photochemical reactions. First, oxygenated compounds only partially elute from the GC column in their underivatized form;17 our estimate does not account for any uneluted organics. Second, oxygenated compounds typically have lower response factors than hydrocarbons,18 but we convert the entire IVOC TIC signal to mass concentrations using hydrocarbon response factors (eq 1). Both of these factors will lead to an underestimate of the IVOC mass. IVOCs directly emitted into the atmosphere (primary IVOCs) have been proposed as an important class of SOA precursors.3 In order to examine the contribution of primary IVOCs to SOA formation, the ambient IVOCs first need to be separated into primary and secondary components. The speciated IVOCs are all primary.19 The mass of the primary IVOC UCM in each bin is determined by the difference between the mass of primary IVOCs and the mass of speciated IVOCs in the same bin. The mass of primary IVOCs is estimated using the signal of m/z 57 measured in atmospheric IVOCs and the fraction of m/ z 57 in the TIC (f m/z57,TIC Bn) measured in mobile source
Figure 1. (a) Chromatograms from a typical CalNex Tenax sorbent sample; (b) average concentrations of quantified primary IVOCs in each bin during the field campaign. 13744
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emissions because this is likely a dominant source of primary IVOCs in the Los Angeles area. During the CalNex study, there was little impact from biomass burning.20,21 Cooking emissions also do not appear to be a major source of IVOCs.20,22 In addition, Chan et al.21 reported that hydrocarbons in the carbon number range of C21 to C24 (at the top end of the IVOC range) were from mobile sources during CalNex. The m/z 57 was chosen as the quantification ion because it was used as the quantification ion for n-alkanes and detected across the entire IVOC retention time. Figure 1b shows the application of this method to the chromatogram in Figure 1a. To estimate the fraction of m/z 57 in primary IVOCs, we measured the fraction of m/z 57 in the TIC derived from GC/ MS analysis of Tenax samples collected in a highway tunnel in Pittsburgh, Pennsylvania and during chassis dynamometer tests of individual on- and off-road engines (Supporting Information). In the tunnel samples (n = 8), m/z 57 contributed 4.0% ± 0.3% of the TIC. The average fraction of m/z 57 in primary emissions from both on- and off-road engines ranged from 2.3% to 5%. Using a weighted average approach based on the fuel consumption in the South Coast Air Basin, which encompasses the CalNex sampling site, the overall fraction of m/z 57 derived from dynamometer source tests was 3.3%. The value is somewhat lower than the tunnel data because of the influence of small off-road gasoline sources. Because the highway tunnel measurements characterized a much larger vehicle fleet with a similar gasoline-diesel split as the Los Angeles area, the fraction of m/z 57 of primary IVOCs in our study was derived from the highway tunnel data.
Figure 2. Weekday and weekend distributions of speciated IVOCs and primary IVOC UCM bins. (a) Concentrations of speciated and primary IVOC UCM bins. (b) Weekend-to-weekday ratio for average concentrations of speciated and primary IVOC UCM. The weekendto-weekday ratios of EC and NOx plotted in panel b are from Bahreini et al.26 Speciated IVOCs are grouped into linear alkanes, branched alkanes, alkylcyclohexanes and aromatics. The “Total” category on the x-axis refers to the sum of speciated IVOCs and the sum of primary IVOC UCM bins. In the box plots, the center line of each box is the median of the data, the top and bottom of the box are 75th and 25th percentiles and top and bottom whiskers are 90th and 10th percentiles.
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RESULTS AND DISCUSSIONS Ambient Concentrations of Primary IVOCs. Ambient IVOCs are a complex mixture of organics contributed by both primary emissions and photochemical oxidation of gas-phase organics. Based on the fraction of m/z 57 measured in a highway tunnel, 60 ± 17% of the IVOC signals measured during CalNex were primary with the balance being contributed by oxygenated compounds formed through photochemical reactions. Dynamometer tests of individual on- and off-road engines suggest that the primary IVOC contribution was even higher during CalNex(∼70%), indicating that the use of the tunnel samples does not likely overstate the contribution of primary IVOCs to SOA. In addition, the average concentration of primary IVOCs calculated using the fraction of m/z 57 measured in a highway tunnel is consistent with the one calculated using the fraction of n-alkanes in primary IVOCs measured in the same highway tunnel. The difference between two campaign-average concentrations of primary IVOCs is less than 10% with the greater one being calculated using the fraction of n-alkanes in primary IVOCs. The campaign-average concentration of primary IVOCs (the sum of primary IVOC UCM and speciated IVOCs) was 6.3 ± 1.9 μg/m3 (average ± one standard deviation), with only 8.6 ± 2.2% of them being speciated (Figure 2). The average concentration of primary IVOCs is similar to that of organic PM1, 7.0 μg/m3,20 and single-ring aromatics (C6−C9), 6.6 ± 0.2 μg/m3, but is only 7.4 ± 1.2% of the total speciated VOCs (Figure 4). Figure 2 presents concentration data for the speciated IVOCs, including n-alkanes, branched alkanes (b-alkanes), alkylcyclohexanes and aromatic hydrocarbons. Campaignaverage concentrations of speciated IVOCs ranged from 2.0 to 81.0 ng/m3, which are less than most primary speciated
VOCs.23 Concentrations of most speciated IVOCs during CalNex are also a factor of 4 lower than those in 1993,9,10 highlighting the substantial reduction in organic emissions that has occurred over the last two decades.24 Figure 3 shows primary IVOCs and VOCs have similar average weekday and weekend diurnal patterns. There is an afternoon peak associated with the transport of morning emissions from downtown Los Angeles to the sampling site and a nighttime peak associated with trapping of fresh emissions in a shallow boundary layer.14 The average mass concentration of primary IVOCs decreases monotonically with decreasing volatility (Figures 1 and 2a), in qualitative agreement with prior studies.9 The total mass concentration of primary IVOCs in Pasadena is consistent with the predictions based on the volatility distribution of Robinson et al.3 and results from factor analysis of ambient aerosol mass spectrometer data.20 However, the mass distribution of measured primary IVOCs in each volatility bin differs somewhat from the prediction (Figure S1, Supporting Information). Weekday/Weekend Patterns and Sources of IVOCs. Figure 2 compares weekday and weekend concentrations of both speciated IVOCs and the primary IVOC UCM. The average weekday-to-weekend ratio for speciated IVOCs ranges from 0.9 to 1.3 and 0.9 to 1.1 for the UCM bins, except for the B21 bin (Figure 2b). The lack of a weekday/weekend pattern provides insight into the possible source(s) of IVOCs. Gentner et al.25 proposed that on-road diesel vehicles were a major source of IVOC emissions in the Los Angeles area. In California’s South Coast Air Basin, which encompasses the 13745
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Pristane, phytane and primary IVOC UCM have similar average concentrations between weekdays and weekends, suggesting that the major IVOC source(s) are likely petroleum-related.9,28 Off-road mobile sources could be one of the candidates for the high contribution to weekend IVOC emissions. The activity of off-road mobile sources such as residential lawn, garden and recreational equipment, is greater on weekends than on weekdays;29 however, more work is needed to characterize their emissions. Estimation of SOA Production. In this section, we estimate the SOA production from measured IVOC and SVOC data. The analysis focuses on the early afternoon time period (12:00−15:00) when the peak CalNex SOA concentrations were observed.20 The next section compares the predicted SOA production to SOA concentration data of Hayes et al.20 The SOA formation from the oxidation of gas-phase organic compounds can be estimated by5,30 ⎛ 1 − e−kOH,i[OH]Δt ⎞ ΔMSOA, i = [HCi] × ⎜ −k [OH]Δt ⎟ × Yi ⎝ e OH,i ⎠
(2)
Where [HCi] is the measured concentration of SOA precursor i; Yi is its SOA yield and kOH,i is its reaction rate constant with OH radicals (cm3 molecule−1 s−1); [OH] is the OH radical concentration; Δt is the OH radical exposure time. The OH exposure, [OH]Δt (molecules cm−3 s), is estimated by the ratio of 1,2,4-trimethylbenzene to benzene.14,20,31 To estimate the SOA formation from speciated compounds (VOCs and IVOCs), the OH reaction rate constant of each compound is taken from Atkinson and Arey32 or estimated on the basis of a structure−reactivity relationship (Tables S5 and S6, Supporting Information).33 SOA mass yields for speciated compounds are taken from published parametrizations of smog chamber results under high-NOx conditions (Hayes et al.20 showed that the high-NOx channel was dominant, with mean branching ratios over 90% during CalNex). These yields (Yi) are estimated at an organic aerosol (OA) concentration of 12 μg m−3, which corresponds to the average OA concentration during the 12:00−15:00 time period.20 SOA formed by primary IVOC UCM is also estimated by eq 2 using SOA yields of surrogate compounds (n-alkanes) assigned to each IVOC UCM bin. These surrogate compounds represent conservative estimates of the SOA yields; they are selected on the basis of the estimated distribution of known balkanes and cyclic compounds in each bin and published highNOx smog chamber data for speciated n-alkanes, b-alkanes and cyclic compounds of similar volatility (Table S7, Supporting Information). Primary IVOC UCM is thought to be dominated by a complex mixture of branched and cyclic hydrocarbons.9,21,25 For species with the same carbon number, branched hydrocarbons generally form less SOA than cyclic hydrocarbons and react at different rates than cyclic alkanes.4,21,34 Therefore, we classified the primary IVOC UCM into two lumped groups, unspeciated b-alkanes and the residual primary IVOC UCM (Figure 1), to provide better observational constraints on OH reaction rate constants and SOA yields of primary IVOC UCM. The unspeciated b-alkane component is calculated by replacing the TAm/z57,Bn and f m/z57,TIC Bn in eq 1 with the abundance of m/z 57 produced by b-alkanes and the average fraction of m/z 57 in the TIC of b-alkanes, respectively. The abundance of m/z 57 produced by b-alkanes is calculated by assuming that all of m/z 57 signal is only produced by n- and b-
Figure 3. Average diurnal patterns of primary IVOCs and single-ring aromatics on weekdays and weekends. The concentrations in each plot are normalized to the campaign average value of each compound. Single-ring aromatics are the sum of benzene, toluene, o-, p-, m-xylene, 1-ethyl-2-methylbenzene, 1-ethyl-3-methylbenzene, 1-ethyl-4-methylbenzene, 1,3,5-trimethybenzene, 1,2,4-trimethylbenezen and 1,2,3trimethylbenzene.
CalNex site, on-road diesel vehicle activity during weekends is less than half that on weekdays.20,26 This pattern drives large weekday/weekend changes in certain pollutant concentrations; for example, Figure 2 shows that there was roughly a factor of 2 difference in weekday/weekend NOx and elemental carbon (EC) concentrations during CalNex.26 The difference in the weekday/weekend pattern of IVOCs compared to NOx and EC strongly suggests that a source other than on-road diesel vehicles dominates the emissions of IVOCs. Or, if on-road diesel vehicle emissions contribute significantly to weekday IVOC concentrations; then an unidentified weekend source of IVOCs compensates for the reduction in the on-road diesel activity. The diurnal pattern of IVOC concentrations is similar to single-ring aromatics (Figure 3), which are dominated by onroad gasoline vehicle emissions.25,26 However, the emission factors of n-alkanes from gasoline vehicles show a steeper decrease in the carbon range of C12 to C1525,27 than our observations. Therefore, the similarity in IVOC and single ring aromatic diurnal patterns may be due to mixing during transport to the Pasadena area from other parts of the air basin. 13746
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alkanes and then subtracting off the abundance of m/z 57 produced by n-alkanes (all of which were individually quantified). The average fraction of m/z 57 in the TIC of balkanes (18% ± 6%) is calculated from the mass spectra of 11 b-alkanes (Tables S1 and S2, Supporting Information). The residual primary IVOC UCM (primary IVOC UCM minus estimated unspeciated b-alkane component) is likely composed of both cyclic alkanes and alkylated aromatics,25 but studies focusing on vehicle emissions suggest that UCM hydrocarbons are dominated by cyclic alkanes.18,19 We therefore classify all of the residual primary IVOC UCM as unspeciated cyclic alkanes. The estimated campaign average concentration of unspeciated b-alkanes is 1.1 ± 0.4 μg/m3; the residual primary IVOC UCM (4.7 ± 1.4 μg/m3) is therefore classified as unspeciated cyclic alkanes. This approach likely overestimates the abundance of b-alkanes because other compounds in addition to n- and b-alkanes also produce m/z 57 in the MS. This, in turn, leads to the underestimation of SOA production from primary IVOC UCM because SOA yields of b-alkanes are lower than cyclic alkanes and n-alkanes with the same carbon number.4,34 Following the separation of unspeciated b-alkanes and cyclic alkanes, surrogate compounds (n-alkanes) are assigned for unspeciated b-alkanes and cyclic components in each UCM bin to estimate the SOA yields and OH reaction rate constants (Table S7, Supporting Information). The SOA yield of the unspeciated b-alkanes in the Bn bin is represented using the nalkane whose carbon number is equal to bin number minus two (e.g., b-alkanes in the B 17 bin are represented using pentadecane). This shift roughly corresponds to a mean balkane structure with four methyl branches and accounts for the effects of branching on the elution time and SOA yield (Supporting Information). This is likely a conservative estimate (lower bound for SOA production) because the analysis of diesel fuel indicates that the maximum (not average) branching is four methyl groups in the carbon number range of 16−21.35 By accounting for the effects of branching on the OH reaction rate constant and retention time,21,33,35 the OH reaction rate constant of unspeciated b-alkanes in the Bn bin is represented by the n-alkane with the carbon number equal to the bin number (e.g., unspeciated b-alkanes in the B17 bin are represented using heptadecane). Both the SOA yield and OH reaction rate constant of unspeciated cyclic components are represented by the n-alkane with the same carbon number as the bin number (Supporting Information). With 60%, on average, of the ambient IVOC signal classified as primary IVOCs based on the fraction of m/z 57, the remaining 40% of the IVOC signal is attributed to oxygenated compounds (grouped as oxygenated IVOC UCM), which represent another class of SOA precursors. These oxygenated IVOCs are likely produced from gas-phase oxidation of organic compounds because they cannot be accounted for by the fraction of m/z 57 derived from highway tunnel sampling (which is overwhelmingly primary). To estimate SOA production from the oxygenated IVOC UCM, we used the average concentration of the oxygenated IVOC UCM during the 6:00−9:00 sample. Although the concentration of the oxygenated IVOC UCM increased during the day, this approach avoids double counting the contribution of sameday hydrocarbon oxidation. The overall SOA yield of the oxygenated IVOC UCM is represented using an n-alkane whose carbon number is equal to the bin number minus three (e.g., oxygenated components in the B17 bin are represented
using tetradecane). However, the OH reaction rate is represented by the n-alkane with the carbon number equal to the bin number. Again these likely are conservative assumptions that will lead to a lower estimate of SOA production (Supporting Information). Primary semivolatile organic compounds (SVOCs) are another important class of SOA precursors.3 The concentration of primary SVOC vapors is estimated to be 0.6 μg m−3 based on the average concentration of the hydrocarbon-like OA factor during the 12:00−15:00 period20 and the volatility distribution from Robinson et al.3 The SOA production from primary SVOCs is estimated assuming that their overall SOA yield is same as the C17 n-alkane and OH reaction rate constant is same as the C21 n-alkane. Given the relatively low concentration of SVOCs compared to both IVOCs and VOCs, these assumptions have relatively little effect on the total predicted SOA. All of the SOA yield estimates used here are based on laboratory smog chamber data measured under high-NOx conditions. These data have been corrected for particle losses to the chamber walls, but not vapor losses. A recent study suggests that high-NOx SOA mass yields used here may be underestimated by a factor of ∼1.25 due to vapor loses (see Table 1 in Zhang et al.36). (Zhang et al.36 estimated that there were much larger biases in SOA yield data from experiments with slow chemistry; all of the yields are from experiments that used HONO as the OH radical source, which leads to more rapid chemistry and lower vapor losses.) Given the uncertainty in vapor losses, we have not corrected yield data used here for them. This means that our calculations likely underestimate the total SOA production. However, correcting for vapor losses will likely not significantly alter the relative contributions of different classes of precursors (e.g., single ring aromatics versus primary IVOCs) to SOA. Vapor losses occur in experiments with all SOA precursors (VOCs and IVOCs); they are not restricted to the relatively few species considered in Zhang et al.36 These losses depend on both the properties of the precursor (vapor pressures) and the relative condensation rate between the particles and the walls (which depends on experimental conditions such as reaction rate, seed particle surface area, turbulence and the accommodation coefficient). In fact, vapor losses may be more significant for IVOCs than for VOCs;37 therefore, if anything, not correcting for vapor losses may result in a larger underestimate for the IVOC yields relative to VOC yields.
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DISCUSSIONS AND ATMOSPHERIC IMPLICATIONS Figure 4 compares the estimated SOA production from VOCs and IVOCs to the semivolatile oxygenated organic aerosol (SVOOA) factor derived from factor analysis of Aerodyne aerosol mass spectrometer data collected during CalNex. The SV-OOA factor is commonly associated with fresh SOA formed in urban environments.20 During CalNex, the concentration of SV-OOA peaked during the early afternoon (12:00−15:00).20 Figure 4 shows that VOCs, IVOCs and SVOCs together are predicted to produce 2.5 μg m−3 of SOA during the 12:00− 15:00 period. This corresponds to 50% of the SV-OOA factor. Only 12% of the predicted SOA is from VOCs (mainly single ring aromatics); 12% is from speciated IVOCs; 45% is from primary IVOC UCM; 19% is from oxygenated IVOC UCM; 12% is from unspeciated primary SVOC. The estimated SOA from primary IVOCs (UCM + speciated compounds) is approximately 5 times that produced from 13747
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precursors, especially IVOCs, into the SOA model in addition to VOCs substantially improves model predictions, helping to move the analysis much closer toward a mass closure. We expect that IVOCs are an important class of SOA precursors in most urban environments. However, the sources of these IVOCs are not well understood. Our results indicate that the petroleum sources other than on-road vehicles likely contribute substantially to primary IVOCs. Given their importance as SOA precursors demonstrated by this study, there is a need for further identification and quantification of IVOC emissions from a range of sources.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 4. Average concentrations of measured and estimated gasphase organics, predicted SOA mass and SV-OOA factor derived by Hayes et al.20 in Pasadena, CA during CalNex.
Detailed description sampling and quantification of IVOCs, estimation of SOA production from both VOCs and IVOCs and data used in the discussions. This material is available free of charge via the Internet at http://pubs.acs.org.
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single-ring aromatics, which is the dominant class of anthropogenic SOA precursors in many models.16 Therefore, primary IVOCs, less than 10% of the mass of VOCs, were a major class of SOA precursors during CalNex. Furthermore, unspeciated precursors (mainly IVOCs, but also SVOCs) contribute the majority of the predicted SOA mass, consistent with recent analyses of smog chamber experiments performed with dilute vehicle exhaust.38 The overall SOA yield of the primary IVOCs are 29%, which is comparable to estimates (10−40%) from Jathar et al.38 for unspeciated organic emissions from combustion sources. Given their unknown composition, the SOA yields of the primary IVOC UCM are uncertain. A lower bound estimate can be made by treating the IVOC UCM entirely as b-alkanes. Under this assumption, the SOA formed from primary IVOCs is 1.0 ± 0.7 μg/m3 during the 12:00−15:00 period with an overall SOA yield of 21% versus 1.4 ± 0.9 μg/m3 μg m−3 with an overall SOA yield of 29% for the best estimate. Even under this lower limit, the amount of SOA from primary IVOC UCM is substantial and dominates the SOA production over speciated IVOCs and VOCs, contributing about 60% of the predicted SOA from primary IVOCs and VOCs. This underscores the need to account for organic compounds of intermediate volatility that cannot be speciated with traditional analysis methods (e.g., 1-D GC). As discussed previously, the predictions shown in Figure 4 have not been corrected for vapor losses. A best correction using the bias estimates listed in Table 1 of Zhang et al.36 increases the predicted SOA production from VOCs from 0.3 to 0.35 μg m−3, which corresponds to 14% of the predicted SOA mass. Even after correcting for vapor losses, primary IVOCs are still predicted to contribute 56% of the predicted SOA (versus 57% for the uncorrected estimate shown in Figure 4). Therefore, the conclusion that primary IVOCs are a dominant class of SOA precursors is not altered by biases in yield estimates associated vapor losses to smog chamber walls. The amount of predicted SOA shown in Figure 4 is conservative. First, we have not corrected the SOA yield data for vapor losses. Second, the SOA yields assigned to each volatility bin are conservative (low). Third, the concentration of oxygenated IVOCs is likely underestimated. Accounting for all these factors could easily close the mass balance between the measured and predicted SOA in Figure 4. The comparison shown in Figure 4 demonstrates that including more quantified and observationally constrained
AUTHOR INFORMATION
Corresponding Author
*A. L. Robinson. E-mail: [email protected]. Phone: 4122683657. Present Addresses ⊥
C. J. Hennigan: Department of Civil, Environmental, and Geodetic Engineering, University of Maryland, Baltimore, Maryland 21250. ∥ A. A. May: Department of Civil, Environmental, and Geodetic Engineering, The Ohio State University, Columbus, Ohio 43210. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Carnegie Mellon University was funded by the U.S. Environmental Protection Agency National Center for Environmental Research through the STAR program (Project RD834554). The views, opinions, and/or findings contained in this paper are those of the authors and should not be construed as an official position of the funding agencies.
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