3.1. Gas and Aerosol Products
 Steady state concentrations of NOy, O3, HNO3, and SO2 for the four stages of the experiment are given in Table 2. As seen in Tables 1 and 2, once the αP/Tol mixture reached steady state, NO and α-pinene had completely reacted while only 30% of the initial toluene (1900 μg m−3; 3.5 ppmC) was consumed. Under these conditions, the steady state concentrations of NOy, O3, and HNO3 were 111, 127, and 79 ppb, respectively. Carbonyl products from the photooxidation of the αP/Tol mixture included formaldehyde, glyoxal, methylglyoxal, benzaldehyde, and pinonaldehyde at concentrations given in Table 3. Steady state mass concentrations of SOA and SOC were measured as 211 μg m−3 and 128 μgC m−3, respectively, as given in Table 4.
Table 2. Steady State Gas Phase and Reacted Hydrocarbon Concentrations During the Irradiations
|Mixture||NOy (ppb)||O3 (ppb)||HNO3 (ppb)||SO2 (ppb)||Δ (αP) (μg m−3)||Δ(Tol) (μg m−3)||Δ(Isop) (μg m−3)|
|αP/Tol||111||127||79.1|| ||590||1900|| |
Table 3. Steady State Carbonyl Concentrations (ppb) During the Irradiations (G: Glyoxal; MG: Methylglyoxal; MA: Methacrolein; MV: Methyl Vinyl Ketone; BZ: Benzaldehyde; PIN: Pinonaldehyde)
|αP/Tol||63||93||83|| || ||14||14|
|αP/Tol/SO2||59||70||76|| || ||13||15|
Table 4. Aerosol Parameters Measured During the Irradiations for the Four Stages
|Mixture||SOA (μg m−3)||Volume (μm3 cm−3)||SOC (μgC m−3)||SO42− (μg m−3)||NO3− (μg m−3)||NH4+ (μg m−3)|
|αP/Tol||211||152||128|| || || |
|αP/Tol/Isop||104||108||71|| || || |
 The addition of 4.1 ppmC isoprene to the αP/Tol mixture substantially changed the gas- and aerosol-phase compositions. In the gas phase, the steady state NOx concentration increased to 195 ppb while at the same time the nitric acid concentration decreased to 62 ppb and O3 increased to 156 ppb. As shown in Table 2, the reacted toluene decreased from 1900 to 1000 μg m−3 with its products, benzaldehyde and glyoxal, decreasing by approximately 25%. The reaction of isoprene led to an increase in formaldehyde by a factor of seven and methylglyoxal by 70% over the values from the αP/Tol mixture. The addition of isoprene to the base-case mixture substantially decreased the SOA produced from 211 to 104 μg m−3 and SOC from 128 to 71 μgC m−3.
 The addition of 0.28 ppm SO2 to the αP/Tol mixture resulted in modest increases in the NOy and nitric acid concentrations and a modest decrease in the O3 concentration. Initiated by its reaction with OH radicals, SO2 was oxidized to form sulfuric acid. The carbonyl and dicarbonyl compounds changed by minor amounts, except for glyoxal, which decreased by 25%. The addition of SO2 to the αP/Tol mixture increased the mass of the aerosol in two ways: (1) from the formation of sulfuric acid condensing onto preexisting particles and (2) from increased SOA. The SOA mass was determined by subtracting the inorganic masses (SO42−, NO3−, NH4+) found in Table 4 from the measured gravimetric mass. The net result was that the SOA mass increased by approximately 16% in going from the αP/Tol to the αP/Tol/SO2 mixture.
 Finally, the addition of both isoprene and SO2 to the αP/Tol mixture led to an increase in the steady state NOy concentration by nearly a factor of two, an increase in O3 by 14%, and a decrease in HNO3 by more than 20%. Similarly, for the carbonyl and dicarbonyl compounds, the addition of both isoprene and SO2 resulted in concentrations that were very close to those when isoprene alone was added. Adding both compounds also left the total SOA and SOC masses largely unchanged. The sulfate concentration was found to increase to 39.8 μg m−3 from that of the base-case mixture, a concentration 63% higher than that measured in the αP/Tol/SO2 mixture.
3.2. Bulk Aerosol Properties
 Figure 1 shows the distribution of molecular fragment ions from the composite aerosol from the (+)LDI-MS spectra in the range of 200 < m/z < 1000 for the four mixtures. The mass spectra can be directly compared because the chamber air sampled and the amount of extract analyzed for each stage were the same. A detailed examination of the (+)LDI-MS mass spectra for ions up to 800 u shows characteristic ion differences of m/z 14, 16, and 18 Da, which suggests the aerosols have an oligomeric character that is consistent with earlier work [Kalberer et al., 2004; Gao et al., 2004a, 2004b; Tolocka et al., 2004]. In mixtures containing SO2, an enhancement in the ion intensities and a shift to higher masses suggest that reactions in the condensed phase involving sulfuric acid could be occurring, which is consistent with previous detection of organosulfates in these samples [Surratt et al., 2007a]. This finding is also consistent with increased OC measured in these stages, as well as increases in OC measured in other studies where SO2 is present in irradiated HC/NOx mixtures [Edney et al., 2005; Kleindienst et al., 2006]. The addition of isoprene to the αP/Tol mixture led to a decrease in the intensities of the ions both in the presence and absence of SO2.
Figure 1. LDI-MS positive ion mode spectra generated from impactor plates collected in each stage of the experiment. The same volume of air was sampled on each impactor.
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 Photographs of the filters (F1–F4) collected from each stage, shown in Figure 2, indicate distinct differences in the products formed for the different mixtures. The filter from the αP/Tol mixture (F1) shows a yellow hue that is close in appearance to filters previously collected in this laboratory from toluene/NOx experiments. Consistent with a decrease in reacted toluene in the αP/Tol/Isop system, filter F2 has lost most of the color seen in F1. The addition of SO2 to the αP/Tol mixture produced material (filter F3) of a deep brown color. Finally, the aerosol from the αP/Tol/Isop/SO2 mixture resulted in filter F4 having a pronounced brown color, although considerably lighter than that from the αP/Tol/SO2 mixture.
Figure 2. Effect of mixture changes in filter appearance in going from the αP/Tol/NOx (base-case) filter (F1) to subsequent mixtures. The same volume of air was sampled on each filter.
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3.3. Nitrogeneous Organic Compounds
 A number of individual organic compounds have been detected in the SOA extracts of the mixtures (Figure 3). Of these, a group of five compounds (N-1 to N-5; Table 5) exhibit odd molecular weights for their BSTFA derivatives, suggesting the presence of a nitrogenous moiety. Compounds N-1 and N-2 were detected only when isoprene, NOx, and SO2 were present in the irradiated mixture, while N-3, N-4, and N-5 were detected only when toluene was present. The common BSTFA fragments for the derivatization of acidic or alcoholic OH groups were detected for these compounds [Jaoui et al., 2004]. Figure 4 shows mass spectra recorded for compounds N-1, N-2, and N-5. The mass spectrum of N-1 shows fragments and adducts consistent with a derivative compound with MW 253 Da., while N-2 fragments and adducts are consistent with a compound with MW of 313 Da [Jaoui et al., 2007]. Compounds N-3, N-4, and N-5 have similar fragmentation patterns, with the ion at m/z M+. + 1 (314) showing the highest intensity. While N-2 shows similar ions as N-3, N-4, and N-5, it also exhibits a strong M+. - 15 ion at m/z 298. Peaks associated with N-3, N-4, and N-5 are also consistent with the results from a combined carbonyl-OH derivatization that suggests the absence of carbonyl (C = O) groups. N-5 was identified as 2-hydroxy-5-nitrobenzyl alcohol (MW of 169 Da) by comparison to a standard of the authentic compound. N-3 and N-4 were tentatively identified as structural isomers of N-5 from the similarity of their mass spectra with 2-hydroxy-5-nitrobenzyl alcohol. Several isomers of 2-nitrophenol and 2-methyl-2-nitrophenol were also detected in the mixtures. Table 6 gives concentrations for the nitrogenated compounds, based on standards or as ketopinic acid where standards are not available.
Figure 3. GC-MS extracted ion chromatograms in CI mode of BSTFA derivatized particle phase collected in each of the four stages of the experiment. Ions extracted: 165, 238, 298, 321, 405, 409, 314, 349, 377, and 495.
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Table 5. Organic Compounds in the Mixtures Identified From the GC-MS Analysis
|Chemical Name||ID||MW||MWa||Precursor VOC|
|2-Hydroxy-4,4-dimethyl glutaric acid||P-2||176||392||αP|
|2,3-Dihydroxy-4-oxo pentanoic acid||T-1||148||364||Tol|
|2-Methylglyceric acid dimerb||I-4||222||510||Isop|
|Nitrogen containing compoundb||N-1||181||253||Isop|
|Nitrogen containing compoundb||N-2||169||313||Isop|
|2-Hydroxy-5-nitrobenzyl alcohol and isomersc||N-5, N-3, N-4||169||313||Tol|
Table 6. Estimated Concentrations (μg m−3) of Nitrogen Containing Compounds Using the Response Factor for the Ketopinic Acid BSTFA Derivative
|αP/Tol|| || ||1.57||0.30||1.80|
|αP/Tol/Isop|| || ||0.25||0.03||0.14|
|αP/Tol/SO2|| || ||1.81||0.36||1.39|
3.4. Organic Tracer Compounds and SOC Contributions
 The oxidation of individual biogenic and aromatic hydrocarbons also leads to a variety of oxygenated compounds which have been identified as tracer compounds for their parent VOC; many of these have also been detected in ambient PM2.5 [Edney et al., 2005; Kleindienst et al., 2004; Jaoui et al., 2005; Surratt et al., 2007a, 2007b; Claeys et al., 2007; Gómez-González et al., 2008]. The organic SOA tracers for the parent VOCs were found in all stages. Table 5 gives compound names, their MWs, and the derivative MWs for the identified tracer compounds and the associated parent VOC [Kleindienst et al., 2007], who also discuss the uncertainties associated with the method.
 The tracers present in the GC-MS extracted ion chromatograms of the four stages are shown in Figure 3. Compounds associated with the photooxidation products of isoprene are 2-methylglyceric acid (I-1), 2-methylthreitol (I-2), and 2-methylerythritol (I-3). Peak I-4 has recently been found to be associated with the NOx photooxidation of isoprene and identified as a 2-methylglyceric acid dimer (2-MGAD) [Szmigielski et al., 2007a]. This compound was detected in the αP/Tol/Isop and αP/Tol/Isop/SO2 mixtures. The concentration of 2-MGAD was found to increase significantly with increasing SO2, as did N-1 and N-2 [Jaoui et al., 2007]. Compounds associated with α-pinene SOA included 3-hydroxyglutaric acid (P-1), 2-hydroxy-4,4-dimethylglutaric acid (P-2), and 3-methyl-1,2,3-butanetricarboxylic (P-4) which are among the largest peaks detected in the GC-MS total ion chromatogram (TIC) and consistent with previous laboratory and field experiments [Jaoui et al., 2005; Szmigielski et al., 2007b]. For toluene, a single compound detected in SOA (2,3-dihydroxy-4-oxopentanoic acid: T-1) is used as the tracer compound. For stages with SO2 present, a strong BSTFA-derivative peak for particle-phase sulfuric acid is also detected at an early retention time. Finally, several compounds, measured in these experiments using ESI-MS but previously reported [Surratt et al., 2007a], were characterized as organosulfates of isoprene and α-pinene [Surratt et al., 2007b; Gómez-González et al., 2008].
 The contribution to the SOC mass from the individual precursors can be determined using an SOC tracer method developed in this laboratory [Kleindienst et al., 2007] and evaluated with binary hydrocarbon mixtures using 14C analysis [Offenberg et al., 2007]. The method consists of measuring, for each hydrocarbon precursor, the concentrations of the organic tracer compounds (Table 5) and SOC from laboratory hydrocarbon/NOx irradiations to obtain an SOC mass fraction, fsoa,hc, for each precursor. To further test this tracer technique for mixtures, the identified organic tracers from Table 5 are used with the previously derived mass fractions (0.23, 0.0079, and 0.16 for α-pinene, toluene, and isoprene SOC, respectively; [Kleindienst et al., 2007]) to obtain estimated SOC masses which can then be compared to the SOC measured in this experiment. Table 7 gives the concentrations of SOC attributed to α-pinene, toluene, and isoprene for each mixture using this tracer method. For the αP/Tol mixture, the estimated and measured SOC concentrations are 123 and 128 μg m−3, respectively. For the αP/Tol/Isop mixture, the α-pinene and toluene contributions decrease significantly and while there is a small SOC contribution from isoprene, the net result is a substantial decrease in the estimated SOC from 123 to 84 μg m−3. With SO2 added to the irradiated αP/Tol mixture, the estimated SOC contribution increased slightly from 123 to 125 μgC m−3. Finally, for the addition of SO2 to the αP/Tol/Isop mixture, the estimated SOC increased from 84 to 95 μgC m−3, which is in the same direction but a lower difference than that measured (71 to 122 μgC m−3).
Table 7. Comparison of the Sum of the Contribution of Individual Hydrocarbon and the Measured OC Concentrations μgC m−3
|Mixture||α-Pinene SOC||Toluene SOC||Isoprene SOC||Estimated SOC||Measured SOC|
 From the SOC contributions in Table 7 and the reacted hydrocarbon concentrations (ΔHC) in Table 2, an SOC yield, Ysoc (i.e., OCHC/ΔHC), can be determined. Table 8 gives the SOC yields for α-pinene, isoprene, and toluene for each mixture. For α-pinene, the largest change in yield occurred (35%) when SO2 was added to the αP/Tol mixture which is a similar increase as that seen by Kleindienst et al.  when SO2 was added to α-pinene/NOx mixtures. For the four mixtures, the mean yields for α-pinene, toluene and isoprene were 0.107, 0.044, and 0.005, respectively. Recent laboratory studies have shown that SOA yields from aromatic hydrocarbons and monoterpenes can be a function of the initial NOx concentration [Ng et al., 2007a, 2007b].
Table 8. Precursor Yields Based on the SOC Masses From Tracer Method Given in Table 7a
|Mixture||α-Pinene SOC||Isoprene SOC||Toluene SOC|