Supporting Information

Transcription

1 Supporting Information Surratt et al. 1.1/pnas SI Text Experimental Details of Chamber peration. Low-N x Experiments. 28 μl of 5% H 2 2 solution (Aldrich) is injected into a glass bulb, vaporized in a 3 C water bath, and introduced into the chamber in an air stream, resulting in an estimated chamber concentration of 4 ppm by volume. 5% of the available blacklights are used to irradiate the chambers. High-N x Experiments. HN is prepared by adding 1 wt% aqueous NaN 2 dropwise into 1 wt% sulfuric acid and introduced into the chamber using an air stream (1). N or N 2 is injected into the chambers from gas cylinders (Scott Marrin). 1% of the available blacklights are used to irradiate the chambers. Procedures to Confirm Purity of MPAN. The purity of the MPAN sample was quantified by comparison of gas-phase FTIR spectra with those published in other studies (see MPAN synthesis below). The major impurity, methacrylic acid, does not contribute to SA. We oxidized methacrylic acid under the same conditions used in the MPAN study, and SA formation was not observed. At the N levels (>1 ppb) in the photooxidation experiments, R 2 þ H 2 and R 2 þ R 2 reactions do not compete with R 2 þ N or R 2 þ N 2 reactions, and aerosol-phase formation of 2-MG from isoprene cannot be attributed to gas-phase R 2 radical-cross reactions of methacrylic acid. Details of the CIMS Technique. perating Conditions. The CIMS was operated in negative ion mode where CF 3 is used as the reagent ion, and in positive ion mode of proton transfer mass spectrometry (PTR-MS) (2). In negative ion mode, CF 3 selectively clusters with compounds with high fluorine affinities to form a complex detected at m z ¼ MW þ 85, whereas highly acidic molecules will mainly form the transfer product detected at m z ¼ MW þ 19 (2). To distinguish between hydroxyisoprene hydroperoxide and isoprene epoxydiols (both of which are detected at m z 23), an MS/MS technique was used (3). In the first quadrupole the parent ion is selected, and in the second quadrupole a small amount of N 2 is present to act as a collision partner producing collision-induced daughter ions, which are subsequently selected in the third quadrupole. Depending on the nature of the ion, different collision-induced daughter ions can be formed. Two daughter ions of particular interest are at m z 63 and m z 183. The daughter ion at m z 63 is selective for the hydroperoxide group, and the isoprene epoxydiols fragment in sufficient yield at m z 183. In PTRMS mode, residual H 2 in N 2 is reacted with N þ 2 ions to form Hþ ðh 2 Þ n reagent ions. Correction of the MS/MS Daughter Ion Signal Associated to IEPX. ISPH and IEPX are isomers and, as a result, are both detected at m z 23. As noted above, using the MS/MS technique of the CIMS, these isomers can be separated from each other due to their production of different characteristic daughter ions. It has also been shown that IEPX does not interfere with the daughter ion at m z 63 associated to ISPH. IEPX is known to fragment in good yield to m z 183; however, ISPH also fragments to this ion to a lesser extent. In order to determine the extent that ISPH fragments to m z 183 in the second quadrupole, a large number of the daughter ions from the parent ion at m z 23 were analyzed and it was determined that only two compounds were present at this ion (i.e., ISPH and IE- PX). Next, the data from an experimental run where IS- PH was only present in the gas phase was analyzed to determine the amount of the daughter ion at m z 183 that was from ISPH. It was found that 2% of the daughter ion at m z 183 was from ISPH, and this amount was subtracted from all MS/MS spectra of m z 183 so that only IEPX was displayed. This corrected signal was also used to determine the concentration of IEPX. Filter Extraction and peration Protocols for the GC/EI-MS Technique Filter extractions for aerosol samples analyzed by the GC/EI-MS technique are similar to the UPLC/( )ESI-TFMS technique; however, following the removal of the methanol extraction solvent with ultra-high purity N 2, the dried residue was trimethylsilylated by the addition of 1 μl ofn,-bis(trimethylsilyl) trifluoroacetamide (BSTFA) plus trimethylchlorosilane (TMCS) [99:1 (vol/vol), Supleco] and 5 μl of pyridine (Sigma-Aldrich, 98%, anhydrous), and the resultant mixture was heated for an hour at 7 C. This latter step converted all isoprene SA constituents containing carboxyl and hydroxyl functions into volatile TMS derivatives. GC/EI-MS analyses of the TMS derivatives were performed with a system comprised of a Hewlett Packard (HP) 589 Series 2 Plus chromatograph, interfaced to an HP 5972 Series mass selective detector. A Restek RTX-5MS fused-silica capillary column (5% diphenyl, 95% dimethyl polysiloxane,.25-mm film thickness, 3 m.25 mm i.d.) was used to separate the TMS derivatives before MS detection. 1-μL aliquots of each sample were injected in the splitless mode onto the column by an HP 76 Series GC injector and autosampler. Helium was used as the carrier gas at a flow rate of.8 ml min 1. The min temperature program of the GC was as follows: isothermal hold at 6 C for 1 min, temperature ramp of 3 C min 1 up to 2 C, isothermal hold at 2 C for 2 min, temperature ramp of 2 C min 1 to 31 C, and isothermal hold at 31 C for 1 min. The MS scan was performed in the m z 5 5 range. A solvent delay time of 7.5 min was employed. The ion source was operated at an electron energy of 7 ev. The temperatures of both the GC inlet and detector were at 25 C. Materials BEPX Synthesis. In general, an aqueous solution of 2-butene-1,4-diol (Fluka, purum, 98.%) is reacted with H 2 2 catalyzed by tungstinic acid, followed by removal of H 2 and other impurities. 1 H-NMR of the final product reveals purity greater than 95% and the product is used as is. MPAN Synthesis. Methanesulfonic acid (Fluka, puriss., 99.%) is used in place of sulfuric acid as suggested by Williams et al. (4) to improve the purity of the product. No column purification is carried out. Instead, the MPAN solution is purified by 2 25 successive washes with deionized water (Millipore, 18.2 MΩ > cm). Spectra of the MPAN sample showed strong peaks at 165, 1297, 174, and 187 cm 1, which are associated with MPAN (5). No peaks associated with methacrylic anhydride and methacrylic acid are observed in spectra obtained from the MPAN sample. 1. Chan AWH, et al. (29) Secondary organic aerosol formation from photooxidation of naphthalene and alkylnaphthalenes: Implications for oxidation of intermediate volatility organic compounds (IVCs). Atmos Chem Phys 9: Crounse JD, McKinney KA, Kwan AJ, Wennberg P (26) Measurements of gas-phase hydroperoxides by chemical ionization mass spectrometry. Anal Chem 78: Paulot F, et al. (29) Unexpected epoxide formation in the gas-phase photooxidation of isoprene. Science 325: Williams J, et al. (2) A method for the airborne measurement of PAN, PPN, and MPAN. J Geophys Res 15(D23): Surratt et al. 1of9

2 5. Bertman SB, Roberts JM (1991) A PAN analog from isoprene photooxidation. Geophys Res Lett 18: Surratt JD, et al. (27) Evidence for organosulfates in secondary organic aerosol. Environ Sci Technol 41: Gómez-González Y, et al. (28) Characterization of organosulfates from the photooxidation of isoprene and unsaturated fatty acids in ambient aerosol using liquid chromatography/( )electrospray ionization mass spectrometry. J Mass Spectrom 43: Surratt JD, et al. (26) Chemical composition of secondary organic aerosol formed from the photooxidation of isoprene. J Phys Chem A 11: Szmigielski R, et al. (27) Characterization of 2-methylglyceric acid oligomers in secondary organic aerosol formed from the photooxidation of isoprene using trimethylsilylation and gas chromatography/ion trap mass spectrometry J Mass Spectrom 42: Surratt et al. 2of9

3 1 A1 A MW TMS-Derivative M [M CH 3 ] MW TMS-Derivative M [M CH 3 ] B1 B2 231 B C1 219 C D Fig. S1. GC/EI-MS mass spectra for isoprene low-n x SA constituents formed in the highly acidified sulfate seed aerosol experiment chemically characterized in Fig 1 G J. (A1 and A2) Mass spectra corresponding to the IEPX compounds characterized for the first time in low-n x isoprene SA. (B1 B3) Mass spectra corresponding to the three isomers of the C 5 -alkene triols. (C1 and C2) Mass spectra corresponding to the diasteroisomeric 2-methyltetrols. (D1) Averaged mass spectrum corresponding to all six major isomers of the previously charaterized hemiacetal dimers. Surratt et al. 3of9

4 [M H] Ion Formula = C 5 H 11 7 S [2M H] Err = -.9 mda i-fit = 42. A B [M H] Ion Formula = C 1 H 21 1 S Err = 1.8 mda i-fit = Fig. S2. UPLC/( )ESI-TFMS mass spectra for isoprene low-n x SA constituents formed in the highly acidified sulfate seed aerosol experiment chemically characterized in Fig 1 K and L, respectively. (A) Mass spectrum corresponding to the organosulfate derivative of the 2-methyltetrols. (B) Mass spectrum corresponding to the organosulfate derivative of the dimer (denoted in prior work as the organosulfate of the hemiacetal dimer). These mass spectra are consistent with prior work (6, 7). Surratt et al. 4of9

5 Absolute Abundance Absolute Abundance 1 1 A1 H D1 H H H H H H MW 14 H A GC/EI-MS Time (min) B1 H H MW 14 H C1 H H H MW 122 H TMS 11 TMS TMS TMS TMS TMS TMS TMS TMS MW TMS-Derivative 248 MW TMS-Derivative 32 MW TMS-Derivative M [M CH 3 ] + 32 M + 41 M + 35 [M CH 3 ] [M CH 3 ] + 23 [M TMSH] + 32 [M TMSH] [M TMSCH ] + 37 [M TMSCH 2 ] MW 226 TMS TMS TMS 217 TMS 321 D2 TMS TMS 555 -TMSH MW TMS-Derivative EICs of 248 EICs of 217 EICs of 217 GC/EI-MS Time (min) GC/EI-MS Time (min) GC/EI-MS Time (min) E1 EICs of 321 EICs of 21 EICs of 35 MW 21 B UPLC/( )ESI-TFMS Time (min) H [M H] Ion Formula = C 4 H 9 7 S [2M H] H S H Err = -1.6 mda I-Fit = 2.2 E2 8 1 F1 C UPLC/( )ESI-TFMS Time (min) H H F2 [M H] Ion Formula = C 8 H 17 1 S [2M H] H - S 3 H MW 35 Err = -2.8 mda I-Fit = 32.2 H 8 1 Fig. S3. (A1 F1) Particle-phase constituents formed from the dark reactive uptake of BEPX in the presence of either neutral (blue lines) or highly acidic (red lines) sulfateseedaerosol. (A2 F2)Corresponding massspectra foreach chemically characterizedparticle-phase constituent shownina1 F1. For simplicity, the mass spectra shown are only those of the most abundant isomers (chromatographic peaks) found in the EICs of A1 F1. All particle-phase constituents shown in A1 F1 are more abundantly formed from the uptake of the BEPX in the presence of highly acidic sulfate seed aerosol, which is consistent with the low-n x isoprene SA constituents shown in Fig. 1. The particle-phase products characterized here were also observed in photooxidation of butadiene under low-n x conditions and in the presence of highly acidified sulfate seed aerosol. These data further confirm the role of IEPX in forming low-n x isoprene SA under acidic conditions. Surratt et al. 5of9

6 EICs of 248 A 1 B Absolute Abundance GC/EI-MS Time (min) Fig. S4. (A) GC/MS EICs of m z 248 from 5 mg of BEPX dissolved in.5 ml of.1 M H 2 S 4 in water (green line) and a reactive uptake experiment of BEPX in the presence of highly acidified sulfate seed aerosol (black line). The two chromatographic peaks differ only slightly in terms of retention time owing to the samples being analyzed by the GC/EI-MS technique on separate days. (B) Corresponding mass spectrum for the chromatographic peak shown in A for the 5-mg BEPX standard dissolved in.5 ml of.1 M H 2 S 4 in water (green line). The mass spectrum corresponding to the chromatographic peak shown in A for the reactive uptake experiment of BEPX in the presence of highly acidified sulfate seed aerosol (black line) is presented in Fig. S3 A2. This comparison shows that some of the BEPX remains unreacted in the particle phase and could have resulted there due to semivolatile partitioning. 3 highly acidified sulfate seed aerosol proposed organosulfate compound neutral sulfate seed aerosol 25 Sulfate (µg m -3 ) 2 1 2,3-epoxy-1,4-butanediol (BEPX) Added Experimental Time (hours) Peak Area of Proposed rganosulfate (x 1) Fig. S5. PILS/IC time traces of sulfate aerosol mass concentrations observed in experiments examining the reactive uptake of BEPX in the presence of either neutral or highly acidified sulfate seed aerosol. In addition, a PILS/IC time trace is shown for the peak area of a tentatively assigned organosulfate compound observed only in the SA formed from the reactive uptake of BEPX in the presence of highly acidified sulfate seed aerosol. In both the neutral and highly acidified sulfate aerosol experiments, the seed aerosol was injected first and allowed to stabilize. Time zero indicates when the BEPX was injected. The sulfate aerosol mass concentration decayed by approximately 58% of its initial loading 1 h after the BEPX was injected in the presence of highly acidified sulfate seed aerosol. The sulfate aerosol mass concentration remained relatively constant after the injection of BEPX in the presence of neutral sulfate seed aerosol. The large decay of sulfate mass in the highly acidified sulfate seed aerosol experiment indicates that it is lost due to reaction with BEPX. Surratt et al. 6of9

7 1.9 min 4.9 min EIC 825 C 28 H S min 4.1 min EIC 721 C 24 H S min EIC 617 C 2 H S min 3.93 min Relative Abundance min.9 min 1.54 min 3.87 min EIC 513 C 16 H S - EIC 49 C 12 H S min EIC 35 C 8 H 17 1 S min EIC 21 C 4 H 9 7 S Time (min) Fig. S6. UPLC/( )ESI-TFMS EICs of organosulfates formed from the reactive uptake of BEPX in the presence of highly acidified sulfate seed aerosol. EICs of m z 49 to 825 indicate the formation of high-order (MW) organosulfates. Accurate mass measurements for each observed ion allowed for the determination and verification for the presence of the latter compounds by providing elemental composition (i.e., molecular formula) information. The elemental compositions for each ion are denoted in red. Numbers marked above the chromatographic peaks are the retention times for these compounds. [N 2 ] initial :[N] initial = 2.3 [N 2 ] initial :[N] initial =.8 [N 2 ] initial :[N] initial =.5 Absolute Abundance A EICs of 323 H H H H H B EICs of 366 H 2 N H H H C EICs of 351 H H H H H UPLC/( )ESI-TFMS Time (min) Fig. S7. UPLC/( )ESI-TFMS EICs associated with three major classes of oligoesters previously observed in isoprene high-n x SA (8, 9). For simplicity, only one structural isomer is shown in each of these EICs. These EICs were obtained from three different experiments in which the photooxidation of the same mixing ratio of MACR was conducted with varying levels of initial ½N 2 Š ½NŠ ratio. Increasing the initial ½N 2 Š ½NŠ ratio for these high-n x MACR experiments shown here results in the enhancement of both the previously characterized high-n x SA constituents and the high-n x SA masses. These enhancements are due to the formation and further reaction of MPAN under high-n 2 conditions. Surratt et al. 7of9

8 Table S1. High-N x MPAN SA constituents ½M HŠ ion* UPLC/ESI-TFMS measured mass TFMS suggested ion formula Error, mda i-fit Structure ligoester Series C 8 H C 12 H C 16 H ligoester Series C 8 H 12 N C 12 H 18 N C 16 H 24 N C 2 H 3 N ligoester Series C 9 H C 13 H C 17 H ligoester Series C 1 H C 14 H C 18 H ligoester Series C 8 H 11 N C 12 H 17 N *ligoester Series 1 ion numbers correspond to the purple numbers in Fig. 4, ligoester Series 2 ion numbers correspond to the green numbers in Fig. 4, ligoester Series 3 ion numbers correspond to the blue numbers in Fig. 4, ligoester Series 4 ion numbers correspond to the red numbers in Fig. 4, and ligoester Series 5 ion numbers correspond to the orange numbers in Fig. 4. For simplicity, only one isomer is shown. This oligoester series involves the esterification with formic acid. This oligoester series involves the esterification with acetic acid. Surratt et al. 8of9

9 Table S2 Summary of experimental conditions for low-n x experiments HC* ½HCŠ, ppb H precursor Seed aerosol Seed volume, μm 3 cm 3 RH, % SA volume, μm 3 cm 3 SA mass, μgm 3 Injection order BEPX 9 none neutral 4 BEPX Isoprene 4 H 2 2 highly acidic h oxidation Isoprene 4 H 2 2 neutral h oxidation Isoprene 4 H 2 2 highly acidic all reactants present at start Isoprene 49 H 2 2 neutral all reactants present at start BEPX 7 none highly acidic 5 BEPX 3-butene- 1,2-diol 1** none highly acidic 12 butenediol BEPX none neutral seed then BEPX BEPX none highly acidic seed then BEPX butadiene 1 g H 2 2 highly acidic h oxidation *HC: hydrocarbon; BEPX: 2,3-epoxy-1,4-butanediol. Temperatures: K. Neutral: ðnh 4 Þ 2 S 4 ; highly acidic: MgS 4 þ H 2 S 4. Not corrected for wall loss. Corrected for wall loss, assuming density of 1.4. Not available owing to order of injection. **Estimated based on amount injected. HC* ½HCŠ, ppb Table S3 Summary of experimental conditions for high-n x experiments H precursor additional ½NŠ, N x ppb ½N 2 Š, ppb ½N 2 Š ½NŠ Seed volume, μm 3 cm 3 RH, % SA volume, μm 3 cm 3 SA mass, μgm 3 methacrolein 277 HN +N methacrolein 285 HN þn methacrolein 257 HN +N MPAN HN none MB 218 H 2 2 þn þ N <2 <2 methacrylic 1 HN none <2 <2 acid isoprene 523 HN none *HC: hydrocarbon; MPAN: methacryloylperoxynitrate; MB: 2-methyl-3-buten-2-ol. Temperatures: K. Not corrected for wall loss. Corrected for wall loss, assuming density of 1.4. Not measured. Surratt et al. 9of9