SOA formation from the atmospheric oxidation of MBO - ACPD

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5 10 15 20 25 induction time given its very large OH rate constant of 5.8 × 10 −11 cm 3 molec −1 s −1 (Carrasro et al., 2007). The profiles of the inorganic species show the characteristics of a standard VOC/NO x irradiation. For experiments in a batch mode, values for the SOA concentrations can also be determined from the SMPS measurements. These measurements have the advantage of having low limits of detections with reasonably good precision. SOA formation is difficult to distinguish at the beginning of the experiment involving only NO due to the presence of seed aerosol. However, once the initial NO is removed from the system due to reaction, SOA concentrations are seen to increase very slightly above the seed aerosol concentration as the ozone builds up in the chamber. This formation; although not quantifiable as a yield; is similar to that observed for monoterpene photooxidation in the presence of NO x. Of greater interest are the carbonyl compounds formed during the irradiation shown in Fig. 1b. It is clear that the carbonyl products from the OH + MBO reaction are detected from the first DNPH sample taken at (0.7 h) with NO is still present in the system at relatively high concentrations. According to conventional reaction kinetics, OH adds to the double bond in MBO to form an RO 2 radical which oxidizes NO to NO 2 with the resultant alkoxy radical producing a carbonyl product. If OH adds to the least substituted position of MBO, glycolaldehyde and acetone result as the major carbonyl products. If OH is added to the most substituent position, scission of the alkoxy radical produces formaldehyde and 2-hydroxy-2-methylpropanal (2-HMP) as the predominant carbonyl products. The elution order of the products was established from the external hydrazone standards mentioned previously. Formaldehyde and acetone formed in the reaction are clearly identified from their retention times. Hydroxycarbons have previously been re- ported (Grosjean and Grosjean, 1995) to have retention times that precede carbonyl compounds of the same carbon number. Thus, glycolaldehyde is detected between the hydrazones of formaldehyde and acetaldehyde and, in fact, is found to nearly coelute with formaldehyde. Since standards are available, the formation of 2-HMP was 24054 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ACPD 11, 24043–24083, 2011 SOA formation from the atmospheric oxidation of MBO M. Jaoui et al. Title Page Abstract Introduction Conclusions References Tables Figures ◭ ◮ ◭ ◮ Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion
5 10 15 20 25 rationalized based on a retention time and elution order of the DNPH derivative between the formaldehyde and acetaldehyde derivatives (Grosjean and Grosjean, 1995). Under the conditions of these experiments, except for acetone, these compounds all have very rapid OH rate constants (> 1×10 −11 cm 3 molec −1 s −1 ) and thus yields for the carbonyls are very difficult to determine in the absence of secondary OH reaction and photolysis. Secondary reactions of glycolaldehyde and acetone are known to produce the dicarbonyl compounds, glyoxal and methyl glyoxal, respectively. Glyoxal (GLY) was detected at relatively high levels during the irradiation, as seen in Fig. 1b, whereas methyl glyoxal (MGLY) was formed at low levels. This finding reflects the factor of 50 differences in OH rate constants for glycolaldehyde and acetone. While photolysis rate constants are less well established, it is possible that photolysis contributed to some degree to the formation of methyl glyoxal by acetone. Finally while glyoxal was identified by retention time with authentic standards, there is the possibility that the derivative for glyoxal might have formed from a partially acidic alcoholic hydrogen from glycolaldehyde. However, previous measurements suggest that the process is too slow to be of much importance (Smith et al., 1989). Nonetheless, this possibility cannot be completely excluded although efforts were made to render this process negligible. Two peaks (unkn 1 and unkn 2: Fig. 1b) were detected with a relatively high concentrations, these compounds could not be identified in this study. The importance of the dicarbonyl compounds rest in their possible significance for the formation of organic aerosol. 3.2.2 Characterization of gas-phase products from denuder sampling A qualitative analysis was performed of the gas-phase products for experiments involving the presence or absence of NO x. The analysis uses the PFBHA + BSTFA double derivatization of a denuder extract. While this technique is an integrated technique that requires long sampling times, it does provide a sensitive method for measuring low concentrations of highly oxidized organic compounds, possibly semivolatile compounds in the gas phase. Thus, products found by this collection technique could be informative 24055 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ACPD 11, 24043–24083, 2011 SOA formation from the atmospheric oxidation of MBO M. Jaoui et al. Title Page Abstract Introduction Conclusions References Tables Figures ◭ ◮ ◭ ◮ Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion
Page 1 and 2: Atmos. Chem. Phys. Discuss., 11, 24
Page 3 and 4: 5 10 15 20 25 the oxidation of MBO,
Page 5 and 6: 5 10 15 20 25 OH and NO 3 radicals,
Page 7 and 8: 5 10 15 20 25 2.2 Gas phase measure
Page 9 and 10: 5 10 15 20 25 All extracts were ana
Page 11: 5 10 15 20 25 formation during the
Page 15 and 16: 5 10 15 20 25 3.3 Particle-phase pr
Page 17 and 18: 5 10 15 20 25 The GC-MS analysis sh
Page 19 and 20: 5 10 15 20 25 small abundance of 2-
Page 21 and 22: 5 10 15 20 25 fraction was 0.05 (C/
Page 23 and 24: 5 10 15 20 25 30 Environ., 33, 2893
Page 25 and 26: 5 10 15 20 25 30 a review, Atmos. C
Page 27 and 28: Geophys. Res. Lett., 34, L19807, do
Page 29 and 30: Table 2. SOA and gas phase compound
Page 31 and 32: Table 2. Continued. ID Rt MW m/z (m
Page 33 and 34: (a) (a) (b) (b) Fig. 1. Time profil
Page 35 and 36: 1 * 1 * 2 3 * 4 5 3 4 2 * Fig. 3. G
Page 37 and 38: ER464-GF6-BSTFA-ei #1431 T: + c Ful
Page 39 and 40: (a): (a): DHIP DHIP (20) (20) in in
Page 41: Scheme 1. Proposed fragmentation pa

SOA formation from the atmospheric oxidation of MBO - ACPD

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