Abstract. While camphene is one of the dominant monoterpenesmeasured in biogenic and pyrogenic emission samples, oxidation of camphenehas not been well-studied in environmental chambers and very little is knownabout its potential to form secondary organic aerosol (SOA). The lack ofchamber-derived SOA data for camphene may lead to significant uncertaintiesin predictions of SOA from oxidation of monoterpenes using existingparameterizations when camphene is a significant contributor to totalmonoterpenes. Therefore, to advance the understanding of camphene oxidationand SOA formation and to improve representation of camphene in air qualitymodels, a series of experiments was performed in the University ofCalifornia Riverside environmental chamber to explore camphene SOA massyields and properties across a range of chemical conditions atatmospherically relevant OH concentrations. The experimental results werecompared with modeling simulations obtained using two chemically detailedbox models: Statewide Air Pollution Research Center (SAPRC) and Generatorfor Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A).SOA parameterizations were derived from the chamber data using both thetwo-product and volatility basis set (VBS) approaches. Experiments performedwith added nitrogen oxides (NOx) resulted in higher SOA mass yields (upto 64 %) than experiments performed without added NOx (up to 28 %).In addition, camphene SOA mass yields increased with SOA mass (Mo) atlower mass loadings, but a threshold was reached at higher mass loadings inwhich the SOA mass yields no longer increased with Mo. SAPRC modelingof the chamber studies suggested that the higher SOA mass yields at higherinitial NOx levels were primarily due to higher production of peroxyradicals (RO2) and the generation of highly oxygenated organicmolecules (HOMs) formed through unimolecular RO2 reactions. SAPRCpredicted that in the presence of NOx, camphene RO2 reacts with NOand the resultant RO2 undergoes hydrogen (H)-shift isomerizationreactions; as has been documented previously, such reactions rapidly addoxygen and lead to products with very low volatility (i.e., HOMs). The endproducts formed in the presence of NOx have significantly lowervolatilities, and higher O : C ratios, than those formed by initial campheneRO2 reacting with hydroperoxyl radicals (HO2) or other RO2.Further analysis reveals the existence of an extreme NOx regime, whereinthe SOA mass yield can be suppressed again due to high NO / HO2 ratios.Moreover, particle densities were found to decrease from 1.47 to 1.30 g cm−3 as [HC]0 / [NOx]0 increased and O : C decreased. Theobserved differences in SOA mass yields were largely explained by thegas-phase RO2 chemistry and the competition between RO2+HO2, RO2+ NO, RO2+ RO2, and RO2 autoxidationreactions.
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Identifying a Missing Link: Confirmation of the Structure and Origin of 4-hydroperoxy-3-methylbut-2-enal (4-HPALD) with an Authentic Standard
Isoprene (C5H8) is the largest non-methane volatile organic compound emitted into the atmosphere. Isoprene reacts rapidly with ambient hydroxyl radicals (OH) and subsequent addition of O2 results in the formation alkyl peroxy (RO2) radicals. The fate of the initially formed RO2 radicals has been the focus of continuing theoretical and experimental research. Under pristine conditions where bimolecular reactions of RO2 are limited, the thermodynamically favored RO2 undergoes an intramolecular H-shift followed by reaction with O2 and elimination of HO2 to yield 4-hydroperoxy aldehyde (4-HPALD, C5H8O3), predicted to account for up to 13% of first-generation isoprene photochemical oxidation products. Mass spectrometric evidence has been reported for 4-HPALD, but lack of an authentic standard has precluded definitive confirmation of both the structure of 4-HPALD and its origin as a first-generation product of OH oxidation of isoprene. We report the synthesis and characterization of 4-HPALD and establish that it is a major product of isoprene oxidation. Synthetic 4-HPALD is isolated as the peroxyhemiacetal. As expected for the 4-hydroperoxy aldehyde, 1H NMR spectra show no evidence for equilibration with the carbonyl form, even in protic solvents, and gas-phase chemical analysis by CIMS also shows only a single form. OH oxidation of isoprene in an oxidation flow reactor coupled to an ion mobility source with an HR-CIMS detector unequivocally demonstrates 4-HPALD (and likely also 1-HPALD) as isoprene oxidation products. Although HPALDs have been discounted as significant contributors to SOA, oxidation of 4-HPALD in a potential aerosol mass (PAM) reactor in the presence of ozone and OH indicates 4-HPALD rapidly undergoes autooxidation reactions forming low-volatility particulate products. We have confirmed highly oxygenated compounds with compositions C5H8O6 and C5H10O6 likely from OH oxidation, and C5H10O7 and C5H10O8 compounds likely products of ozonolysis. The PAM oxidation experiment further
demonstrates that the highly oxygenated, low-volatility products efficiently nucleate particles.
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- Award ID(s):
- 2001027
- NSF-PAR ID:
- 10392022
- Date Published:
- Journal Name:
- AAAR 40th Annual Conference
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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