Direct Aqueous Photochemistry of Isoprene High-NOx Secondary Organic Aerosol
Secondary organic aerosol (SOA) generated from the high-NOx photooxidation of isoprene was dissolved in water and irradiated with {lambda} > 290 nm light to simulate direct photolytic processing of organics in atmospheric water droplets. High-resolution mass spectrometry was used to characterize the composition at four time intervals (0, 1, 2, and 4 h). Photolysis resulted in the decomposition of high molecular weight (MW) oligomers, reducing the average length of organics by 2 carbon units. Approximately 65% by count of SOA molecules decomposed during photolysis, accompanied by the formation of new products. An average of 30 % of the organic mass was modified after 4 h of direct photolysis. In contrast, only a small fraction of the mass (<2 %), belonging primarily to organic nitrates, decomposed in the absence of irradiation by hydrolysis. We observed a statistically-significant increase in average O/C, decrease in H/C, and increase in N/C ratios resulting from photolysis. Furthermore, the concentration of aromatic compounds increased significantly during photolysis. Approximately 10 % of photodegraded compounds and 50 % of the photoproducts contain nitrogen. Organic nitrates and multifunctional oligomers were identified as compounds degraded by photolysis. Low-MW 0N (compounds with 0 nitrogen atoms in their structure) and 2N compounds were the dominant photoproducts. Fragmentation experiments using tandem mass spectrometry (MSn, n = 2-3) indicate that the 2N products are likely heterocyclic/aromatic and are tentatively identified as furoxans. Although the exact mechanism is unclear, these 2N heterocyclic compounds are produced by reactions between photochemically-formed aqueous NOx species and SOA organics.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1047387
- Report Number(s):
- PNNL-SA-89184; KP1704020
- Journal Information:
- Physical Chemistry Chemical Physics, Journal Name: Physical Chemistry Chemical Physics Journal Issue: 27 Vol. 14; ISSN 1463-9076
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
AEROSOLS
AROMATICS
ATOMS
CARBON
DECOMPOSITION
DROPLETS
FRAGMENTATION
HETEROCYCLIC COMPOUNDS
HYDROLYSIS
IRRADIATION
ISOPRENE
LENGTH
MASS
MASS SPECTROSCOPY
MOLECULAR WEIGHT
MOLECULES
NITRATES
NITROGEN
PHOTOCHEMISTRY
PHOTOLYSIS
PROCESSING
UNITS
VISIBLE RADIATION
WATER