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 Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1047387
- Report Number(s):
- PNNL-SA-89184; KP1704020; TRN: US201216%%246
- Journal Information:
- Physical Chemistry Chemical Physics, Vol. 14, Issue 27; ISSN 1463-9076
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
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