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Published August 1, 2008 | Published
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Secondary organic aerosol (SOA) formation from reaction of isoprene with nitrate radicals (NO₃)


Secondary organic aerosol (SOA) formation from the reaction of isoprene with nitrate radicals (NO₃) is investigated in the Caltech indoor chambers. Experiments are performed in the dark and under dry conditions (RH < 10%) using N₂O₅ as a source of NO₃ radicals. For an initial isoprene concentration of 18.4 to 101.6 ppb, the SOA yield (defined as the ratio of the mass of organic aerosol formed to the mass of parent hydrocarbon reacted) ranges from 4.3% to 23.8%. By examining the time evolutions of gas-phase intermediate products and aerosol volume in real time, we are able to constrain the chemistry that leads to the formation of low-volatility products. Although the formation of ROOR from the reaction of two peroxy radicals (RO₂) has generally been considered as a minor channel, based on the gas-phase and aerosol-phase data it appears that RO₂+RO₂ reaction (self reaction or cross-reaction) in the gas phase yielding ROOR products is a dominant SOA formation pathway. A wide array of organic nitrates and peroxides are identified in the aerosol formed and mechanisms for SOA formation are proposed. Using a uniform SOA yield of 10% (corresponding to Mₒ ≅ 10 μg m⁻³), it is estimated that ~2 to 3 Tg yr⁻¹ of SOA results from isoprene + NO₃. The extent to which the results from this study can be applied to conditions in the atmosphere depends on the fate of peroxy radicals in the nighttime troposphere.

Additional Information

© Author(s) 2008. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 3 January 2008 – Published in Atmos. Chem. Phys. Discuss.: 15 February 2008. Revised: 3 July 2008 – Accepted: 3 July 2008 – Published: 1 August 2008. This research was funded by US Department of Energy Biological and Environmental Research Program DE-FG02-05ER63983. This material is based in part on work supported by the National Science Foundation (NSF) under grant ATM-0432377. The Waters LCT Premier XT time-of-flight mass spectrometer interfaced to a Waters UPLC system was purchased in 2006 with a grant from the National Science Foundation, Chemistry Research Instrumentation and Facilities Program (CHE-0541745). The LCQ Ion Trap mass spectrometer was purchased in 1997 with funds from the National Science Foundation through the CRIF program (CHE-9709233). J.D. Surratt is supported in part by the US EPA under the STAR Graduate Fellowship Program. A.J. Kwan and H.O.T. Pye acknowledge the support of NSF graduate research fellowships. The authors would like to thank C.D. Vecitis, J. Cheng, and M.R. Hoffmann for use of and aid with their ozonizer and UV-VIS spectrometer; to K. Takematsu and M. Okumura for helpful advice on preparing N₂O₅; to J.H. Kroll and M. Claeys for helpful discussions and suggestions; to M.N. Chan for assistance with filter sample collection; to H.G. Kjaergaard and F. Paulot for performing the quantum calculations and estimating the sensitivities of CIMS to various gas-phase products; and to Y. Yu and the reviewers for helpful comments on the manuscript.

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