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Published October 15, 2013 | Supplemental Material
Journal Article Open

Observational Insights into Aerosol Formation from Isoprene


Atmospheric photooxidation of isoprene is an important source of secondary organic aerosol (SOA) and there is increasing evidence that anthropogenic oxidant emissions can enhance this SOA formation. In this work, we use ambient observations of organosulfates formed from isoprene epoxydiols (IEPOX) and methacrylic acid epoxide (MAE) and a broad suite of chemical measurements to investigate the relative importance of nitrogen oxide (NO/NO_2) and hydroperoxyl (HO_2) SOA formation pathways from isoprene at a forested site in California. In contrast to IEPOX, the calculated production rate of MAE was observed to be independent of temperature. This is the result of the very fast thermolysis of MPAN at high temperatures that affects the distribution of the MPAN reservoir (MPAN / MPA radical) reducing the fraction that can react with OH to form MAE and subsequently SOA (F_(MAE formation)). The strong temperature dependence of F_(MAE formation) helps to explain our observations of similar concentrations of IEPOX-derived organosulfates (IEPOX-OS;~1 ng m^(–3)) and MAE-derived organosulfates (MAE-OS;~1 ng m^(–3)) under cooler conditions (lower isoprene concentrations) and much higher IEPOX-OS (~20 ng m^(–3)) relative to MAE-OS (<0.0005 ng m^(–3)) at higher temperatures (higher isoprene concentrations). A kinetic model of IEPOX and MAE loss showed that MAE forms 10−100 times more ring-opening products than IEPOX and that both are strongly dependent on aerosol water content when aerosol pH is constant. However, the higher fraction of MAE ring opening products does not compensate for the lower MAE production under warmer conditions (higher isoprene concentrations) resulting in lower formation of MAE-derived products relative to IEPOX at the surface. In regions of high NO_x, high isoprene emissions and strong vertical mixing the slower MPAN thermolysis rate aloft could increase the fraction of MPAN that forms MAE resulting in a vertically varying isoprene SOA source.

Additional Information

© 2013 American Chemical Society. Received: March 12, 2013. Revised: August 1, 2013. Accepted: September 4, 2013. Published: September 4, 2013. Funding for UCB was provided by the National Science Foundation (NSF, Grants ATM-0922562 and ATM-0639847). DKF, KSD, MJC and JLJ were supported by NSF ATM-0919189. JBG and JDG were partially supported by NSF ATM-0516610. GMW acknowledges support from NASA Earth Systems Science Fellowship NNG-05GP64H and U.S.-EPA STAR Fellowship (FP-91698901). MRB, JMC, JC, and POW were supported by NSF ATM-0934408 and ATM-0934345. Analyses at the University of Aarhus were partially funded by the NSF US-NORDIC BSOA workshop program. The authors acknowledge Sierra Pacific Industries for the use of their land and the Blodgett Forest Research Station for cooperation in facilitating this research. The authors thank S. S. Cliff (University of California, Davis) for the loan of the high volume filter sampler, N. C. Bouvier-Brown (Loyola Marymount University, Los Angeles) for assistance with filter collection, J. Murphy (University of Toronto) for initial discussions about the E-AIM model, N. Eddingsaas (Caltech) for providing the BEPOX-derived organosulfate standard, D. Covert (University of Washington) for providing the SMPS measurements in 2009 and M. Chan and K. Schilling (Caltech) for their assistance with filter extractions and UPLC/ESI-HR-TOFMS analyses. The authors declare no competing financial interest.

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