Articles | Volume 16, issue 18
Research article
19 Sep 2016
Research article |  | 19 Sep 2016

Volatility and lifetime against OH heterogeneous reaction of ambient isoprene-epoxydiols-derived secondary organic aerosol (IEPOX-SOA)

Weiwei Hu, Brett B. Palm, Douglas A. Day, Pedro Campuzano-Jost, Jordan E. Krechmer, Zhe Peng, Suzane S. de Sá, Scot T. Martin, M. Lizabeth Alexander, Karsten Baumann, Lina Hacker, Astrid Kiendler-Scharr, Abigail R. Koss, Joost A. de Gouw, Allen H. Goldstein, Roger Seco, Steven J. Sjostedt, Jeong-Hoo Park, Alex B. Guenther, Saewung Kim, Francesco Canonaco, André S. H. Prévôt, William H. Brune, and Jose L. Jimenez

Abstract. Isoprene-epoxydiols-derived secondary organic aerosol (IEPOX-SOA) can contribute substantially to organic aerosol (OA) concentrations in forested areas under low NO conditions, hence significantly influencing the regional and global OA budgets, accounting, for example, for 16–36 % of the submicron OA in the southeastern United States (SE US) summer. Particle evaporation measurements from a thermodenuder show that the volatility of ambient IEPOX-SOA is lower than that of bulk OA and also much lower than that of known monomer IEPOX-SOA tracer species, indicating that IEPOX-SOA likely exists mostly as oligomers in the aerosol phase. The OH aging process of ambient IEPOX-SOA was investigated with an oxidation flow reactor (OFR). New IEPOX-SOA formation in the reactor was negligible, as the OFR does not accelerate processes such as aerosol uptake and reactions that do not scale with OH. Simulation results indicate that adding  ∼  100 µg m−3 of pure H2SO4 to the ambient air allows IEPOX-SOA to be efficiently formed in the reactor. The heterogeneous reaction rate coefficient of ambient IEPOX-SOA with OH radical (kOH) was estimated as 4.0 ± 2.0  ×  10−13 cm3 molec−1 s−1, which is equivalent to more than a 2-week lifetime. A similar kOH was found for measurements of OH oxidation of ambient Amazon forest air in an OFR. At higher OH exposures in the reactor (>  1  ×  1012 molec cm−3 s), the mass loss of IEPOX-SOA due to heterogeneous reaction was mainly due to revolatilization of fragmented reaction products. We report, for the first time, OH reactive uptake coefficients (γOH =  0.59 ± 0.33 in SE US and γOH =  0.68 ± 0.38 in Amazon) for SOA under ambient conditions. A relative humidity dependence of kOH and γOH was observed, consistent with surface-area-limited OH uptake. No decrease of kOH was observed as OH concentrations increased. These observations of physicochemical properties of IEPOX-SOA can help to constrain OA impact on air quality and climate.

Short summary
IEPOX-SOA is biogenically derived secondary organic aerosol under anthropogenic influence, which has been shown to comprise a substantial fraction of OA globally. We investigated the lifetime of ambient IEPOX-SOA in the SE US and Amazonia, with an oxidation flow reactor and thermodenuder coupled with MS-based instrumentation. The low volatility and long lifetime of IEPOX-SOA against OH radicals' oxidation (> 2 weeks) was observed, which can help to constrain OA impact on air quality and climate.
Final-revised paper