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Published March 4, 2016 | Supplemental Material + Published
Journal Article Open

SOA formation from the photooxidation of α-pinene: systematic exploration of the simulation of chamber data


Chemical mechanisms play an important role in simulating the atmospheric chemistry of volatile organic compound oxidation. Comparison of mechanism simulations with laboratory chamber data tests our level of understanding of the prevailing chemistry as well as the dynamic processes occurring in the chamber itself. α-Pinene photooxidation is a well-studied system experimentally, for which detailed chemical mechanisms have been formulated. Here, we present the results of simulating low-NO α-pinene photooxidation experiments conducted in the Caltech chamber with the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) under varying concentrations of seed particles and OH levels. Unexpectedly, experiments conducted at low and high OH levels yield the same secondary organic aerosol (SOA) growth, whereas GECKO-A predicts greater SOA growth under high OH levels. SOA formation in the chamber is a result of a competition among the rates of gas-phase oxidation to low-volatility products, wall deposition of these products, and condensation into the aerosol phase. Various processes – such as photolysis of condensed-phase products, particle-phase dimerization, and peroxy radical autoxidation – are explored to rationalize the observations. In order to explain the observed similar SOA growth at different OH levels, we conclude that vapor wall loss in the Caltech chamber is likely of order 10^(−5) s^(−1), consistent with previous experimental measurements in that chamber. We find that GECKO-A tends to overpredict the contribution to SOA of later-generation oxidation products under high-OH conditions. Moreover, we propose that autoxidation may alternatively resolve some or all of the measurement–model discrepancy, but this hypothesis cannot be confirmed until more explicit mechanisms are established for α-pinene autoxidation. The key role of the interplay among oxidation rate, product volatility, and vapor–wall deposition in chamber experiments is illustrated.

Additional Information

© Author(s) 2016. This work is distributed under the Creative Commons Attribution 3.0 License. Received: 2 November 2015 – Published in Atmos. Chem. Phys. Discuss.: 25 November 2015. Revised: 17 February 2016 – Accepted: 23 February 2016 – Published: 4 March 2016. This work was supported by National Science Foundation grant AGS-1523500. Renee C. McVay acknowledges support by a National Science Foundation Graduate Research Fellowship under grant no. DGE-1144469. Bernard Aumont acknowledges support from the French National Research Agency (ANR) under the MAGNIFY project (ANR-14-CE01-0010).

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Published - acp-16-2785-2016.pdf

Supplemental Material - acp-16-2785-2016-supplement.pdf


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