Articles | Volume 16, issue 5
Atmos. Chem. Phys., 16, 2785–2802, 2016
Atmos. Chem. Phys., 16, 2785–2802, 2016

Research article 04 Mar 2016

Research article | 04 Mar 2016

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

Renee C. McVay1, Xuan Zhang2,a, Bernard Aumont3, Richard Valorso3, Marie Camredon3, Yuyi S. La3, Paul O. Wennberg2,4, and John H. Seinfeld1,2 Renee C. McVay et al.
  • 1Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA 91125, USA
  • 2Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA 91125, USA
  • 3LISA, UMR CNRS/INSU 7583, Université Paris Est Créteil et Université Paris Diderot, Institut Pierre Simon Laplace, 94010 Créteil CEDEX, France
  • 4Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA
  • anow at: Center for Aerosol and Cloud Chemistry, Aerodyne Research, Billerica, MA, USA

Abstract. 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.

Short summary
Secondary organic aerosol (SOA) affects climate change, human health, and cloud formation. We examine SOA formation from the biogenic hydrocarbon α-pinene and observe unexpected experimental results that run contrary to model predictions. Various processes are explored via modeling to rationalize the observations. The paper identifies the importance of further constraining via experiments various steps in the chemical mechanism in order to accurately predict SOA worldwide.
Final-revised paper