Role of ozone in SOA formation from alkane photooxidation 1Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, USA
14 Feb 2014
2Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, CA, USA
Received: 15 Aug 2013 – Published in Atmos. Chem. Phys. Discuss.: 24 Sep 2013 Abstract. Long-chain alkanes, which can be categorized as intermediate volatility
organic compounds, are an important source of secondary organic
aerosol (SOA). Mechanisms for the gas-phase OH-initiated oxidation of
long-chain alkanes have been well documented; particle-phase chemistry,
however, has received less attention. The δ-hydroxycarbonyl, which is
generated from the isomerization of alkoxy radicals, can undergo
heterogeneous cyclization and dehydration to form substituted dihydrofuran.
Due to the presence of C=C bonds, the substituted dihydrofuran is predicted
to be highly reactive with OH, and even more so with O3 and NO3,
thereby opening a reaction pathway that is not usually accessible to alkanes.
This work focuses on the role of substituted dihydrofuran formation and its
subsequent reaction with OH, and more importantly ozone, in SOA formation
from the photooxidation of long-chain alkanes. Experiments were carried out
in the Caltech Environmental Chamber using dodecane as a representative
alkane to investigate the difference in aerosol composition generated from
"OH-oxidation-dominating" vs. "ozonolysis-dominating" environments. A
detailed mechanism incorporating the specific gas-phase photochemistry,
together with the heterogeneous formation of substituted dihydrofuran and its
subsequent gas-phase OH/O3 oxidation, is used to evaluate the importance
of this reaction channel in dodecane SOA formation. We conclude that (1) the
formation of δ-hydroxycarbonyl and its subsequent heterogeneous
conversion to substituted dihydrofuran is significant in the presence of
NOx; (2) the ozonolysis of substituted dihydrofuran dominates over
the OH-initiated oxidation under conditions prevalent in urban and rural air;
and (3) a spectrum of highly oxygenated products with carboxylic acid, ester,
and ether functional groups are produced from the substituted dihydrofuran
chemistry, thereby affecting the average oxidation state of the SOA.
Revised: 02 Jan 2014 – Accepted: 09 Jan 2014 – Published: 14 Feb 2014
Citation: Zhang, X., Schwantes, R. H., Coggon, M. M., Loza, C. L., Schilling, K. A., Flagan, R. C., and Seinfeld, J. H.: Role of ozone in SOA formation from alkane photooxidation, Atmos. Chem. Phys., 14, 1733-1753, doi:10.5194/acp-14-1733-2014, 2014.