May 17 2017 03:00 PM
May 17 2017 04:00 PM
A seminar on "Gas-phase autoxidation of endocyclic alkenes as a direct source of organic highly oxidized material" by Matti P. Rissanen, Department of Physics, University of Helsinki was hosted this Wednesday by Prof. Mani Sarathy of the Clean Combustion Research Center.
oxidation of certain volatile organic compounds (VOC) leads to
highly-oxidized and very low volatile
material that is capable of acting as a direct source of atmospheric
secondary organic aerosol (SOA) [1, 2]. Contrary to previous perception
of slow oxidation by consecutive reactions in the atmospheric gas
mixture, the generation of highly-oxidized multifunctional
compounds (HOM) can be tremendously fast. HOM form by pseudo-one-step
reactions, even in a sub-second time-scale, by a process called
autoxidation . Autoxidation proceeds by sequential peroxy radical (RO2)
hydrogen-shift isomerization reactions and subsequent O2 addition steps, until a termination occurs by unimolecular dissociation
or by bimolecular reaction [2-4].
Endocyclic alkenes are cyclic compounds with one or more unsaturated bonds within an organic ring structure
(see Figure 1). They constitute a large fraction of biogenically produced terpenes (i.e. compounds that are multiples of C5H8
structural units) and are emitted to the atmosphere in vast quantities
. Cyclohexene is a common structural part of many abundant biogenic
monoterpenes such as α-pinene and limonene. Its symmetric and
relatively simple structure has made it a
common surrogate species for tracking detailed oxidation mechanisms and
for studying formation of secondary organic material. Previously, we
have investigated the autoxidation sequence of cyclohexene leading to
gas-phase products with an O/C ratio up to 1.5
, and applied the rules generated to understand the autoxidation
mechanisms of methylated cyclohexenes . Since then, the relevant
autoxidation reaction steps have been updated to include the latest
findings on, for example, ultrafast hydrogen shifts scrambling
the peroxy radical distribution , and by further unimolecular
dissociation processes. Currently we are investigating the influence of
autoxidation perturbation by addition of common atmospheric gas-phase
co-reactants NO, NO2,
SO2 and HO2,
as well as RO2 derived from other smaller VOCs. These reagents are seen to interfere with the oxidation pathways, mainly
terminating autoxidation prematurely and forming products which are indicative of the type of perturbing reaction.
To study the autoxidation phenomena, we perform flow reactor investigations of ozonolysis initiated oxidation
under ambient conditions utilizing chemical ionization mass spectrometry (CIMS) with nitrate (NO3-)
and iodide (I-) reagent
ions to detect products with varying levels of oxidation. By careful
selection of reacting gas molecules and by perturbing the reaction
mixture with certain gas-phase co-reagents (e.g. NO, NO2,
SO2 and HO2,
as well as RO2)
it is possible to infer mechanistic insight into the important reaction
steps that transform a completely
volatile precursor molecule into a very low volatile molecule that is
able to act as a substrate for atmospheric particle formation.
Rissanen completed his undergraduate studies in Chemistry at the
University of Helsinki with honors,
where he later also received his PhD in Physical Chemistry with the
highest possible grade, Laudatur. Since then he has been a Post-Doctoral
Researcher at the Division of Atmospheric Sciences at the Physics
Department, also at the Univeristy of Helsinki, with
research focus on gas-phase reactions creating low volatile material
that serves as a substrate for atmospheric new particle formation.
Recently he was appointed in a position of Finnish Academy Post-Doctoral
Researcher. Rissanen has also remained active with
the previous post in Chemistry Department and is still frequently
publishing papers on chemical kinetics mostly relevant to low
temperature combustion and atmospheric chemistry.
 Jimenez, J. et al, Science 2009, 326, 1525.
 Ehn, M. et al. Nature 2014, 506, 476.
 Crounse, J. D. et al. J. Phys. Chem. Lett. 2013, 4, 3513.
 Rissanen, M. P. et al. J. Am. Chem. Soc. 2014, 136, 15596.
 Sakulyanontvittaya, T. et al. Environ. Sci. Technol. 2008, 42, 1623.
 Rissanen, M. P. et al. J. Phys. Chem. A 2015, 119, 4633.
 Knap, H. C. and Jørgensen, S. J. Phys. Chem. A 2017, 121, 1470.