ACCOUNTING FOR CHEMICAL COMPLEXITY IN THE FORMATION OF HIGHLY
OXIDIZED COMPOUNDS BY ATMOSPHERIC AUTOXIDATION
T. KURTÉN1, K. H. MØLLER2, M. P. RISSANEN3, K. TIUSANEN1, P. ROLDIN4, N. HYTTINEN1, R.
V. OTKJÆR2, J. THORNTON5, M. EHN3, J. FRY6, H. G. KJAERGAARD3
1
University of Helsinki, Department of Chemistry, Helsinki, Finland
University of Copenhagen, Department of Chemistry, Copenhagen, Denmark
3
University of Helsinki, Department of Physics, Helsinki, Finland
4
Lund University, Division of Nuclear Physics, Lund, Sweden
5
University of Washington, Department of Atmospheric Sciences, Seattle, Washington U.S.A.
6
Reed College, Department of Chemistry, Portland, Oregon, U.S.A.
2
Keywords: secondary organic aerosol, autoxidation, volatility, saturation vapor pressure
Peroxyradical - based autoxidation mechanisms have recently been established as a potentially
important source of highly oxidised, and thus potentially extremely condensable, monoterpene degradation
products (Ehn et al., 2014). Due to their high polarity and molecular weight, such compounds are precisely
the type of vapors needed to explain the secondary organic aerosol (SOA) formation missing from current
atmospheric chemistry models (Riipinen et al., 2011).
Most work on atmospheric monoterpene oxidation (including autoxidation) in the context of SOA
formation has focused on α-pinene, as its emissions accounts for about 35% of total global monoterpene
emissions (Griffin et al., 1999). Other monoterpenes such as -pinene, limonene, and 3-carene have also
been studied, though to a lesser extent. Similarly, OH- and O3-based oxidation have (in this context) been
studied more often than NO3-based oxidation. From a chemical perspective, it is hardly surprising that
different monoterpene-oxidant combinations display different behavior with respect to for example the yield
of autoxidation products (and/or SOA), as well as the elemental compositions of the products. Some of these
differences are easily understood based on already established reaction mechanisms: for example, OHoxidation generally (Jokinen et al., 2015) has smaller yields of autoxidation products than ozonolysis,
probably due to the (on average) greater number of oxygen atoms in the peroxyradicals formed in the latter.
On the other hand, OH-oxidation typically leads to a larger variety of structures, as OH can react with
monoterpenes both via hydrogen abstraction and addition mechanisms, while O3 and NO3 react solely by
addition. In comparing different alkenes reacting with the same oxidant, exocyclic alkenes such as -pinene
have generally lower autoxidation product yields than analogous endocyclic alkenes such as α-pinene, likely
due to fragmentation in the initial oxidant attack. In contrast, compounds with multiple double bonds such
as limonene are, as expected, more reactive, and tend to have higher yields (Jokinen et al., 2015).
Some of the observed differences between monoterpenes are more difficult to explain. For example,
the NO3-inititated oxidation of α-pinene in the absence of seed aerosol leads to essentially no SOA
formation. In contrast, the SOA yield from NO3-initiated oxidation of 3-carene – which has an almost
identical structure, as shown in Figure 1 – can be up to 72% (Fry et al., 2014). Our preliminary modeling
work, using a recently developed cost-effective approach for addressing conformational complexity (Møller
et al., 2016) suggests that the difference is related to relative branching ratios of alkoxyradical ring-breaking
reactions. O3-initiated autoxidation of 3-carene is also likely to be more effective than that of α-pinene, as
the C3 ring in the former hinders the peroxyradical hydrogen shifts to a lesser degree than the C4 ring in the
latter (Kurtén et al., 2015). Neglecting chemical complexity thus proves to be dangerous: extrapolating
results from α-pinene to all monoterpene, not to mention sesquiterpenes, very likely leads to serious errors
in e.g. predictions of global SOA yield.
Figure 1. α-pinene (left) and 3-carene (right) have almost identical chemical structures. Despite this, αpinene forms very little low-volatility compounds in NO3-initiated oxidation, while 3-carene forms
significant amounts.
The products of peroxyradical autoxidation likely contain multiple peroxy acid or hydroperoxide
groups, and represent a new and hitherto unknown class of chemical compounds in the atmosphere.
Estimates of their thermodynamic properties such as vapor pressures and solubilities are urgently needed to
determine their ultimate fate, and impact on SOA formation. Unfortunately, the basis datasets of existing
empirical group contribution methods for determining saturation vapor pressures do not contain complex
polyhydroperoxides, and their predictions for these compounds are therefore unreliable. Our recent results
(Kurtén et al., 2016) indicate that due to the strong intramolecular H-bonding of the hydroperoxide groups,
autoxidation products originating from ozonolysis are much more volatile than previously assumed, and
most of them are likely to be “LVOC” (low volatility organic compounds) or even “SVOC” (semi-volatile
organic compounds) rather than “ELVOC” (extremely low volatility organic compounds). While LVOCs
and SVOCs will still play a role in SOA formation, they will preferentially condense onto existing particles
rather than form new clusters. Preliminary calculations (Berndt et al., 2016) indicate that products of OHinitiated autoxidation may be less volatile, and could thus play a larger role for SOA, and especially newparticle formation, despite smaller overall yields compared to O3-initiated autoxidation.
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