The oxidation regime and SOA composition in limonene ozonolysis: roles of different double bonds, radicals, and water
Authors: Gong Yiwei , Chen Zhongming , Li Huan 
Published: October 2018

Abstract. Volatile organic compounds play an important role in air quality and climate
change, largely because they contribute to the formation of oxidizing
compounds and secondary organic aerosol (SOA). In this study, a series of
products, including peroxides and carbonyl compounds in both gaseous and
particulate phases, were simultaneously detected to investigate the oxidation
regime and SOA composition in limonene ozonolysis. The roles of different
double bonds (DBs), radicals, and water were also examined. In our first
investigation, we focused on representative oxidizing compounds produced in
limonene ozonolysis, including stabilized Criegee intermediates (SCIs), OH
radicals, and peroxides. The dependence of H2O2 and
hydroxymethyl hydroperoxide (HMHP) formation on RH demonstrates that the
reaction with water is an important reaction pathway for limonene SCIs, and
the lower limit SCI yields of endocyclic and exocyclic DBs were estimated to
be ∼0.24 and ∼0.43, respectively. The OH yield was determined by
adding sufficient amounts of an OH scavenger, and the OH yields of endocyclic
and exocyclic DBs were ∼0.65 and ∼0.24, respectively. These
results indicate that in limonene ozonolysis the endocyclic DB is inclined to
generate OH radicals through the hydroperoxide channel, while the exocyclic
DB has a higher fraction of forming SCIs. Additionally, other gas-phase and
particle-phase peroxides were also studied in this work. The formation of
performic acid (PFA) and peracetic acid (PAA) was promoted significantly by
increasing RH and the degree of oxidation, and the discrepancy between the
experimental and model results suggested some missing formation pathways.
Considerable generation of H2O2 from SOA in the aqueous phase
was observed, especially at a high [O3] ∕ [limonene] ratio,
which was mainly attributed to the hydration and decomposition of unstable
peroxides in SOA such as peroxycarboxylic acids and peroxyhemiacetals.
Different DBs and OH scavengers had a large impact on the particulate
peroxides, and their stability indicated that the types of peroxides in SOA
changed under different conditions. As for the contribution of peroxides to
SOA, the results demonstrated that the mass fraction of particulate peroxides
in limonene SOA was less than 0.2 at a low [O3] ∕ [limonene]
ratio, while the mass fraction was 0.4–0.6 at a high
[O3] ∕ [limonene] ratio. The partitioning behavior of peroxides
showed that multi-generation oxidation helped produce more low-volatility
peroxides, which partially explained the higher SOA yield. The partitioning
behavior of carbonyls was also examined and the experimental partitioning
coefficients (Kp) were found to be typically several orders of
magnitude higher than the theoretical values. This study provided new
insights into the oxidation regime and SOA composition in limonene
ozonolysis, and limonene showed its specificity in many aspects when both
endocyclic and exocyclic DBs were ozonated. We suggest that the atmospheric
implications of terpenes containing more than one DB and the SOA composition,
especially particulate peroxides, need further study.