Persistence of Monoterpene-Derived Organic Peroxides in the Atmospheric Aqueous Phase

Abstract

Organic peroxides are essential reactive species formed during the atmospheric oxidation of organic compounds and can effectively partition into aqueous aerosols and cloud droplets, yet their aqueous-phase fate in aerosol and cloud water remains poorly characterized. Here, we report the first investigation of the isomer-resolved aqueous-phase chemical behaviors of a large suite of organic peroxides in dissolved monoterpene secondary organic aerosol (SOA). We find that about half of individual peroxides undergo noticeable hydrolysis, while the other half mainly react via nonhydrolysis unimolecular or bimolecular pathways. The chemical complexity of organic peroxides, as well as the effects of coexisting organic compounds in dissolved SOA, results in diverse chemical behaviors of individual peroxides and a complicated dependence on the water content, pH, and even organic concentration in the aqueous phase. Compared to the previously investigated α-substituted hydroperoxides that undergo hydrolysis on timescales of minutes to a few hours, the vast majority of organic peroxides studied herein are significantly more persistent in the aqueous phase, with decay rate constants of 1.8 × 10 −6 to 2.4 × 10 −4 s −1 and the corresponding e-folding lifetimes of 1.1−153 h under atmospherically relevant conditions, which would have distinctly different impacts on atmospheric chemistry and human health.

Organic Peroxides in Aerosol: Key Reactive Intermediates for Multiphase Processes in the Atmosphere

“…Over the past decades, the multiphase fate of POs in air has become of great interest to the atmospheric chemistry community. As indicated in Figure , POs have been observed to undergo hydrolysis, ,,, photolysis, interaction with SO 2 , ,, and transition metal ions (TMIs), , as well as many other bimolecular reactions , upon partitioning from the gas phase to the condensed phase. In this section, we discuss the multiphase chemical transformation behaviors of POs that have been investigated so far with an evaluation of their relative importance in the atmosphere.…”

“…In addition, the hydrolysis behaviors of other types of POs have been rarely studied so far. Yet, a very recent study has shown that substantial amounts of non-α-substituted POs detected in α-pinene SOA are relatively stable (with lifetimes up to days) in the aqueous phase, which would have distinctly different implications on atmospheric aqueous chemistry compared to the fast hydrolysis of α-substituted hydroperoxides, as summarized in Figure . Also, many of the studies reviewed here are focused on the kinetics of PO hydrolysis, with only a few studies clearly reporting the reaction products, and proposed the detailed mechanisms .…”

“…The photolysis lifetime was derived by scaling the experimentally determined lifetime under UVB exposure under room temperature to atmospheric actinic flux at a solar zenith angle of 65° (mimicking summer in Los Angeles) . The hydrolysis lifetimes were estimated by the PO decomposition kinetics measured under both heated conditions (308 K) and room temperature (288 K). , With the kinetics between TMI and POs measured for MHP and ISOPOOH, the lifetime of POs was estimated based on a TMI concentration of 10 –6 –10 –3 M . Lifetimes of POs against interactions with SO 2 were estimated with a SO 2 mixing ratio of 1 ppb. , The unresolved multiphase lifetimes of isomer-resolved POs were measured to be 1.7 h to 1.6 days for PO monomers and minutes to 1.6 days for PO dimers .…”

“…The lifetimes of different types of POs against hydrolysis span a wide range. Our most recent work indicates that POs in α-pinene SOA-diluted solution undergo e-folding lifetimes of 1.1–153 h . While most of the hydrolysis rates were measured in dilute aqueous solutions, the hydrolysis rate for POs in organic-rich SOA where less water is available might be different (slower than the kinetics measured in water so far).…”

“…The photolysis lifetime was derived by scaling the experimentally determined lifetime under UVB exposure under room temperature to atmospheric actinic flux at a solar zenith angle of 65°(mimicking summer in Los Angeles) 52. The hydrolysis lifetimes were estimated by the PO decomposition kinetics measured under both heated conditions (308 K)400 and room temperature (288 K) 356,396. With the kinetics between TMI and POs measured for MHP422 and ISOPOOH,50 the lifetime of POs was estimated based on a TMI concentration of 10 −6 −10 −3 M 424.…”