Formation of highly oxidized multifunctional compounds: autoxidation of peroxy radicals formed in the ozonolysis of alkenes – deduced from structure–product relationships

Abstract

Abstract. It has been postulated that secondary organic particulate matter plays a pivotal role in the early growth of newly formed particles in forest areas. The recently detected class of extremely low volatile organic compounds (ELVOC) provides the missing organic vapors and possibly contributes a significant fraction to atmospheric SOA (secondary organic aerosol). The sequential rearrangement of peroxy radicals and subsequent O 2 addition results in ELVOC which are highly oxidized multifunctional molecules (HOM). Key for efficiency of such HOM in early particle growth is that their formation is induced by one attack of the oxidant (here O 3 ), followed by an autoxidation process involving molecular oxygen. Similar mechanisms were recently observed and predicted by quantum mechanical calculations e.g., for isoprene. To assess the atmospheric importance and therewith the potential generality, it is crucial to understand the formation pathway of HOM.To elucidate the formation path of HOM as well as necessary and sufficient structural prerequisites of their formation we studied homologous series of cycloalkenes in comparison to two monoterpenes. We were able to directly observe highly oxidized multifunctional peroxy radicals with 8 or 10 O atoms by an Atmospheric Pressure interface High Resolution Time of Flight Mass Spectrometer (APi-TOF-MS) equipped with a NO − 3 -chemical ionization (CI) source. In the case of O 3 acting as an oxidant, the starting peroxy radical is formed on the so-called vinylhydroperoxide path. HOM peroxy radicals and their termination reactions with other peroxy radicals, including dimerization, allowed for analyzing the observed mass spectra and narrowing down the likely formation path. As consequence, we propose that HOM are multifunctional percarboxylic acids, with carbonyl, hydroperoxy, or hydroxy groups arising from the termination steps. We figured that aldehyde groups facilitate the initial rearrangement steps. In simple molecules like cycloalkenes, autoxidation was limited to both terminal C atoms and two further C atoms in the respective α positions. In more complex molecules containing tertiary H atoms or small, constrained rings, even higher oxidation degrees were possible, either by simple H shift of the tertiary H atom or by initialization of complex ring-opening reactions.

Relationship between chemical composition and oxidative potential of secondary organic aerosol from polycyclic aromatic hydrocarbons

“…Organic peroxides have been identified to be major components in both laboratory and ambient OA (Jokinen et al, 2014;Lin et al, 2016;Surratt et al, 2010;Zhang et al, 2015Zhang et al, , 2017. They can play important roles in forming high MW oligomers (Docherty et al, 2005) and highly oxygenated molecules (Mentel et al, 2015). Recent studies have shown that peroxides may also be important for OP.…”

Nanoparticle growth by particle-phase chemistry

“…(1) to reestablish equilibrium. The equilibrium point (Pankow, 1994) expressed as the volume ratio of SVOC in the particle phase…”

“…Absorptive partitioning (Barsanti et al, 2017;Pankow, 1994) of gasphase products to the particle phase forms secondary organic aerosol (SOA), which includes both non-volatile (NVOCs) and semi-volatile (SVOCs) organic compounds (Kroll and Seinfeld, 2008;Riipinen et al, 2012). NVOCs have a negligible evaporation rate after partitioning to the particle phase, and therefore cause particles to grow at a rate given by the condensation rate.…”

“…Gas-phase oxidation of volatile organic compounds occurs through a complex set of reaction pathways involving both the gas and particle phases to yield particle-phase products that often number in the hundreds or thousands based on accurate mass measurements (Bateman et al, 2008;Mentel et al, 2015;Reinhardt et al, 2007;Tu et al, 2016). Absorptive partitioning (Barsanti et al, 2017;Pankow, 1994) of gasphase products to the particle phase forms secondary organic aerosol (SOA), which includes both non-volatile (NVOCs) and semi-volatile (SVOCs) organic compounds (Kroll and Seinfeld, 2008;Riipinen et al, 2012).…”

Detection of RO2 radicals and other products from cyclohexene ozonolysis with NH4+ and acetate chemical ionization mass spectrometry

“…Two recent studies focused on the formation of these closed shell products starting from the ozonolysis of cycloalkenes (Rissanen et al, 2014;Mentel et al, 2015). Especially cyclohexene serves as model substance with an endocyclic double bond, which represents the reactive structural element of the most important monoterpenes α-pinene and limonene.…”