Abstract
Introduction
Volatile organic compounds (VOCs) of both anthropogenic and biogenic origin are oxidized in the atmosphere to form particulate matter commonly referred to as secondary organic aerosol (SOA). Given the prominant role that SOA plays in air quality and global climate, understanding the composition and formation mechanism of SOA has been a major focus in the field of Atmospheric Chemistry. In the global scale, a large fraction of SOA arisese from oxidation of monoterpenes (e.g., α-pinene) which are oxidized readily by major atmospehric oxidants, such as the OH radical and O3 . Despite the number of studies focusing on SOA from α-pinene, our understanding of the reaction mechanisms and products from this reaction system still remains incomplete. Particularly, the measurement and identification of organic peroxides represent important missing aspects to complete the picture. Organic peroxides are reservoirs of the OH radical which governs the oxidative environment in the gas and condensed phases \cite{Ervens_2015,Crounse_2013}. As well, organic peroxides belong to reactive oxygen species (ROS) which is linked to the health effect of air pollution \cite{Tao_2003} . Quantification and identification of organic peroxides present in SOA are of great benefit in understanding the multifaceted role that SOA plays in the atmospheric environment.
It has been known that SOA arising from α-pinene contains a substential amount of total organic peroxides \cite{Docherty_2005}, quantified by chemical assays such as iodometry and \cite{Mutzel_2013} dichlorofluoresin (DCFH) assay \cite{Wang_2011,Badali_2015}. It is well established that organic hydroperoxides (ROOH) forms in both the \cite{Ziemann_2012} gas and aqueous \cite{VONSONNTAG_1997} phases, via reaction of a peroxy radical (RO2) with a hydroperoxy radical (HO2). The recent development of analytical techniques, particularly the application of liquid chromatography - electrospray ionization - time of flight mass spectrometry (LC-ESI-ToF-MS) on extracted SOA components, has lead to proposals of novel chemical mechanisms and has significantly advanced our understanding of particle-phase organic compounds, with many of which been proposed to be organic peroxides. (Groups to check out: Kalberer, Kristensen, Suratt, Nizkorodov, Johnston) These studies have shown that organic peroxides constitute a key component of particle-phase highly oxygenated multifunctional organic compounds (HOMs) which likely arises from repeated intra-molecular hydrogen-abstraction reactions and exhibit extremely low volatility \cite{Ehn_2014}. Alternatively, the stabilized Criegee intermediate (SCI) formed during gas-phase ozonolysis can react with organic acids and form a class of dimer estes that contains a hydroperoxy group \cite{Kristensen_2014,Kristensen_2016}. When present in sufficiently high mixing ratios in the gas phase, RO2 radicals can likely react with another RO2 radical to form ROOR species \citet{Kwan_2012,Ng_2008,Zhang_2015} .