Chemical characterization and quantification of organic aerosols: addressing storage effects and peroxide quantification

Organic aerosols are crucial constituents of atmospheric particulate matter, significantly influencing the Earth’s climate and human health. Despite extensive research, large uncertainties remain in the molecular-level chemical characterization of aerosols, particularly regarding the effects of sample storage during offline analysis and the quantification of specific compounds, such as organic
peroxides. This dissertation addresses these challenges through two main objectives: characterizing the effects of storage conditions and time on the molecular-level
chemical composition of aerosol samples and developing a novel method to identify and quantify individual peroxides in aerosols.
To evaluate the effects of storage conditions on the chemical composition of aerosols, β-pinene secondary organic aerosol (SOA), naphthalene SOA, and urban atmospheric aerosol were collected on filters and investigated. To characterize temporal changes in aerosol composition, all samples were extracted and analyzed immediately, or stored as aqueous extracts or filters for 24 h, 1 week, 2 weeks,
or 4 weeks at either +20°C, -20°C, or -80°C. Analysis was conducted using ultrahigh-performance liquid chromatography-high-resolution mass spectrometry (UHPLC-HRMS). Targeted and non-targeted data analysis combined with principal component analysis were used to identify changes in composition over time. The study highlights that all samples should be kept frozen as soon as possible after sampling to best retain their chemical composition compared to the fresh
collected samples. In contrast, storage of both aqueous extracts and filters at room temperature led to significant compositional changes even at short storage times of
only 1 day. In cases where immediate frozen storage is not possible, authors should mention in detail how the samples were stored and how much time passed between collection and analysis to reduce uncertainties.
The significant compositional changes observed in samples stored at room temperature (i.e. +20°C) were further investigated and characterized. β-Pinene SOA filter and extract samples show distinct temporal concentration changes for monomers and oligomers. In aqueous SOA extracts a significant increase is observed for monomers, while dimers decay at the same time. The inverse can be seen on filters, a strong and persistent increase for dimers, while many of the monomers decrease. Additionally, new dimer compounds are formed over time in SOA samples stored on filters. These observed trends are proposed to be due to hydrolysis
of dimers in aqueous extracts, and a continuous formation of oligomers in SOA formed through reactions of monomers on filters. Further experiments were done to confirm dimer formation through esterification of monomers. It is important to consider such on-filter reaction artifacts when detailed composition of organic aerosol is studied. These continuous reactions of SOA components over days and
weeks on filters can also mimic dark aging particle phase processes in particles with low-water content in the ambient atmosphere over their entire lifetime. Such long-term experiments of many days are not possible with conventional laboratory chamber studies.
The second main objective of this thesis shifts the focus away from storage effects to the quantification of peroxides, which have been identified as an important class
of SOA components contributing to aerosol toxicity and new particle formation. Despite their importance, there are large uncertainties about their contribution to
the mass of SOA. One source of uncertainty may be the differences in detection methods, such as iodometric titration, which is often used to determine the total
peroxide concentration in aerosol samples. A major drawback of such methods is the inability to identify and quantify individual peroxide concentrations in organic aerosol. Therefore a novel high-performance liquid-chromatography (HPLC) in-column derivatization method is presented to identify and quantify individual organic peroxides in SOA through chemiluminescence of luminol catalyzed by cytochrome c. Three different sample types were measured: commercially available peroxide standards, samples generated through liquid-phase ozonolysis of α-Pinene and 3-Carene, and laboratory generated SOA from α-Pinene, 3-Carene, naphthalene, and a 3-Carene and naphthalene mix. The results presented highlight the methods capability of differentiating between different samples. All samples are additionally analyzed by traditional iodometry with UV-Vis to obtain a total peroxide concentration. A clear linear correlation is observed between the HPLC chemiluminescence
method and iodometry for peroxide quantification. This allows for quantification of individual peaks in the chromatograms. A unique cross-product peroxide peak
in the 3-Carene/naphthalene mix SOA is identified and quantified to contribute 5.5% of the total peroxide concentration, illustrating the additional complexity when
several SOA precursors are oxidized simultaneously, as is the case in the ambient atmosphere.