Temporal Evolution of Isomer-Specific Reactivity in Dark-Aged β-Pinene Secondary Organic Aerosols

Collection of secondary organic aerosol (SOA) onto filters is often used in combination with LC-MS for detailed chemical characterization. Studies have shown that a large fraction of laboratory-generated SOA can be attributed to high molecular weight oligomeric compounds. Often there is a significant delay between sample collection and analysis (e.g., during automated filter collection) which may lead to changes in the chemical profile of the samples. In this study β-pinene SOA was generated by O3 and OH oxidation and collected onto filters, which were either extracted in a mixture of water and acetonitrile or left on the filter and stored at room temperature, to investigate changes in the overall chemical composition profile over time up to one month. The samples were analyzed by UHPLC-ESI-MS in negative polarity mode. An untargeted analysis led to several thousand detected compounds and principal component analysis indicated significantly different compositional changes between the samples stored on filters or as extracts. In order to understand these differences, further focus was put on previously identified carboxylic acids, dimer esters and other oligomers. The concentration of several hundred compounds in the monomer mass range increased in extracts over the 4-week time span, whereas the opposite was observed for samples stored on filters, where the concentration of a large number of monomers decreased. For dimers in samples stored on filters, a large number of compounds increased in concentration, while extracts show the opposite behavior. A possible explanation for these trends could be the decomposition through hydrolysis of dimer esters and other components in extracts and the formation of oligomers on filters. This indicates the continuous chemical aging of the SOA particles deposited and stored on filters. To test this hypothesis, we nebulized a solution of carboxylic acid standards in excess onto filters where β-pinene SOA was deposited and monitored the temporal evolution of dimer esters which have been reported to form from diaterpenylic acid and carboxylic acids. A stronger increase of the dimers (and a stronger decrease of the monomer precursor products) was observed for these “spiked” samples in comparison to the “non-spiked” SOA filter controls. This study not only highlights that the persistent changes observed on filters are due to previously overlooked on-filter reactions, which can lead to misinterpretation of the detailed chemical composition of samples collected on filters for offline analyses. These on-filter reactions also mimic particle phase non-oxidative aging of SOA over the entire lifetime of SOA particles in the atmosphere of days or weeks.