Aging of α-Pinene Secondary Organic Aerosol by Hydroxyl Radicals in the Aqueous Phase: Kinetics and Products

Abstract

The reaction of hydroxyl radicals (OH) with a water-soluble fraction of the α-pinene secondary organic aerosol (SOA) was investigated using liquid chromatography coupled with negative electrospray ionization mass spectrometry. The SOA was generated by the dark ozonolysis of α-pinene, extracted into the water, and subjected to chemical aging by the OH. Bimolecular reaction rate coefficients (k OH) for the oxidation of terpenoic acids by the OH were measured using the relative rate method. The unaged SOA was dominated by the cyclobutyl-ring-retaining compounds, primarily cis-pinonic, cis-pinic, and hydroxy-pinonic acids. Aqueous oxidation by the OH resulted in the removal of early-stage products and dimers, including well-known oligomers with MW = 358 and 368 Da. Furthermore, a 2- to 5-fold increase in the concentration of cyclobutyl-ring-opening products was observed, including terpenylic and diaterpenylic acids and diaterpenylic acid acetate as well as some of the newly identified OH aging markers. At the same time, results obtained from the kinetic box model showed a high degree of SOA fragmentation following the reaction with the OH, which indicates that non-radical reactions occurring during the evaporation of water likely contribute to the high yields of terpenoic aqSOAs reported previously. The estimated atmospheric lifetimes showed that in clouds, terpenoic acids react with the OH exclusively in the aqueous phase. Aqueous OH aging of the α-pinene SOA results in a 10% increase of the average O/C ratio and a 3-fold decrease in the average k OH value, which is likely to affect the cloud condensation nuclei activity of the aqSOA formed after the evaporation of water.

Prolonged Dark Chemical Processes in Secondary Organic Aerosols on Filters and in Aqueous Solution

“…These results highlight that these temporal effects illustrated in Figure 3, and especially the persistent growth of dimers on filters, are observed for hundreds of compounds in complex SOA samples. Additionally, this overall trend reinforces the hypothesis that such temporal behaviors could be explained by a general mechanism, e.g., hydrolysis leading to decomposition of dimers and formation of monomers in SOA aqueous extracts, as also observed by Witkowski et al 51 In contrast, on filters, continuous SOA processing occurs and therefore results in a removal of monomers and formation of dimers. We acknowledge that with the high SOA concentrations used in this study, some constituents may have partitioned more from the gas phase into the particle phase compared to atmospheric conditions.…”

Photo-Oxidation of α-Pinene Oxidation Products in Atmospheric Waters – pH- and Temperature-Dependent Kinetic Studies

“…Witkowski used a similar approach to find that the major components of α-pinene SOA, like cis-pinonic acid, undergo oxidation by • OH primarily in the aqueous phase, when the LWC is ≥1 × 10 −3 /g m −3 , albeit that LWC is rather unrealistic. 63 Moreover, they calculated the lifetimes for PA and found the same dependencies as those in our studies. Similar findings were made for limononic acid, the oxidation product of limonene, indicating that in fogs and clouds with LWC = 0.3−0.5/g m −3 limononic acid will reside entirely in the aqueous phase.…”

“…MBTCA, which has better solubility in water, was effectively removed in all systems with the longest lifetime of ∼90 min. Witkowski used a similar approach to find that the major components of α-pinene SOA, like cis -pinonic acid, undergo oxidation by • OH primarily in the aqueous phase, when the LWC is ≥1 × 10 –3 /g m –3 , albeit that LWC is rather unrealistic . Moreover, they calculated the lifetimes for PA and found the same dependencies as those in our studies.…”