Primary and secondary organics in biogenic atmospheric aerosol : The chiral study of 2-methyltetrols from method development to application
Sammanfattning: Distinguishing between the primary and secondary origin of the organic components in atmospheric aerosols is one of the key parameters in assessing the relevance and role of different chemical species in the overall chemistry of the atmosphere. Currently available methods used in field measurements are not able to fully discriminate between the primary or secondary origin of compounds detected in atmospheric aerosols. This thesis presents a new analytical method for examining the primary and secondary origin of certain organic constituents, 2-methyltetrols, in biogenic atmospheric aerosol based on chirality. Chirality is a molecular structure property that makes a pair of compounds to be mirror images of each other. Consequently, compounds with chirality represent challenges for their study as individual species. This is of importance because 2-methyltetrols have been studied so far solely as two diastereomeric constituents, 2-methylerythritol and 2-methylthreitol, when in reality they are four different compounds: (2R,3S)-methylerythritol, (2S,3R)-methylerythritol, (2S,3S)-methylthreitol and (2R,3R)-mehtylthreitol. Furthermore, 2-methylerythritol and 2-methylthreitol are currently used as the main tracers for the contribution of isoprene to the formation of secondary organic aerosols in the atmosphere. Therefore, a new method directed to the chiral analysis of 2-methyltetrols was developed and applied to atmospheric samples collected at Aspvreten, Sweden, the Amazon, Brazil, and Hyytiälä, Finland. The results indicate that 2-methyltetrols in samples from the three sites have both primary and secondary origins with variability among sites as well as seasons. The estimates of the minimum primary origin contribution represent a significant fraction of the total mass of 2-methyltetrols in the samples. In some samples the contribution was as high as 30% of the total mass of these compounds. Even though 2-methyltetrols have secondary origin, their use as tracers for the atmospheric oxidation of isoprene should be discouraged, unless relative primary contributions are considered, as this study has demonstrated that they have primary origin as well. Moreover, the development and application of chiral analytical methods should be prioritized in order to improve the prevailing understanding of the real chemical composition and origin of chiral compounds in atmospheric aerosols.
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