Abundance of pinic acid in its two mirror forms

Mirror, mirror in the forest…

Some organic compounds exist in two mirror-image forms. This is called chirality, and such chiral forms are identical in most physical and chemical properties. However, some properties such as melting point and water solubility can differ between chiral forms. Therefore, the abundance of one form over the other in the atmosphere can have far-reaching consequences and might even affect cloud formation processes.

That is because of the formation of so-called secondary organic aerosols (SOA) from chiral precursors. SOA formation describes the process when organic particles are oxidized in the atmosphere to create tiny particles, which then act as seeds for cloud formation.

A chiral source

However, there are very few studies that distinguish between chiral forms. Thus, researchers still know little about their relevance to atmospheric processes. A few years ago, Nora Zannoni and her colleagues performed a study at ATTO focusing on the chiral compound α-pinene. Among other things, they found that its two chiral forms are neither equally abundant nor is the ratio of the two forms constant over time, season or height above the forest floor.

In a new study, Denis Leppla, Thorsten Hoffmann and their colleagues looked at pinic acid and its chiral forms. Pinic acid forms in the atmosphere through SAO formation from α-pinenes. The team wanted to find out how the chemical reactions in the atmosphere affect the chirality of its product pinic acid.

Sampling setup on the tall tower. © Stefanie Hildmann/Uni Mainz
Sampling setup on the tall tower. © Stefanie Hildmann/Uni Mainz
Denis Leppla at work on the tall tower. © Maria Prass/MPI-C
Denis Leppla at work on the tall tower. © Maria Prass/MPI-C

A chiral product

They found that the abundance of the two chiral forms remains the same. Similar to α-pinenes, their analysis shows a gradient in the ratios of the two forms of pinic acid with increasing height. These results were consistent over three measurement campaigns at ATTO in 2018 and 2019. They conclude that the chiral information of the precursor molecule α-pinene is merely transferred to pinic acid. That means that large-scale emission processes of the two precursor chiral forms mainly determine their chiral ratio. Meteorological, chemical, or physicochemical processes, on the other hand, do not play a particular role.

Overall, the results show that the chiral relationship of the biogenic precursor compound α-pinene is preserved in the oxidation products. Thus, future studies can use it to interpret the biogenic emission sources. Because pinic acid exists in the atmosphere in particle form and because it has a longer lifetime before it reacts to something else again, it provides a larger-scale picture of precursor emissions, while also revealing local and regional influences. Thus, this study by Denis Leppla and his team offers a better alternative for studying chirality.

Denis Leppla et al. published the study “Varying chiral ratio of pinic acid enantiomers above the Amazon rainforest” Open Access in Atmos. Chem. Phys.

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