Forest emissions: A chiral surprise in the rainforest

Date:
August 27, 2020
Source:
Max Planck Institute for Chemistry
Summary:
Reversed ratio of chiral volatile organic compounds over the
Amazon rainforest reveal insects as unexplored important source
of forest emissions.



FULL STORY ========================================================================== Forests such as the Amazon rainforest emit huge amounts of biogenic
volatile organic compounds (BVOC) into the atmosphere. These compounds
impact the physical and chemical properties of the atmosphere and also
our climate. The molecules react rapidly with ambient OH radicals and
ozone, thereby influencing the oxidation capacity of the atmosphere
for pollutants such as carbon monoxide and greenhouse gases such as
methane. Furthermore, BVOC are precursors to secondary organic aerosols,
which affect the Earth's radiative budget. Many BVOCs such as a-pinene
are chiral. This means that they exist in two non- superimposable mirror
image forms just like our left and right hands.

Scientists speak of enantiomers, or plus and minus forms. However, all
physical properties such as their boiling point, mass and their reaction
rate with atmospheric oxidizing agents like OH and ozone are identical.


========================================================================== Despite the chemical similarity of these chiral pairs, insects and plants
can distinguish enantiomeric forms of pheromones and phytochemicals,
although little attention has been paid to the mixing ratio of the two separated forms in forests. Previous measurements reported minus a-pinene
to be the dominant chiral molecule of the tropical forest. Scientists from
the Max Planck Institute for Chemistry, the Johannes Gutenberg-University
Mainz and from Brazil have now made a surprising discovery: from the 325-meter-high measuring tower in the Amazon rainforest, they were
able to show that the ratio of the a- pinene enantiomers varies in the
vertical by a factor of ten. The team around the Max Planck researcher
Nora Zannoni was also able to demonstrate that the concentrations are altitude-dependent and vary with the time of day and in both wet and
dry seasons.

While plus-a-pinene dominates at 40 meters anytime and at 80 meters during
the night, the minus form predominates at 80 meters during the day and
at all other higher heights anytime. The team also observed that the
minus a-pinene concentration depends on temperature at 80 meters while
plus a-pinene does not.

"The photosynthetic activity of the vegetation depends on temperature
and stomatal opening. It thus drives the emissions of minus a-pinene, demonstrating that leaves are the main source of emission of this isomer,
and that the two isomers are released from leaves through different
pathways," says Zannoni, who is first author of a study recently published
in the science magazine Communications Earth & Environment.

Termites as unknown source of plus a-pinene in the canopy? During the
dry season, the chiral ratio of the two forms reverses at 80 meters.

"This indicates a strong, uncharacterized source of plus a-pinene in
the canopy," says Jonathan Williams, group leader at the institute in
Mainz and last author of the study. Since the researchers could rule out atmospheric sinks such as the chiral-selective degradation of pinene by
OH radicals and ozone or deposition onto aerosols as well as the influence
of wind direction and sunlight, they instead suspect that insect stresses
such as herbivores feeding and termites emissions are responsible for
the plus a- pinene higher values. In order to test a possible impact
of insects the researchers conducted additional measurements above
termite nests which confirmed that such emissions can overturn the
ambient chiral ratio of a- pinene. As termite populations are expected
to increase significantly in the future with continued deforestation
and climate warming, their influence needs to be considered in forest
emission models and forest signaling.

"We also know that plants can release large amounts of plus a-pinene
when injured or eaten," Williams adds. This is supported by measurements
of volatile compounds associated with leaf wounding that even revealed
when the herbivores were most active. The atmospheric chemists Zannoni
and Williams conclude that they need to rethink how canopy emissions of volatile organic compounds are simulated, and take the whole ecosystem
into account.

The research was co-financed by the H2020 project "ULTRACHIRAL" of the
European Union.


========================================================================== Story Source: Materials provided by
Max_Planck_Institute_for_Chemistry. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Nora Zannoni, Denis Leppla, Pedro Ivo Lembo Silveira de Assis,
Thorsten
Hoffmann, Marta Sa', Alessandro Arau'jo, Jonathan
Williams. Surprising chiral composition changes over the Amazon
rainforest with height, time and season. Communications Earth &
Environment, 2020; 1 (1) DOI: 10.1038/ s43247-020-0007-9 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200827105911.htm

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