Tropical forests are the world’s largest source of biogenic volatile organic compounds (BVOCs). Those compounds are exchanged between the forest and the atmosphere. In the atmosphere, they react with oxidants, like the OH radical, to form new compounds. Thereby BVOCs alter the concentration of those oxidants. In turn, this affects the capacity of the atmosphere to oxidize other trace gases, like methane for example. VOC chemistry also leads to the nucleation and growth of particles, which are crucial for forming clouds and precipitation. Prior studies show that the types of particles can have a strong impact on these processes. A better understanding the atmospheric chemistry is therefore important for understanding the hydrological cycle.
Carbonyl compounds are an interesting subclass of BVOCs. They are emitted by the forest for various reasons but are also formed during chemical reactions in the atmosphere. They are divided into the chemical groups of aldehydes and ketones. Aldehydes and ketones are isomeric compounds, which means they consist of the same atoms but are arranged differently, like using the same building blocks to form different shapes. However, aldehydes exist only for hours to days before they are transformed into other chemicals. Ketones, on the other hand, persist for up to several weeks and can be transported over wider distances. Thus, both have different implications for atmospheric chemistry. However, scientists commonly observe them together due to their chemical likeness.
The VOC working group of Jonathan Williams specializes in identifying many VOCs with a time resolution of seconds using a mass spectrometer (called “chemical ionization time-of-flight mass spectrometer”). They used this instrument to measure aldehydes and ketones separately. To achieve this, they had to switch out the chemical used to ionize the samples. Akima Ringsdorf, a scientist of the VOC working group, measured carbonyl compounds in the atmosphere of the Amazon rainforest with the adopted instrumentation. For about two weeks in the wet-to-dry transition and the dry season of 2019, she took air measurements at 80, 150, and 325 meters along the tall tower at ATTO. This allowed her to observe the vertical distribution of different aldehydes and ketones separately.
The effort paid off because the team made quite some unexpected discoveries. They had expected Ketones would be distributed fairly equally at all heights due to their long lifetime. However, some showed decreasing concentrations between 80 and 150 m and a higher variability at 80 m compared to higher altitudes. This suggests a large source in or above the canopy balanced by an uptake process inside the forest. The team now plans a follow-up study with continuous vertical measurements on an elevator between the canopy and 320 m. This could give them more detailed insights into the exchange of these ketones.
The scientists also found rapidly decreasing carbonyl concentrations at night. This is most likely due to the deposition of these chemicals to the foliage and uptake through leaf surfaces, thereby removing them from the atmosphere.
Overall, the team could show that studying the variety of carbonyl compounds separately is extremely worthwhile. It helps to understand the huge diversity of compounds that are exchanged between the forest and the atmosphere. They recommend colleagues to periodically switch reagents to allow for more specific detection of biogenic emissions.
Ringsdorf et al. published the study “Investigating carbonyl compounds above the Amazon rainforest using a proton-transfer-reaction time-of-flight mass spectrometer (PTR-ToF-MS) with NO+ chemical ionization” Open Access in the journal Atmospheric Chemistry and Physics.
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