Fingerprinting sources of emissions of volatile organic compounds in the Amazonian rain forest
Many volatile organic compounds occur in nature as isomers and enantiomers (or chirals), but little is known about their relative abundance and behavior because the commonly used atmospheric measurement techniques cannot detect them individually.
The Amazonian region is the Earth’s richest natural biodiverse heritage. At least 40000 different plant species populate the Amazonian ecosystem, and their photosynthetic activity is known to uptake large amounts of CO2 each year.
Volatile organic compounds (VOC) emitted from the Amazonian biota have the ability to react rapidly with the main atmospheric oxidants, mainly OH, and their emission plays a crucial role in the atmospheric oxidative capacity as well as in aerosol and cloud formation. Many VOC occur in nature as isomers and enantiomers (or chirals), but little is known about their relative abundance and behavior because the commonly used atmospheric measurement techniques cannot detect them individually. One interesting aspect of chiral VOC is that they smell differently, therefore plants and insects can make use of them for communicating.
From 2017, I started conducting regular fieldwork from a permanently installed remote site located in central Amazonia (Brazil) as part of the Amazonian Tall Tower Observatory (ATTO) German-Brazilian scientific project, in order to investigate the VOC in air. My colleagues and I, started atmospheric measurements of VOC and aerosols from the 325 m ATTO tall tower to investigate diel, seasonal and vertical changes in air composition. We installed at four tower heights (40, 80, 150, 320m) some custom built automatic samplers where VOC are collected into sorbent tubes. Each day we climbed the tower top to install fresh sorbent tubes and remove the sampled ones, then packed and stored the samples in a dry and cooled container before being analyzed in our laboratory in Germany.
Walking up 1500 steps in the hot sunny Amazons was rewarded each day by a new adventure, either spotting couples of macaws flying from a tree to another, finding a snake on one platform, watching the sun rising above the jungle or the next rainstorm approaching fast. Back from wildlife, we analysed the samples using gas chromatography mass spectrometry equipped with a chiral column and a validated method for separating chiral VOC. The first results were exciting: chirality of VOC had a clear diel and vertical pattern. We conducted lab experiments with aerosol sampled filters to understand that the vertical changing ratio was not depending on a selective chiral sink, either it was driven by a selective chiral source.
Leaf measurements of stomatal conductance and air temperature permitted to understand the chiral VOC diel profile: air temperature drives canopy emission of the (-) enantiomer.
In the next campaigns we examined potential sources of emissions of the (+) enantiomer. We sampled air far from the tower, in the main wind direction reaching the tower (NE), from the shorter tower (80m), along the nearby river, from the understory below the tall tower and outside two arboreal termite nests. Only the samples taken next to the nests showed the (+) enantiomer being more abundant than the (-) one, suggesting termites as a potential strong and overlooked chiral VOC source in the forest.
Original publication: https://www.nature.com/articles/s43247-020-0007-9