Abstract
Many biogenic volatile organic compounds (BVOC) are chiral, existing in two mirror image forms called enantiomers. The most abundant atmospheric chiral BVOC is α-pinene (C10H16), whose enantiomeric ratio has been reported to be regiospecific. Here we show with measurements made on a 325 m tower in the Amazon rainforest that the enantiomeric ratio varies unexpectedly (by a factor of ten) with (+)-α-pinene dominating at canopy level and (−)-α-pinene at tower top. The ratio is independent of wind direction, speed and sunlight but shows diurnal temperature dependent enrichment in the (−)-α-pinene enantiomer at the lowest 80 m height. These effects cannot be caused by atmospheric reaction with oxidants, or aerosol uptake. The reversal of chiral ratio at 80 m reveals the presence of a potent uncharacterized local (+)-α-pinene rich source, possibly linked to herbivory and termites. These results suggest the presence of a strong uncharacterized BVOC source that is overlooked in current emission models.
Highlights
Many biogenic volatile organic compounds (BVOC) are chiral, existing in two mirror image forms called enantiomers
This study shows that all the collected air samples at the ATTO field site contain chiral species, and the most abundant chiral compound is the monoterpene α-pinene
This study highlights that the enantiomeric ratio of α-pinene is not constant as previously expected, instead it inverts over the vertical profile of the tower height and generates a diel profile at 80 m
Summary
Many biogenic volatile organic compounds (BVOC) are chiral, existing in two mirror image forms called enantiomers. The ratio is independent of wind direction, speed and sunlight but shows diurnal temperature dependent enrichment in the (−)-α-pinene enantiomer at the lowest 80 m height These effects cannot be caused by atmospheric reaction with oxidants, or aerosol uptake. Each year an estimated 760 TgC of biogenic volatile organic compounds (BVOCs) enters the atmosphere, primarily from tropical forests[1] These compounds react rapidly with ambient OH radicals and O3, thereby influencing the atmospheres oxidation capacity for pollutants (e.g., CO) and greenhouse gases (e.g., CH4). Recent studies within a monoculture maritime pine forest have shown chiral signatures can vary between canopy, stem, and litter[9] This suggests that accurate modelling of BVOC emissions will require chirally and compartment specific measurements in future. In order to understand better the emissions of chiral BVOC compounds at the ecosystem scale, as well as their vertical and diel distribution, we performed measurements of chiral monoterpenes at several heights on the 325 m ATTO
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