Surprising chiral composition changes over the Amazon rainforest with height, time and season

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. The chiral compositions of biogenic volatile organic compounds over the Amazon tropical rainforest vary with height, time of day and season, according to measurements from a 325 m tall tower.