AbstractMany 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.