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

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 (C 10 H 16), 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.

Varying chiral ratio of Pinic acid enantiomers above the Amazon rainforest

“…It ranges from 4.2 ± 0.1 near the ground to 5.8 ± 0.1 at 320 m altitude. Remarkably, as reported by Zannoni et al (2020), there was little variation in the two α-pinene enantiomers in the gas phase during the 2018 wet season. This finding is not consistent with the results from the particulate phase as shown in Fig.…”

“…Consequently, lower ground level concentrations of pinic acid are expected. The α-pinene emissions exhibit a pronounced day-night cycle and reach their maximum together with photosynthetically active radiation (PAR) and temperature at about 11:00 to 14:00 local time (Zannoni et al, 2020). In comparison, a temporal shift of the diel concentration maxima is expected for the particulate phase, caused by mixing and chemical aging.…”

“…Recently, Zannoni et al (2020) have reported significant variances of the enantiomeric ratio of α-pinene with increasing altitude at the ATTO site in the Amazon rainforest. This unexpected result was attributed to strong local sources, such as insects, which may be responsible of changing the predominant ambient ratio of (−)-and (+)-α-pinene.…”

Total OH reactivity over the Amazon rainforest: variability with temperature, wind, rain, altitude, time of day, season, and an overall budget closure

“…For monoterpene and sesquiterpene speciation, air samples were collected on adsorbent filled tubes equipped with ozone scrubbers at different heights on the ATTO tower (Zannoni et al, 2020a). Sampling occurred every three hours for two weeks at 80 m and 150 m (March 2018) and every hour for three days at 80 m, 150 m and 320 m (October 2018).…”

“…Other factors not monitored, such as herbivore activity, can vary strongly between years (Velasque and Del-Claro, 2016), and can elicit the release of certain BVOCs that might contribute to inter-annual differences. Additionally, the VOC composition measured at the same height can differ between the walk-up tower used in 2013 and the tall tower used in 2018-2019 due to local emissions (Zannoni et al, 2020a).…”