Seasonality of isoprenoid emissions from a primary rainforest in central Amazonia

Abstract

Tropical rainforests are an important source of isoprenoid and other volatile organic compound (VOC) emissions to the atmosphere. The seasonal variation of these compounds is however still poorly understood. In this study, vertical profiles of mixing ratios of isoprene, total monoterpenes and total sesquiterpenes, were measured within and above the canopy, in a primary rainforest in central Amazonia, using a proton transfer reaction – mass spectrometer (PTR-MS). Fluxes of these compounds from the canopy into the atmosphere were estimated from PTR-MS measurements by using an inverse Lagrangian transport model. Measurements were carried out continuously from September 2010 to January 2011, encompassing the dry and wet seasons. Mixing ratios were higher during the dry (isoprene – 2.68 ± 0.9 ppbv, total monoterpenes – 0.67 ± 0.3 ppbv; total sesquiterpenes – 0.09 ± 0.07 ppbv) than the wet season (isoprene – 1.66 ± 0.9 ppbv, total monoterpenes – 0.47 ± 0.2 ppbv; total sesquiterpenes – 0.03 ± 0.02 ppbv) for all compounds. Ambient air temperature and photosynthetically active radiation (PAR) behaved similarly. Daytime isoprene and total monoterpene mixing ratios were highest within the canopy, rather than near the ground or above the canopy. By comparison, daytime total sesquiterpene mixing ratios were highest near the ground. Daytime fluxes varied significantly between seasons for all compounds. The maximums for isoprene (2.53 ± 0.5 µmol m−2 h−1) and total monoterpenes (1.77 ± 0.05 µmol m−2 h−1) were observed in the late dry season, whereas the maximum for total sesquiterpenes was found during the dry-to-wet transition season (0.77 ± 0.1 µmol m−2 h−1). These flux estimates suggest that the canopy is the main source of isoprenoids emitted into the atmosphere for all seasons. However, uncertainties in turbulence parameterization near the ground could affect estimates of fluxes that come from the ground. Leaf phenology seemed to be an important driver of seasonal variation of isoprenoid emissions. Although remote sensing observations of changes in leaf area index were used to estimate leaf phenology, MEGAN 2.1 did not fully capture the behavior of seasonal emissions observed in this study. This could be a result of very local effects on the observed emissions, but also suggest that other parameters need to be better determined in biogenic volatile organic compound (BVOC) models. Our results support established findings that seasonality of isoprenoids are driven by seasonal changes in light, temperature and leaf phenology. However, they suggest that leaf phenology and its role on isoprenoid production and emission from tropical plant species needs to be better understood in order to develop mechanistic explanations for seasonal variation in emissions. This also may reduce the uncertainties of model estimates associated with the responses to environmental factors. Therefore, this study strongly encourages long-term measurements of isoprenoid emissions, environmental factors and leaf phenology from leaf to ecosystem scale, wi...

Observations of sesquiterpenes and their oxidation products in central Amazonia during the wet and dry seasons

“…6). This is in contrast with typically observed daytime maxima for isoprene and monoterpenes (Alves et al, 2016;Yáñez-Serrano et al, 2015), for which reactive loss is dominated by reaction with OH and with OH / O 3 , respectively (Atkinson, 1997;Kesselmeier and Staudt, 1999), and whose daytime emissions are likely more associated with immediate release following production as a function of solar radiation input (Alves et al, 2014;Bracho-Nuñez et al, 2013;Harley et al, 2004;Jardine et al, 2015;Kuhn, 2002;Kuhn et al, 2004). To better understand the reactive loss of O 3 , the estimated cumulative loss of O 3 by reaction with sesquiterpenes compared to that by reaction with isoprene and monoterpenes is shown for the wet and dry seasons in Figs.…”

“…This is also evident for monoterpenes, for which concentrations and the associated O 3 reactivity at the top of the canopy at an upwind forest site (Jar- , 2015) are both approximately 10 times higher than those at the measurement site in this study. Further, the ratio between monoterpene and sesquiterpene concentrations typical within canopy sites upwind during the wet (7.4) and dry (2.4) seasons (Alves et al, 2016) and that observed at our measurement site for both seasons (∼ 17) indicates that the majority of sesquiterpenes have reacted away before our measurement. This is consistent with the fact that lifetimes of all 30 sesquiterpene species detected in SV-TAG with respect to loss via reaction with O 3 tend to be > 20 min, whereas more highly reactive and more commonly studied sesquiterpenes were below detection (Table 1; unobserved or below detection).…”

“…First, by comparing relative concentrations of isoprene, monoterpenes, and sesquiterpenes at T3 to those made within the canopy at an upwind Amazon forest site (Alves et al, 2016), we confirm that the majority of sesquiterpene oxidation must occur within or near the canopy. Second, differences in chemical composition between ambient samples at our site away from the canopy and that of sesquiterpene-rich essential oils (used as a proxy for emission profile within the canopy) reveals that the most reactive sesquiterpenes in the oils (e.g., β-caryophyllene) are reacted away to levels below detection by SV-TAG before reaching the sampling location at T3.…”

“…This demonstrates that the estimate of O 3 reactivity via reactive loss with sesquiterpenes at our measurement site is a significant underestimate and not representative of near-field O 3 chemistry. For example, taking typical terpenoid concentrations within canopy as reported in Alves et al (2016) and assuming monoterpene and sesquiterpene speciation within canopy to be that of Jardine et al (2015; for monoterpenes) and that of copaiba essential oil (for sesquiterpenes), near-field or in-canopy loss of O 3 is dominated (> 50 %) by reaction with sesquiterpenes for both seasons (Fig. S2).…”

“…Chemical characterization of tropical plant tissue shows the presence of an abundance of sesquiterpenes (F. Courtois et al, 2012) and suggests their widespread emission from such vegetation (F. Courtois et al, 2009). Previous branch enclosure measurements of native Amazon saplings found that sesquiterpene emission was below detection limits (Bracho-Nuñez et al, 2013) even though sesquiterpenes have been observed within and outside the canopy of the central Amazon (Alves et al, 2016;Jardine et al, 2012). Further, F. observed higher sesquiterpene emissions from wounded seedlings of the tropical tree Copaifera officinalis, with similar composition and quantity to that in the leaves.…”