Articles | Volume 16, issue 6
Atmos. Chem. Phys., 16, 3903–3925, 2016

Special issue: Observations and Modeling of the Green Ocean Amazon (GoAmazon2014/5)...

Atmos. Chem. Phys., 16, 3903–3925, 2016

Research article 23 Mar 2016

Research article | 23 Mar 2016

Seasonality of isoprenoid emissions from a primary rainforest in central Amazonia

Eliane G. Alves1, Kolby Jardine2, Julio Tota3, Angela Jardine1, Ana Maria Yãnez-Serrano1,4, Thomas Karl5, Julia Tavares6, Bruce Nelson6, Dasa Gu7, Trissevgeni Stavrakou8, Scot Martin9, Paulo Artaxo10, Antonio Manzi1,11, and Alex Guenther7 Eliane G. Alves et al.
  • 1Climate and Environment Department, National Institute for Amazonian Research (INPA) and State University of Amazonas (UEA), Av. André Araújo 2936, CEP 69067-375, Manaus-AM, Brazil
  • 2Climate Science Department, Earth Science Division, Lawrence Berkeley National Laboratory (LBNL), One Cyclotron Rd, building 64-241, Berkeley, CA 94720, USA
  • 3Institute of Engineering and Geoscience, Federal University of West Para (UFOPA), Rua Vera Paz s/n, CEP 68035-110, Santarem-PA, Brazil
  • 4Biogeochemistry Department, Max Planck Institute for Chemistry, P.O. Box 3060, 55128, Mainz, Germany
  • 5Institute for Meteorology and Geophysics, University of Innsbruck, Innrain 52, 6020, Innsbruck, Austria
  • 6Ecology Department, National Institute for Amazonian Research (INPA), Av. André Araújo 2936, CEP 69067-375, Manaus-AM, Brazil
  • 7Department of Earth System Science, University of California, Irvine, USA
  • 8Belgian Institute for Space Aeronomy, Avenue Circulaire 3, 1180 Uccle, Brussels, Belgium
  • 9School of Engineering and Applied Sciences, Department of Earth and Planetary Sciences, Harvard University, 29 Oxford St, Cambridge, MA 02138, USA
  • 10Institute of Physics, University of Sao Paulo, Rua Matão, Travessa R, 187 – Cidade Universitária, CEP 05508-900, Sao Paulo-SP, Brazil
  • 11National Institute for Spatial Research, Center of Weather Forecasting and Climate Studies, Rod. Presidente Dutra, km 40, Cachoeira Paulista/SP, Brazil

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, with the purpose of improving BVOC model approaches that can characterize seasonality of isoprenoid emissions from tropical rainforests.

Short summary
For a long time, it was thought that tropical rainforests are evergreen forests and the processes involved in these ecosystems do not change all year long. However, some satellite retrievals have suggested that ecophysiological processes may present seasonal variations mainly due to variation in light and leaf phenology in Amazonia. These in situ measurements are the first showing of a seasonal trend of volatile organic compound emissions, correlating with light and leaf phenology in Amazonia.
Final-revised paper