Amazonian Forest Isoprene Emissions Vary with Terrain Elevations
Research identifies a key factor governing the variability of isoprene emissions over the Amazonian forest.
Isoprene dominates global non-methane volatile organic compound (VOC) emissions and impacts tropospheric chemistry by influencing oxidants and aerosols (small atmospheric particles). This work, performed by a team including Department of Energy (DOE) scientists and DOE’s Atmospheric Radiation Measurement (ARM) Aerial Facility, identifies for the first time a key factor that governs isoprene emission rates within the Amazonian forest. Analyzing aircraft eddy covariance measurements during the GoAmazon 2014-5 field campaign, this research finds that isoprene emissions strongly correlate with terrain elevations, most likely due to varying plant species distributions at different elevations. These findings are consistent with similar correlations derived from analysis of satellite data.
This work demonstrates the value of aircraft-derived measurements during the DOE-supported GoAmazon 2014-2015 field campaign and produces new insights on the isoprene emissions rate. The findings provide key clues for improving the representation of isoprene emissions within regional and global Earth system models (ESMs). The study demonstrates that current modeling estimates of isoprene emissions may be too low over the Amazonian forest, especially during the dry season.
Isoprene is the most abundant short-lived, reactive VOC emitted by terrestrial vegetation and therefore affects the oxidation capacity of the atmosphere, the formation of ozone, and production of secondary organic aerosols (SOAs). Accurate model representation of isoprene emission rates is critical to understand global impacts on regional chemistry and aerosols. The research analyzed eddy covariance measurements based on a proton-transfer reaction mass spectrometry (PTR-MS) instrument onboard the Gulfstream-1 research aircraft, and showed that levels of isoprene emissions strongly correlate with terrain elevation, a finding not presently represented in current ESMs. The study also analyzed results from the regional Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model that uses simple mechanistic algorithms to estimate biogenic emissions fluxes based on the Model of Emissions of Gases and Aerosols from Nature (MEGAN). The research showed that the model underestimates isoprene emissions fluxes by ~35% compared to aircraft-derived estimates. Furthermore, these observations showed that biogenic isoprene emissions are much higher during the dry season compared to the wet season over the Amazonian forest. The study highlights the need for further measurements of leaf and canopy-scale isoprene emissions—at multiple sites along elevation gradients—to determine the cause and generality of these findings in other geographic locations.
Pacific Northwest National Laboratory
Institutional support was provided by the Central Office of the Large-Scale Biosphere Atmosphere Experiment in Amazonia (LBA), the (Brazilian) National Institute of Amazonian Research and National Institute for Space Research, Amazonas State University, and Brazilian Space Agency. The work was conducted for the Brazilian National Council for Scientific and Technological Development. We acknowledge the Atmospheric Radiation Measurement (ARM) Climate Research Facility, a user facility of the U.S. Department of Energy, Office of Science, sponsored by the Office of Biological and Environmental Research (BER), and support from BER’s Atmospheric System Research (ASR) program. A.B.G. was partially supported by the National Aeronautics and Space Administration’s Atmospheric Composition Campaign Data Analysis and Modeling program.
Gu, D., et al. 2017. “Airborne Observations Reveal Elevational Gradient in Tropical Forest Isoprene Emissions,” Nature Communications 8(15541), DOI:10.1038/ncomms15541