9 September 2020
adapted from the story by University of Minnesota Communications
A new study led by a team of University of Minnesota researchers, with CSL/CIRES co-authors, provides the first global satellite measurements of one of the most important chemicals affecting Earth's atmosphere.
Isoprene is a natural hydrocarbon emitted to the atmosphere in vast quantities—approximately 500 billion kg per year—by plants and trees. Isoprene is chemically reactive, and once in the atmosphere it combines with human-caused pollutants to adversely affect air quality. Isoprene also reacts with the main atmospheric oxidizing agent – called OH radicals – and therefore reduces the capacity of the atmosphere to scrub itself of pollutants and greenhouse gases.
Scientists look to atmospheric models to predict current and future atmospheric composition and air quality, as well as to diagnose the atmosphere's ability to remove greenhouse gases and air pollutants. But isoprene emission rates are highly uncertain due to sparse ground-based measurements, and scientists are also unsure of the extent to which isoprene acts to suppress or sustain the abundance of OH radicals in the atmosphere.
Now, researchers have developed the first-ever global measurements of isoprene from space. Using observations from the Cross-track Infrared Sounder (CrIS) sensor on Suomi NPP, a NOAA operated satellite, researchers developed a retrieval method that uses machine learning to determine the atmospheric concentration of isoprene over different parts of the world. They combined these measurements with atmospheric modeling to test current scientific understanding of global isoprene emissions and of how isoprene affects atmospheric oxidation. The research was published today in the journal Nature. CIRES scientists Joost de Gouw and Carsten Warneke are co-authors on the paper.
"Isoprene is one of the most important drivers of global atmospheric chemistry," said lead author Dylan Millet, a professor in the University of Minnesota's Department of Soil, Water, and Climate. "These satellite measurements provide new understanding of how Earth's biosphere and atmosphere interact."
Wells, K.C., D.B. Millet, V.H. Payne, M.J. Deventer, J.A. de Gouw, M. Graus, C. Warneke, A. Wisthaler, and J.D. Fuentes, Satellite isoprene retrievals constrain emissions and atmospheric oxidation, Nature, doi:10.1038/s41586-020-2664-3, 2020.
Isoprene is the dominant non-methane organic compound emitted to the atmosphere. It drives ozone and aerosol production, modulates atmospheric oxidation and interacts with the global nitrogen cycle. Isoprene emissions are highly uncertain, as is the nonlinear chemistry coupling isoprene and the hydroxyl radical, OH – its primary sink. Here we present global isoprene measurements taken from space using the Cross-track Infrared Sounder (CrIS). Together with observations of formaldehyde, an isoprene oxidation product, these measurements provide constraints on isoprene emissions and atmospheric oxidation. We find that the isoprene–formaldehyde relationships measured from space are broadly consistent with the current understanding of isoprene–OH chemistry, with no indication of missing OH recycling at low nitrogen oxide concentrations. We analyse these datasets over four global isoprene hotspots in relation to model predictions, and present a quantification of isoprene emissions based directly on satellite measurements of isoprene itself. A major discrepancy emerges over Amazonia, where current underestimates of natural nitrogen oxide emissions bias modelled OH and hence isoprene. Over southern Africa, we find that a prominent isoprene hotspot is missing from bottom-up predictions. A multi-year analysis sheds light on interannual isoprene variability, and suggests the influence of the El Ninño/Southern Oscillation.