Isoprene is the dominant non-methane organic compound emitted to the atmosphere1–3. It drives ozone and aerosol production, modulates atmospheric oxidation and interacts with the global nitrogen cycle4–8. Isoprene emissions are highly uncertain1,9, as is the nonlinear chemistry coupling isoprene and the hydroxyl radical, OH—its primary sink10–13. Here we present global isoprene measurements taken from space using the Cross-track Infrared Sounder. 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 Niño/Southern Oscillation.
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Acknowledgements This work was supported by the NASA Atmospheric Composition Modeling and Analysis Program (Grant Number NNX17AF61G) and by the Minnesota Supercomputing Institute. We thank D. Fu for providing optimal estimation isoprene retrievals over Amazonia and input on this manuscript; C. Barnet, E. Manning and R. Monarrez for providing CLIMCAPS HNO3 retrievals; M. Alvarado, K. Cady-Pereira, D. Gombos, J. Hegarty and I. Strickland for generating and testing isoprene absorption look-up tables employed here; and E. Edgerton for providing isoprene data from the SouthEastern Aerosol Research and CHaracterization (SEARCH) network. The SEARCH network was sponsored by the Southern Company and the Electric Power Research Institute. Isoprene measurements aboard the NASA DC-8 during SEAC4RS were supported by the Austrian Federal Ministry for Transport, Innovation and Technology (bmvit) through the Austrian Space Applications Programme (ASAP) of the Austrian Research Promotion Agency (FFG). T. Mikoviny is acknowledged for his support during SEAC4RS. We thank S. Springston for GoAmazon T3 data, which were supported by the ARM Climate Research Facility, the Central Office of the Large-Scale Biosphere Atmosphere Experiment in Amazonia (LBA), the Instituto Nacional de Pesquisas da Amazonia (INPA) and the Universidade do Estado do Amazonia (UEA). Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA.
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