Satellite isoprene retrievals constrain emissions and atmospheric oxidation

Abstract

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. Direct satellite measurements of atmospheric isoprene are compared with model predictions, showing broad agreement but highlighting spatial and temporal biases in modelled isoprene and nitrogen oxide emissions.