Satellite isoprene retrievals constrain emissions and atmospheric oxidation

SUMMARY:

Isoprene is the dominant non-methane organic compound emitted to the atmosphere ^{1– 3} . It drives ozone and aerosol production, modulates atmospheric oxidation, and interacts with the global nitrogen cycle ^{4– 8} . Isoprene emissions are highly uncertain ^{1, 9} , as is the non-linear chemistry coupling isoprene and the hydroxyl radical, OH—its primary sink ^{10– 13} . Here we present the first global isoprene measurements from space, using the Cross-track Infrared Sounder (CrIS). These isoprene measurements, together with observations of its oxidation product formaldehyde, provide new constraints on isoprene emissions and atmospheric oxidation. We find that isoprene:formaldehyde relationships measured from space are broadly consistent with current understanding of isoprene-OH chemistry, with no indication of missing OH recycling at low-NO _x. We analyze these datasets over four global isoprene hotspots in relation to model predictions, and present a first demonstration of isoprene emission quantification based directly on satellite measurements of isoprene itself. A major discrepancy emerges over Amazonia, where current underestimates of natural NO _x emissions bias modeled 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 role of El Niño.