Deriving cropland N2O emissions from space-based NO2 observations

Abstract

Croplands are the largest anthropogenic source of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance. Agricultural emissions produce small atmospheric signals with high spatiotemporal variability presenting a large observational challenge. If capable, space-based observations could characterize cropland N2O emissions from farmlands across the world. No current satellite can resolve near-surface N2O variations from cropland emissions. However, satellite observations of nitrogen dioxide (NO2), a component of NOx along with nitric oxide (NO), capture cropland emissions. NO, which quickly converts to NO2 in the atmosphere, and N2O are co-emitted from soils. Both gases are produced by microbial soil processes and are emitted in large amounts as a result of excess nitrogen from applied fertilizer. Given their co-emission in croplands, we ask: Can satellite NO2 observations be used to infer N2O emissions? We examine coincident airborne N2O and NOx measurements downwind of California croplands to characterize N2O:NOx emission relationships from farms. We use these emission ratios to transform estimates of agricultural NOx emissions derived from space-based TROPOMI NO2 observations to N2O emissions. We compare these estimates to independent ground and airborne studies in the US Corn Belt and Mississippi River Valley. Space-based estimates are broadly consistent with these ground and airborne studies, suggesting that satellite NO2 observations can be used to infer cropland N2O emissions. Further refinement of a NO2 proxy approach for cropland N2O emissions has the potential to expand observational capabilities to constrain regional and global cropland N2O emissions and inform process models.