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 NO_x 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 NO₂ measurements downwind of California croplands to characterize N₂O:NO_x emission relationships from farms. We use these emission ratios to transform estimates of agricultural NO_x 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.