Vertical Profile Corrections Explain Satellite-Inventory Ammonia Discrepancies and Reveal Concentrated Agricultural Sources in China.

Persistent discrepancies exist between bottom-up inventories and satellite-based ammonia (NH3) emission estimates, with satellites typically reporting values one-third higher. These discrepancies prevent accurate targeting of NH3 control policies for reducing air pollution and ecosystem nitrogen deposition. Here, we demonstrate that systematic biases in satellite vertical profile assumptions substantially explain these long-standing discrepancies. By replacing default vertical profile in satellite retrievals with spatially and temporally resolved atmospheric profiles, we reduced satellite-model discrepancies from 71 to 18%. Our hybrid inversion analysis across China reveals that baseline satellite retrievals overestimated growing season emissions by up to 44% due to systematic overestimation of near-surface NH3 concentrations, while our corrected estimates show close agreement with bottom-up inventories (7.9% difference). Critically, our analysis reveals that China's NH3 emissions are more spatially concentrated than what the a priori inventory indicates, with the top 10% of high-emitting areas contributing 54-56% of national emissions. This concentration reflects agricultural intensification patterns inadequately captured by bottom-up inventories. Independent validation confirms improved accuracy with 1-27% error reductions across all months. These findings provide essential insights for targeted emission control policies in the most concentrated agricultural regions while resolving methodological uncertainties that have long complicated NH3 management strategies.