Seasonal Amplification and Spatial Reorganization of Atmospheric Ammonia (NH3) in Northeast China

Regional assessments of atmospheric ammonia must distinguish changes in annual abundance from changes in seasonality and hotspot organization. We analyzed monthly Infrared Atmospheric Sounding Interferometer (IASI) total columns over Liaoning, Jilin and Heilongjiang during 2008–2024 on a 0.2° grid. We constructed a Unified Spatiotemporal Bayesian Additive Regression Trees model (UST-BART) for 2008–2022 using nonnegative complete observations, combining location means and within-location departures with geographical and calendar backgrounds. Regional columns increased at 4.52 × 10−5 mol m−2 decade−1, and positive trends covered 93.6% of represented area. The seasonal amplitude of atmospheric NH3 columns increased by 119.7% between 2008–2012 and 2019–2024; relative hotspots remained concentrated in Liaoning, with southwestward centroid trends despite start-year-sensitive endpoint distances (477.4 vs. 46.8 km). Liaoning retained the largest absolute seasonal cycle, whereas Heilongjiang showed the largest proportional provincial amplification. UST-BART identified a positive temperature association, a U-shaped soil-moisture association and a broadly inverse-U association with boundary layer height. Temperature contrasts varied jointly with moisture and boundary layer conditions, with the largest provincial temperature response in Liaoning. Temporal and spatial holdouts yielded R2 values of 0.414 and 0.286. These findings reveal strengthening seasonality and spatial reorganization of NH3, with context-dependent environmental associations that support regionally focused monitoring.