Monthly Patterns of Ammonia Over the Contiguous United States at 2-km Resolution

Monthly, high-resolution (∼2 km) ammonia (NH₃) column maps from the Infrared Atmospheric Sounding Interferometer (IASI) were developed across the contiguous United States and adjacent areas. Ammonia hotspots (95th percentile of the column distribution) were highly localized with a characteristic length scale of 12 km and median area of 152 km². Five seasonality clusters were identified with k-means++ clustering. The Midwest and eastern United States had a broad, spring maximum of NH₃ (67% of hotspots in this cluster). The western United States, in contrast, showed a narrower midsummer peak (32% of hotspots). IASI spatiotemporal clustering was consistent with those from the Ammonia Monitoring Network. CMAQ and GFDL-AM3 modeled NH₃ columns have some success replicating the seasonal patterns but did not capture the regional differences. The high spatial-resolution monthly NH₃ maps serve as a constraint for model simulations and as a guide for the placement of future, ground-based network sites.