Mapping and quantification of medium-scale ammonia plumes from HyTES observations on the high-altitude ER-2 flights

Observations from the thermal infrared HyTES instrument deployed on NASA's high-altitude ER-2 aircraft during the 2019 HySPIRI field campaign are used to characterize and quantify ammonia (NH₃) enhancements in California's Imperial Valley. The objective is to demonstrate the value of high-altitude airborne HyTES measurements for resolving sub-kilometer spatial variability in NH₃ associated with heterogeneous surface sources—variability that cannot be captured by current satellite products with spatial resolutions of 15 km or larger. Major NH₃ sources in California include livestock operations, composting and fertilized soils, and industrial activities. Ammonia also plays a central role in secondary aerosol formation through reactions with sulfuric and nitric acids, producing ammonium sulfate and ammonium nitrate that contribute substantially to PM₂.₅. This is of particular relevance to the California Air Resources Board (CARB), as mitigation strategies targeting dairy methane (CH₄) emissions may inadvertently increase NH₃ emissions and exacerbate PM₂.₅ air quality. This work presents an analysis of sensitivity and retrievability of NH 3 from ER-2 HyTES observations (20 km swath with 34.73 m spatial resolution) collected over densely distributed multi-sources in the California Imperial Valley on September 12, 2019. The information content of HyTES observations from the ER-2 altitudes is discussed in context of earlier studies of NH 3 HyTES retrievals from a low-flying Twin Otter aircraft. The collocated satellite NH 3 product from Cross-track Infrared Sounder (CrIS) is shown to demonstrate that the spatially up-scaled HyTES ER-2 retrievals are mostly within the range of estimated retrieval uncertainties. By adjusting spatial resolutions of the retrieved NH 3 maps, we assess the added value of high-resolution NH 3 observations. Our analysis demonstrates that a spatial resolution of at least 1.6 km is necessary to preserved information on NH 3 point sources in the California Imperial Valley. • High-resolution HyTES NH₃ retrievals capture km-scale variability, improving regional emission inventories. • Spatial resolution of ≤1.6 km is required to resolve NH₃ point sources in the California Imperial Valley. • Sub-km NH₃ heterogeneity is unresolved by current satellite products, highlighting the value of HyTES observations.