Validation of IASI Satellite Ammonia Observations at the Pixel Scale Using In Situ Vertical Profiles

Satellite ammonia (NH₃) observations provide unprecedented insights into NH₃ emissions, spatiotemporal variabilities and trends, but validation with in situ measurements remains lacking. Here, total columns from the Infrared Atmospheric Sounding Interferometer (IASI) were intercompared to boundary layer NH₃ profiles derived from aircraft- and surface-based measurements primarily in Colorado, USA, in the summer of 2014. IASI-NH₃ version 3 near real-time data set compared well to in situ derived columns (windows ±15 km around centroid, ±1 h around overpass time) with a correlation of 0.58, a slope of 0.78 ± 0.14 and an intercept of 2.1 × 10¹⁵±1.5 × 10¹⁵ molecules cm⁻². Agreement degrades at larger spatiotemporal windows, consistent with the short atmospheric lifetime of NH₃. We also examined IASI version 3R data, which relies on temperature retrievals from the ERA Reanalysis, and a third product generated using aircraft-measured temperature profiles. The overall agreement improves slightly for both cases, and neither is biased within their combined measurement errors. Thus, spatiotemporal averaging of IASI over large windows can be used to reduce retrieval noise. Nonetheless, sampling artifacts of airborne NH₃ instruments result in significant uncertainties of the in situ-derived columns. For example, large validation differences exist between ascent and descent profiles, and the assumptions of the free tropospheric NH₃ profiles used above the aircraft ceiling significantly impact the validation. Because short-lived species like NH₃ largely reside within the boundary layer with complex vertical structures, more comprehensive validation is needed across a wide range of environments. More accurate and widespread in situ NH₃ data sets are therefore required for improved validations of satellite products.