ALBERT

Description: Ammonia measurements from a vehicle-based, mobile open-path sensor and those from aircraft were compared with TES NH 3 columns at the pixel scale during the NASA DISCOVER-AQ field experiment. Spatial and temporal mismatches were reduced by having the mobile laboratory sample in the same areas as the TES footprints. To examine how large heterogeneities in the NH 3 surface mixing ratios may affect validation, a detailed spatial survey was performed within a single TES footprint around the overpass time. The TES total NH 3 column above a single footprint shows excellent agreement with the in-situ total column constructed from surface measurements with a difference of 2% (within the combined measurement uncertainties). The comparison was then extended to a TES transect of nine footprints where aircraft data (5-80 ppbv) were available in a narrow spatiotemporal window (〈10 km, 〈 1 hour). The TES total NH 3 columns above the nine footprints agreed to within 6% of the in-situ total columns derived from the aircraft-based measurements. Finally, to examine how TES captures surface spatial gradients at the inter pixel scale, ground-based, mobile measurements were performed directly underneath a TES transect, covering nine footprints within ± 1.5 hours of the overpass. The TES total columns were strongly correlated (R 2  = 0.82) with the median NH 3 mixing ratios measured at the surface. These results provide the first in-situ validation of the TES total NH 3 column product, and the methodology is applicable to other satellite observations of short-lived species at the pixel scale.