4. Validation of the CrIS Fast Physical NH3 Retrieval with ground-based FTIR

Presented here is the validation of the CrIS Fast Physical Retrieval (CFPR) NH3 column and profile measurements using ground-based Fourier Transform Infrared (FTIR) observations. We use the total columns and profiles from seven FTIR sites in the Network for the Detection of Atmospheric Composition Change (NDACC) to validate the satellite data products. The overall FTIR and CrIS total columns have a positive correlation of r = 0.77 (N=218) with very little bias (a slope of 1.02). Binning the comparisons by total column amounts, for concentrations larger than 10.0 x 1015 molecules cm-2, i.e. ranging from moderate to polluted conditions, the relative difference is on average ~ 0 5% with a standard deviation of 25-50%, which is comparable to the estimated retrieval uncertainties in both CrIS and the FTIR. For the smallest total column range (<10.0x1015 molecules cm-2) where there are a large number of observations at or near the CrIS noise level (detection limit) the absolute differences between CrIS and the FTIR total columns show a slight positive column bias. The CrIS and FTIR profile comparisons differences are mostly within the range of the estimated retrieval uncertainties single level retrieved profile values, showing average differences in the range of ~20 to 40%. The CrIS retrievals typically show good vertical sensitivity down into the boundary layer that typically peaks at ~850 hPa (~1.5 km), at this level the median absolute difference is 0.87 (std = ±0.08) ppb, corresponding to a median relative difference of 39% (std = ±2%). Most of the absolute and relative profile comparison differences are in the range of the estimated retrieval uncertainties. At the surface, where CrIS typically has lower sensitivity, it tends to overestimate under low concentrations conditions, and underestimate under higher atmospheric concentration conditions. Published as: Dammers, E., Shephard, M. W., Palm, M., Cady-Pereira, K., Capps, S., Lutsch, E., Strong, K., Hannigan, J. W., Ortega, I., Toon, G. C., Stremme, W., Grutter, M., Jones, N., Smale, D., Siemons, J., Hrpcek, K., Tremblay, D., Schaap, M., Notholt, J., and Erisman, J. W.: Validation of the CrIS Fast Physical NH3 Retrieval with ground-based FTIR, Atmospheric Measurement Techniques Discussions, doi:10.5194/amt-2017-38, in review, 2017.