Validation of the CrIS Fast Physical NH ₃ Retrieval with ground-based FTIR

Abstract. Global reactive nitrogen emissions into the air have increased to unprecedented levels. Limiting the loss of reactive nitrogen into the environment is one of the major challenges for humankind. At the current levels ammonia (NH3) is a threat to both the environment and human health. However, relatively little is known about the total nitrogen budget and distribution around the world, due in part to the sparseness of observations over most of the globe. Recent advances in the capabilities of measuring NH3 with satellite instruments have improved the situation with sensors such as the Infrared Atmospheric Sounding Interferometer (IASI) and the Cross-Track Infrared Sounder (CrIS) making twice daily observations with global coverage. However, these require validation to be truly useful, and one of the main challenges in the validation of the satellite NH3 profile and total column data products is the scarcity of measurements that can be directly compared. 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 compare well with a 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 1.0 x 1016 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 where there are a large number of observations at or near the CrIS noise level (detection limit) and the FTIR total columns are smaller than 1.0 x 1016 molecules cm−2, the absolute differences between CrIS and the FTIR total columns are small with CrIS showing a slight positive column bias around +2.4 x 1015 (standard deviation = 5.5 x 1015) molecules cm−2, which corresponds to a relative difference of ~ +50 % (std = 100 %). The CrIS retrievals for these comparisons typically show good vertical sensitivity down to ~850 hPa, and at this level the retrieved profiles also compare well with the median absolute difference of 0.87 (±0.08) ppb and a corresponding median relative difference of 39 (±2) %. Most of the absolute and relative profile comparison differences are in the range of the estimated retrieval uncertainties. However, the CrIS retrieval does tend to overestimate the concentrations in the levels near the surface at low concentrations, most probably due to the detection limit of the instrument, and at higher concentrations shows more of an underestimation of the concentrations in these lower levels.

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