Abstract
Abstract. Presented here is the validation of the CrIS (Cross-track Infrared Sounder) fast physical NH3 retrieval (CFPR) 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 1.0 × 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 (< 1.0 × 1016 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 comparison differences are mostly within the range of the single-level retrieved profile values from estimated retrieval uncertainties, showing average differences in the range of ∼ 20 to 40 %. The CrIS retrievals typically show good vertical sensitivity down into the boundary layer which 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 in low-concentration conditions and underestimate in higher atmospheric concentration conditions.
Highlights
The disruption of the nitrogen cycle by the human creation of reactive nitrogen has created one of the major challenges for humankind (Rockström et al, 2009)
The retrieval is based on an optimal estimation approach (Rodgers, 2000) that minimizes the differences between CrIS spectral radiances and simulated forward model radiances computed from the Optimal Spectral Sampling method (OSS) OSS-CrIS (Moncet et al, 2008), which is built from the well-validated Line-By-Line Radiative Transfer Model (LBLRTM) (Clough et al, 2005; Shephard et al, 2009; Alvarado et al, 2013) and uses the HITRAN database (Rothman et al, 2013) for its spectral lines
In the smaller total column range the CrIS retrieval shows a positive bias with larger relative differences 49.0 % that mostly seem to follow from observations near the CrIS detection limit
Summary
The disruption of the nitrogen cycle by the human creation of reactive nitrogen has created one of the major challenges for humankind (Rockström et al, 2009). Global reactive nitrogen emissions into the air have increased to unsurpassed levels (Fowler et al, 2013) and are currently estimated to be four times larger than pre-industrial levels (Holland et al, 1999). As a consequence the deposition of atmospheric reactive nitrogen has increased causing ecosystems and species loss (Rodhe et al, 2002; Dentener et al, 2006; Bobbink et al, 2010). Atmospheric aerosols impact global climate directly through their radiative forcing effect and indirectly through the formation of clouds (Adams et al, 2001; Myhre et al, 2013). Deposition of NH3 and other reactive nitrogen compounds plays a key role in the sequestration of carbon dioxide (Oren et al, 2001)
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