2.2 Cross-track Infrared Sounder NH3
The first CrIS sensor launched into low-Earth polar sun synchronous orbit in October 2011 is onboard the NOAA Suomi-NPP satellite. Like IASI, CrIS observes the Earth twice daily, though in the early afternoon (13h30 LST) and after midnight (01h30 LST) (Goldberg et al., 2013). It has the same swath width as IASI and similar ground pixel resolution (14 km circular pixels at nadir). The fast physical retrieval (CFPR) approach used to retrieve NH3 columns is described in detail in Shephard & Cady-Pereira (2015) and Shephard et al. (2020). Briefly, it is based on conventional optimal estimation that involves minimizing the difference between observed and calculated outgoing spectral radiances with a priori vertical profiles of NH3 (Rodgers, 2000). CFPR uses three prior NH3 profiles representing polluted, moderately polluted, and remote conditions (Shephard et al., 2020) that are selected based on the ammonia spectral signal. This is different to standard optimal estimation that uses prior information that is independent of the observations and imposes spatial and temporal information. The CFPR retrieval generates averaging kernels that quantify the vertical sensitivity of the retrieval. These typically peak between 900 and 750 hPa (~1-2.5 km altitude) (Dammers et al., 2017; Shephard & Cady-Pereira, 2015).
We use the Level 2 CrIS NH3 CFPR version 1.6 product for 2013-2018. The predecessor product (version 1.5) exhibited a positive bias for NH3 < 1 × 1016molecules cm-2, as values were only retrieved over scenes exceeding the instrument detection limit of ~2 × 1015 molecules cm-2 (Dammers et al., 2017; Shephard & Cady-Pereira, 2015). This approach filtered out cloud-free scenes below the instrument detection limit and indirectly removed cloudy scenes when the NH3 signal below clouds could not be detected. In version 1.6 clouds are explicitly identified with information from the space-based Visible Infrared Imaging Radiometer Suite (VIIRS) (White et al., 2021). We use daytime cloud-free CrIS observations with quality flag ≥ 4 (Shephard et al., 2020) and thermal contrast > 0 K, where thermal contrast is the difference between the reported temperatures at the surface and the lowest atmospheric layer. We identify and correct for a positive trend in the CrIS baseline that appears to be erroneous, as it is not apparent in the IASI data. We do this by estimating a statistically significant (p-value = 0.03) increase in monthly mean background NH3columns over Scotland (Figure S1) of 2.21 × 1013molecules cm-2 per month (amounting to 1.6 × 1015 molecules cm-2 over the whole record) and subtract this from individual CrIS NH3column retrievals. We grid the corrected data to 0.1° × 0.1° using the same tessellation code used for IASI, but without error weighting. The individual total column errors include measurement and representative errors and cover a much narrower range (5-55% (Shephard et al., 2020)) than those for IASI (5% to >100%). As a result, higher relative weighting would be applied to low column densities, leading to anomalously low gridded values in the CrIS multiyear means. For consistency with IASI, and because of weak spectral signal in autumn and winter, we only consider CrIS retrievals in March to September.
Figure 2 shows the gridded March-September CrIS NH3multiyear monthly mean columns. As with IASI, we filter for extreme values in the multiyear means (column densities < -1 × 1016 molecules cm-2 and > 1 × 1017 molecules cm-2), removing <0.1% of the gridded data. Observations during the July 2018 heatwave only increase the July multiyear mean by 1.6%, but for consistency with IASI these days are also removed. The number of CrIS retrievals in each grid ranges from 11 to 96. The CrIS multiyear means are roughly double those for IASI (Figure 1; Figure S2), in part because CrIS passes overhead at midday when higher ambient temperatures lead to greater volatilization of NH3. Differences in vertical sensitivity and distinct retrieval approaches likely also contribute. Difference are particularly large in September when background NH3 is 5.3 × 1015 molecules cm-2 more in CrIS than IASI, obtained as the intercept from regressing CrIS against IASI. The spatial correlation between CrIS and IASI multiyear means isR < 0.5 in most months (March, June-September),R = 0.53 in May, and R = 0.55 in April. If extreme values in the gridded products are retained, the spatial correlation degrades to R = 0.42 in April and R = 0.29 in May.