Figure 5. CrIS-derived NH3 emissions for March-September. Maps are 24-hour total emissions at 0.1° × 0.1°. Inset values are monthly emissions that sum to 389.4 Gg.
Maps of the resultant top-down monthly NH3 emissions are shown in Figure 4 for IASI and Figure 5 for CrIS. Qualitatively, both estimates exhibit spatial patterns similar to the NAEI (Figure 3). This includes relatively low emissions along the Welsh border, and peak emissions in Northern Ireland, the northern portion of the English side of the Welsh border, and in Norfolk in the east. Emissions for retained grid squares total 271.5 Gg for IASI, whereas these are 43% more from CrIS (389.4 Gg). CrIS monthly emissions are 20-38% more than IASI for March-July. This is similar in magnitude to the 25-50% low bias in IASI columns, though for an earlier IASI product (Dammers et al., 2017; Whitburn et al., 2016a). The percentage difference increases to 57% for August and >100% for September. The large difference in September is due to 5.4 × 1015 molecules cm-2 greater background NH3 in CrIS, even after correcting for the baseline trend (Section 2.2, Figure S1). CrIS emissions excluding September are 33% more than IASI. Differences in sampling periods (2008-2018 for IASI, 2013-2018 for CrIS) only has a small effect on satellite-derived emissions, but leads to data gaps over Scotland and Northern England. IASI-derived emissions obtained for 2013-2018 are only 6% more (288.3 Gg) than those in Figure 4.
For comparison of monthly top-down and bottom-up emissions, we estimate monthly bottom-up emissions as the product of the annual NAEI emissions in Figure 3 and GEOS-Chem seasonality. The latter we obtain as ratios of GEOS-Chem monthly to annual 24-hour NH3 emissions interpolated onto the 0.1° × 0.1° grid. Figure 6 shows the resultant monthly bottom-up NH3 emissions for April and July. The other months are in the supplementary (Figure S4). The bottom-up emissions peak in April (~14% of the annual total) coincident with fertilizer application (Hellsten et al., 2007; Paulot et al., 2014). The gridded difference between top-down and bottom-up emissions are also shown in Figure 6 for April and July and in Figure S4 for the other months. Locations where bottom-up emissions exceed those from the top-down approach (red grids) mostly occur where emissions are low. The largest difference is in July when top-down emissions are 30 Gg more (IASI) and 46 Gg more (CrIS) than the bottom-up emissions. Pronounced regional differences include lower bottom-up values in eastern England, particularly in April, where fertilizer use and pigs and poultry farming are dominant sources, as well as in western England and Northern Ireland, particularly in July, where dairy cattle farming dominates (Hellsten et al., 2008). The spatial correlation between top-down and bottom-up gridded emissions in general ranges from R= 0.5 to R = 0.7, except for IASI in September (R = 0.34) when dynamic range in emissions is low.
The bottom-up emissions for March-September total 198.7 Gg. This is 27% less than IASI and 49% less than CrIS. It is unlikely that the relatively low bottom-up emissions is due to the time period (1981-2020) of the 30-year meteorology used to determine agricultural NH3 emissions for the NAEI. We find that 2-metre temperature from the NASA long-term consistent relanalysis product, Modern-Era Retrospective analysis for Research and Applications Version 2 (MERRA-2), is similar for 1981-2010 (282.750 K) and 1991-2020 (282.957 K). Bottom-up emissions in March-September are 67% of the annual total, similar to ~60% for the monthly bottom-up NH3 emissions estimated by Hellsten et al. (2007). If we use this relative contribution (60-67%) to scale IASI and CrIS to annual totals, this suggests annual NH3 emissions of 405-453 Gg according to IASI and 581-664 Gg according to CrIS. Subtracting the UK annual natural NH3 emissions of ~22 Gg (Section 3) yields top-down annual anthropogenic NH3 emissions of 383-431 Gg according to IASI and 559-642 Gg according to CrIS. Both top-down estimates exceed annual total anthropogenic emissions from the NAEI of 276 Gg (Section 3) and the Gothenburg protocol emissions ceiling of 297 Gg.