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.