Figure 6. Comparison of bottom-up and top-down
NH3 emissions for April and July. Panels are bottom-up
emissions (left), and the difference between top-down and bottom-up
emissions for IASI (middle) and CrIS (right) in April (top row) and July
(bottom row). Grids are blue for bottom-up < top-down and red
for bottom-up > top-down. Values inset are bottom-up total
(left) and differences in (middle and right) monthly emissions and the
Pearson’s spatial correlation (R ) between top-down and bottom-up
emissions.
Figure 7 compares regional seasonality in UK NH3emissions from bottom-up and top-down estimates as the percent change in
emissions in each month relative to those in June. Regional seasonality
in the top-down emissions is very similar in March-August in all regions
except Northern Ireland. The mismatch between IASI and CrIS in September
is due to the at least 2-times greater CrIS than IASI columns in that
month (Section 2). The July peak in emissions in Northern Ireland is
more pronounced in IASI than CrIS. This is also apparent in the
seasonality in the column densities (Figure S5). Northern Ireland has
experienced dramatic changes in agricultural activity that includes
increases in livetock numbers of 45% for pigs and 42% for table
chickens and a decline in nitrogen fertilizer of 37% from 2000 to 2016
(DEFRA, 2020b). We find though that the that top-down emissions
estimates are relatively insensitive to differences in temporal coverage
of the two sensors (2008-2018 for IASI, 2013-2018 for CrIS). All
emission estimates exhibit a spring peak in April due to intensive
fertilizer and manure application in March-April (Hellsten et al.,
2007). Paulot et al. (2014) also identified this April peak in
NH3 emissions inferred from ammonium wet deposition
measurements, though a recent study questions the utility of these
measurements for constraining NH3 emissions (Tan et al.,
2020). A second summer peak in the top-down emissions in July that is
not present in the bottom-up emissions could be due to the timing of
manure spreading, dairy farming practices, or enhanced volatilization
and suppressed dry deposition due to warm summer temperatures (Hellsten
et al., 2007; Sutton et al., 1994). Spatial consistency between the July
top-down emissions (Figures 4 and 5) and locations dominated by
emissions from dairy cattle (Hellsten et al., 2008) suggests a it is due
to dairy farming, but this requires further investigation.