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.