Figure 7. Regional seasonality in March-September
NH3 emissions. Points are the percentage change in
emissions in each month relative to those in June for top-down emissions
from IASI (black) and CrIS (blue), and from the bottom-up emissions
(red). Regions sampled are in Figure 3. Inset values are March-September
totals for each region from each estimate.
In Figure 8, we compare March-September 2016 mean modelled and observed
surface concentrations of NH3 to determine if the model
driven with NAEI NH3 emissions and prior assumptions of
NH3 seasonality and diurnal variability corroborates the
results obtained with the satellite observations. Monthly means from
model grids coincident with the surface sites are reasonably spatially
consistent with the surface observations (R = 0.54) and the model
is 38.3% less than the observations. This is midway between the NAEI
comparison to the top-down emissions of 27% less than IASI and 49%
less than CrIS. There are also low-cost passive sampler measurements of
NH3 concentrations at 39 rural sites, but these have
relatively low precision, are not as extensively distributed as the
observations in Figure 8, and are only reliable (within ±10% of
reference measurements) at NH3 ≥ 2 µg
m-3 (Martin et al., 2019; Sutton et al., 2001). Even
so, the model is similarly biased low (by 41.5%) compared to these
measurements (not shown).