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).