Figure 1. Monthly multiyear (2008-2018) mean IASI NH3. Data are gridded to 0.1° × 0.1° using oversampling (see text for details). Grey grids, limited to Scotland, have < 10 observations.
Figure 1 shows the gridded March-September multiyear monthly mean IASI NH3 columns. The number of observations in each grid ranges from 11 to 128. Values over Scotland are very low (typically < 2 × 1015 molecules cm-2) due to weak signal, lower agricultural activity than the rest of the UK, and greater distance from sources in mainland Europe. The range in IASI NH3 over the rest of the country of 4-8 × 1015 molecules cm-2 is much less than the NH3 hotspots in other parts of the world. Columns over global hotspots such as North China, West Africa, the Po Valley (Italy), and the Indo-Gangetic Plain (India) exceed 2 × 1016 molecules cm-2 (Cady-Pereira et al., 2017; Dammers et al., 2019; Van Damme et al., 2014b; 2018). These are associated with industrial and agricultural activity in India and China, and intense seasonal open burning of biomass and relatively low abundance of acidic aerosols in West Africa and northern India. Warm temperatures in these regions also increase NH3emissions, suppress partitioning of NH3 to aerosols, and enhance the spectral signal.