Plain Language Summary
Emissions of ammonia, mostly from agriculture, are often a dominant contributor to fine particles in countries with well-established policies that have led to large reductions in other precursors of such pollutants detrimental to our health. Here we use a model and observations of ammonia from two space-based sensors to estimate emissions in the UK where there are no direct policies regulating agricultural sources of ammonia. The satellite-derived emissions, limited to March-September when conditions are ideal for viewing ammonia from space, total 272 kilotonnes from an instrument that passes overhead in the morning and 389 kilotonnes from an instrument with a midday overpass. Though the emissions estimates differ for the two instruments, both exhibit a spring (April) peak due to fertilizer and manure use and summer (July) peak likely associated with dairy cattle farming. The summer peak is missing in bottom-up emissions and total March-September emissions from these inventories are also 27-49% less than those derived with satellites. Further research is needed to address these discrepancies, as such inventories are widely used for developing policies and assessing environmental damage caused by ammonia.
1 Introduction
Agricultural practices such as synthetic fertilizer and manure use and livestock farming release large quantities of ammonia (NH3) to the atmosphere. Once emitted, NH3 partitions to acidic aerosols to form ammonium that contributes to mass concentrations of fine particles (PM2.5) hazardous to health (Cohen et al., 2017; Dockery et al., 1993; Vohra et al., 2021b). NH3 and ammonium also deposit to the Earth’s surface and drastically alter the natural nitrogen balance of terrestrial and aquatic ecosystems (Galloway, 1998; Johnson & Carpenter, 2010; Vitousek et al., 1997).
In the UK, agriculture is the dominant (>80%) source of NH3 emissions (Ricardo, 2018b), mostly from nitrogen fertilizer use, manure management, and farming of dairy and beef cattle (DEFRA, 2019). Modelling studies suggest that the largest and most extensive decline in PM2.5 in the UK would be achieved by targeting NH3 sources (Vieno et al., 2016), but only large pig and poultry farms are required to adopt best practices and technologies that reduce NH3 emissions (DEFRA, 2019). There are additional policy options under consideration, such as limiting the use of solid urea fertilizer, a large source of NH3 in the UK (DEFRA, 2020a). The UK is a signatory of the United Nations Economic Commission for Europe (UNECE) Gothenburg protocol, lesgislated through the UK National Emission Ceilings Regulations adopted in 2018 (UK, 2018). This commits the UK to an anthropogenic NH3 emission ceiling of 297 Gg, informed by annual emissions estimates from the UK National Atmospheric Emissions Inventory (NAEI). The UK is also required as part of the protocol to reduce NH3 emissions by 8% in 2020 and beyond relative to emissions in 2005 (UNECE, 2019). The estimated decline in NH3 emissions from 1980 to 2017 is 0.2% a-1 due to a steep decline in vehicular emissions of NH3 in 1998-2007 and a recent increase in agricultural emissions since 2013 mostly due to increased use of urea-based fertilizers (Ricardo, 2020). Any future policies targeting NH3 emissions would also need to consider increases in emissions as the atmosphere warms (Sutton et al., 2013).
Estimates of the contribution of NH3 emissions to PM2.5 and mobilization of nitrogen in aquatic and terrestrial ecosystems, assessment of attainment of emissions ceilings commitments and targets, and decisions on effective mitigation measures demand accurate estimates of NH3 emissions. The NAEI of annual total and mapped UK NH3 emissions is published each year. These are obtained at high spatial resolution (1 km) with a model that uses climatological environmental factors and incorporates detailed information about farming activities that contribute to NH3 emissions. The ability to validate the inventory is challenging, as there are no long-term measurements of NH3 fluxes. There is a network of very reliable measurements of rural 24-hour mean surface concentrations of NH3 that cover the full latitudinal extent of the UK from Cornwall in the south to Shetland in the north (Tang et al., 2018), but there are large monitoring gaps in-between. Individual sites are also unlikely to be representative of inventory grid cells for an emission source with large spatial variability. Satellite observations of NH3 retrieved from infrared spectral measurements offer complete coverage of the UK and routine daily measurements in the absence of clouds and under good retrieval conditions. Satellites observe NH3 molecules throughout the atmospheric column, but the majority are within the planetary boundary layer and most of the variability in the column is typically due to NH3 at or near the surface (Clarisse et al., 2010; Nowak et al., 2010; Schiferl et al., 2016; Vohra et al., 2021a).
Retrieval of NH3 from space-based instruments was first described by Beer et al. (2008) for the Tropospheric Emission Spectrometer (TES) instrument. Satellite NH3 retrieval products have since undergone substantial retrieval development (Clarisse et al., 2009; Shephard et al., 2011; 2020; Shephard & Cady-Pereira, 2015; Van Damme et al., 2014a; 2017; 2021; Whitburn et al., 2016a), intercomparisons (Dammers et al., 2019), and validation against ground-based observations of total atmospheric column densities and surface concentrations of NH3 (Dammers et al., 2016; 2017; Van Damme et al., 2015a; Vohra et al., 2021a). These products have also seen extensive use in characterizing NH3 emissions. This includes detecting global and regional NH3 emission hotspots (Cady-Pereira et al., 2017; Clarisse et al., 2019; Dammers et al., 2019; Shephard et al., 2020; Van Damme et al., 2018), constraining NH3 emissions from biomass burning (Adams et al., 2019; Whitburn et al., 2016b), evaluating regional emission inventories (Chen et al., 2021; Fortems-Cheiney et al., 2020), identifying underestimated or missing NH3 sources in widely used global and regional emission inventories and models (Heald et al., 2012; Hickman et al., 2018; Van Damme et al., 2014b), and determining long-term local and regional trends and variability in NH3 (Hickman et al., 2020; Van Damme et al., 2015b; 2021; Vohra et al., 2021a).
Here we use satellite observations of NH3 and the GEOS-Chem chemical transport model (CTM) to derive top-down NH3 emissions for the UK and evaluate the NAEI inventory and current understanding of seasonality in emissions as represented in GEOS-Chem. This includes the use of surface observations from the UK monitoring network to evaluate the model driven with the NAEI to corroborate findings from the satellite observations.
2 Space-based observations of column densities of NH3
Satellite observations of NH3 retrieved in the infrared portion of the light spectrum rely on the spectral signal that depends on the atmospheric state, such as abundance and vertical distribution of NH3 and thermal contrast between the surface of the Earth and the overlying atmosphere (Clarisse et al., 2010; Shephard et al., 2011). Two prominent products are available from contemporary space-based instruments that pass overhead in the morning (the Infrared Atmospheric Sounding Interferometer or IASI) and midday (the Cross-track Infrared Sounder or CrIS). These products use distinct retrieval approaches, offering two independent datasets to assess the potential to use satellite observations to constrain the magnitude and seasonality of UK NH3 emissions.