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.