The National Ammonia Monitoring Network

Ammonia (NH₃) emissions are estimated to have at least doubled over the last century across Europe. This rise has been primarily due to the intensification of agriculture, together with the increased use of nitrogen-based fertilizers. The main sources of NH₃ in the atmosphere are from the decomposition and volatilisation of animal wastes. Other sources include direct volatilisation from synthetic fertilizers (particularly urea), and a wide range of non-agricultural sources such as sewage, catalytic converters, wild animals and industrial processes. It is recognised that deposition of atmospheric NH₃ contributes to acidification and eutrophication processes, which can cause damage to sensitive ecosystems.

Under the UNECE Gothenburg Protocol and the National Emissions Ceilings Directive of the EU, the target of both these agreements is that NH₃ emissions should not exceed 297 kt as NH₃ by 2010, with a particular focus on reducing the extent of critical loads exceedence for acidification and eutrophication effects. Abatement of NH₃ emissions is also included in EU Integrated Pollution Prevention and Control (IPPC) for the intensive pig and poultry sectors.

In the UK, the Defra-funded National Ammonia Monitoring Network (NAMN, http://www.uk-pollutantdeposition.ceh.ac.uk/networks) was established in 1996 to quantify temporal and spatial changes in air concentrations and deposition in gaseous NH₃ and aerosol NH₄+ (included since 1999) on a long-term basis. The monitoring provides a baseline in reduced nitrogen (NH_x) species, which is necessary for examining responses to changes in the agricultural sector and to verify compliance with targets set by international agreements. Data from the network are also used to test the performance of an atmospheric chemistry and transport model (FRAME) that was developed at the same time with a special focus on NH_x, and to contribute to national Nitrogen (N) deposition estimates.

There are currently 95 sites in the NAMN and the high spatial variability of ammonia concentrations demonstrates that this large number of sites is necessary. At 58 of these sites, where power is available, an active diffusion denuder methodology using the CEH DELTA (DEnuder for Long Term Atmospheric sampling) system is used to provide the main spatial and temporal patterns of NH₃ across the UK. Aerosol NH₄+, a secondary product is spatially more even and is monitored at a subset of DELTA sites (currently 46).

The DELTA network is complemented by the implementation of a high sensitivity passive diffusion sampler, the ALPHA (Adapted Low-cost Passive High-Absorption) sampler at a further 49 sites to assess regional and local scale variability in air NH₃ concentrations in source regions. In the first phase of the network, the Gradko 3.5 cm membrane diffusion tube was used, but, owing to limitations in the sensitivity of the method (Limit of Detection = approx. 1-2 µg NH₃ m^-3), this was replaced by the new ALPHA sampler in the second phase of the network (since 2000). The ALPHA sampler was designed and developed specifically for monitoring ambient concentrations of NH₃, with a detection limit of around 0.02 µg NH₃ m^-3. To provide an ongoing validation of the ALPHA sampler, its performance is continuously assessed against the DELTA system at 12 sites within the network.

The DELTA system used in the NAMN also provides a foundation for monitoring of other atmospheric components at a subset of sites, including monthly measurements of gaseous HNO₃, SO₂, HCl and aerosol NO_3-, SO₄^2-, Cl-, plus base cations Na⁺, Ca²⁺ and Mg²⁺, as part of the UK Nitric Acid Monitoring Network.

Overall, the UK NAMN structure currently consists of the following sites:

Non-automatic monitoring stations for acid deposition, nitric acid and ammonia during 2007