The UK National Ammonia Monitoring Network (NAMN, Sutton et al. 2001a) was established in 1996 to quantify the spatial distribution and long-term trends in concentrations of atmospheric ammonia (NH3) and also aerosol ammonium (NH4 +) (since 1999). There are currently 94 sites. At 59 of these sites, the CEH DELTA methodology (Sutton et al. 2001b) is used to provide the spatial and temporal patterns of NH3 and aerosol NH4 + across the UK, while passive diffusion samplers (Tang et al. 2001) are used to assess regional and local scale variability in air NH3 concentrations in source regions. Monitoring is on a monthly timescale, which is optimal to provide information on seasonality and for estimating annual mean in air concentrations. \ud Emissions of NH3 in the UK have fallen by 12% between 1990 and 2004 (http://www.naei.org.uk). The long-term dataset from the UK NAMN, which comprises 9 years of gaseous NH3 data (since September 1996) and 7 years of aerosol NH4 + data (since 1999) may therefore be analysed to assess trends in air concentrations. The data can also be used to compare with critical levels of NH3 concentrations set in the UK to protect vegetation and ecosystem
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