A unique long-term dataset from the UK National Ammonia Monitoring Network (NAMN) is used here to assess spatial, seasonal and long-term variability in atmospheric ammonia (NH3: 1998-2014) and particulate ammonium (NH4+: 1999-2014) across the UK. Extensive spatial heterogeneity in NH3 concentrations is observed, with lowest annual mean concentrations at remote sites (< 0.2 µg m 3) and highest in the areas with intensive agriculture (up to 22 µg m-3), while NH4+ concentrations show less spatial variability (e.g. range of 0.14 to 1.8 g m-3 annual mean in 2005). Temporally, NH3 concentrations are influenced by environmental conditions and local emission sources. In particular, peak NH3 concentrations are observed in summer at background sites (defined by 5 km grid average NH3 emissions <1 kg N ha 1 y-1) and in areas dominated by sheep farming, driven by increased volatilization of NH3 in warmer summer temperatures. In areas where cattle, pig and poultry farming is dominant, the largest NH3 concentrations are in spring and autumn, matching periods of manure application to fields. By contrast, peak concentrations of NH4+ aerosol occur in spring, associated with long-range transboundary sources. An estimated decrease in NH3 emissions by 16 % between 1998 and 2014 was reported by the UK National Atmospheric Emissions Inventory. Annually averaged NH3 data from NAMN sites operational over the same period (n = 59) show an indicative downward trend, although the reduction in NH3 concentrations is smaller and non-significant (6.3 % (Mann-Kendall, MK); 3.1 % (linear regression, LR)). In areas dominated by pig and poultry farming, a significant reduction in NH3 concentrations between 1998 and 2014 (22 % (MK); 21 % (LR): annually averaged NH3) is consistent with, but not as large as the decrease in estimated NH3 emissions from this sector over the same period (39 %). By contrast, in cattle-dominated areas there is a slight upward trend (non-significant) in NH3 concentrations (+12%, (MK); +3.6% (LR): annually averaged NH3), despite the estimated decline in NH3 emissions from this sector since 1998 (¬¬11%). At background and sheep dominated sites, NH3 concentrations increased over the monitoring period. These increases (non-significant) at background (+17 % (MK); +13 % (LR): annually averaged data) and sheep dominated sites (+15 % (MK); +19 % (LR): annually averaged data) would be consistent with the concomitant reduction in SO2 emissions over the same period, leading to a longer atmospheric lifetime of NH3, thereby increasing NH3 concentrations in remote areas. The observations for NH3 concentrations not decreasing as fast as estimated emission trends are consistent with a larger downward trend in annual particulate NH4+ concentrations (1999-2014: 47 % (MK); 49 % (LR), p < 0.01, n = 23), associated with a slower formation of particulate NH4+ in the atmosphere from gas-phase NH3.
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