Drivers for spatial, temporal and long-term trends in atmospheric ammonia and ammonium in the UK

Abstract. 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 µg 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 yr⁻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: Mann–Kendall (MK), −6.3 %; linear regression (LR), −3.1 %. In areas dominated by pig and poultry farming, a significant reduction in NH₃ concentrations between 1998 and 2014 (MK: −22 %; LR: −21 %, 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 (MK: +12 %; LR: +3.6 %, 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 (MK: +17 %; LR: +13 %, annually averaged data) and sheep-dominated sites (MK: +15 %; LR: +19 %, 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: MK: −47 %; LR: −49 %, p < 0.01, n =  23), associated with a lower formation of particulate NH₄⁺ in the atmosphere from gas phase NH₃.