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

摘要

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 (NHsub3/sub: 1998–2014) and particulate ammonium (NHsub4/subsup+/sup: 1999–2014) across the UK. Extensive spatial heterogeneity in NHsub3/sub concentrations is observed, with lowest annual mean concentrations at remote sites (&?0.2?μg?msup?3/sup) and highest in the areas with intensive agriculture (up to 22?μg?msup?3/sup), while NHsub4/subsup+/sup concentrations show less spatial variability (e.g. range of 0.14 to 1.8?μg?msup?3/sup annual mean in 2005). Temporally, NHsub3/sub concentrations are influenced by environmental conditions and local emission sources. In particular, peak NHsub3/sub concentrations are observed in summer at background sites (defined by 5?km grid average NHsub3/sub emissions &?1?kg?N?hasup?1/sup?yrsup?1/sup) and in areas dominated by sheep farming, driven by increased volatilization of NHsub3/sub in warmer summer temperatures. In areas where cattle, pig and poultry farming is dominant, the largest NHsub3/sub concentrations are in spring and autumn, matching periods of manure application to fields. By contrast, peak concentrations of NHsub4/subsup+/sup aerosol occur in spring, associated with long-range transboundary sources. An estimated decrease in NHsub3/sub emissions by 16?% between 1998 and 2014 was reported by the UK National Atmospheric Emissions Inventory. Annually averaged NHsub3/sub data from NAMN sites operational over the same period (in/i?=??59) show an indicative downward trend, although the reduction in NHsub3/sub 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 NHsub3/sub concentrations between 1998 and 2014 (MK: ?22?%; LR: ?21?%, annually averaged NHsub3/sub) is consistent with, but not as large as the decrease in estimated NHsub3/sub emissions from this sector over the same period (?39?%). By contrast, in cattle-dominated areas there is a slight upward trend (non-significant) in NHsub3/sub concentrations (MK: +12?%; LR: +3.6?%, annually averaged NHsub3/sub), despite the estimated decline in NHsub3/sub emissions from this sector since 1998 (?11?%). At background and sheep-dominated sites, NHsub3/sub 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 SOsub2/sub emissions over the same period, leading to a longer atmospheric lifetime of NHsub3/sub, thereby increasing NHsub3/sub concentrations in remote areas. The observations for NHsub3/sub concentrations not decreasing as fast as estimated emission trends are consistent with a larger downward trend in annual particulate NHsub4/subsup+/sup concentrations (1999–2014: MK: ?47?%; LR: ?49?%, ip/i?&?0.01, in/i?=??23), associated with a lower formation of particulate NHsub4/subsup+/sup in the atmosphere from gas phase NHsub3/sub.