Abstract
Long-term change and shorter-term variability in the atmospheric deposition of pollutants and marine salts can have major effects on the biogeochemistry and ecology of soils and surface water ecosystems. In the 1980s, at the time of peak acid deposition in the UK, deposition loads were highly dependent on prevailing weather types, and it was postulated that future pollution recovery trajectories would be partly dependent on any climate change-driven shifts in weather systems. Following three decades of substantial acidic emission reductions, we used monitoring data collected between 1992 and 2015 from four UK Environmental Change Network (ECN) sites in contrasting parts of Great Britain to examine the trends in precipitation chemistry in relation to prevailing weather conditions. Weather systems were classified on the basis of Lamb weather type (LWT) groupings, while emissions inventories and clustering of air mass trajectories were used to interpret the observed patterns. Concentrations of ions showed clear differences between cyclonic-westerly-dominated periods and others, reflecting higher marine and lower anthropogenic contributions in Atlantic air masses. Westerlies were associated with higher rainfall, higher sea salt concentrations, and lower pollutant concentrations at all sites, while air mass paths exerted additional controls. Westerlies therefore have continued to favour higher sea salt fluxes, whereas emission reductions are increasingly leading to positive correlations between westerlies and pollutant fluxes. Our results also suggest a shift from the influence of anthropogenic emissions to natural emissions (e.g., sea salt) and climate forcing as they are transported under relatively cleaner conditions to the UK. Westerlies have been relatively frequent over the ECN monitoring period, but longer-term cyclicity in these weather types suggests that current contributions to precipitation may not be sustained over coming years.
Highlights
Atmospheric deposition is the process by which chemical species are transported from the atmosphere to terrestrial and aquatic surfaces (Pacyna, 2008)
When classifying the trajectories into those that travelled from the east or the west of a site, up to 20% only of westerly trajectories originated from the east, in contrast to up to 40% for the other Lamb weather type (LWT)
Our analysis, based on long-term weekly monitoring data collected at sites in contrasting locations across the UK over the past 25 years demonstrates that weather types have continued to exert a significant influence on precipitation chemistry until close to the present day, despite the fact that atmospheric pollution loads have declined dramatically over this period
Summary
Atmospheric deposition is the process by which chemical species are transported from the atmosphere to terrestrial and aquatic surfaces (Pacyna, 2008). It contributes acidity and other toxic contaminants, provides nutrients to plants and is attributed with maintaining key biogeochemical cycles in more nutrient poor environments (Tipping et al, 2014). The deposition loads of major ions and nutrients can induce both acidification and eutrophication. Deposited acidity in upland areas depleted soils of base cations, and resulted in the chronic acidification of soils and acid-sensitive freshwater ecosystems over centennial time scales (Battarbee and Charles, 1986), and is linked to forest decline (Grennfelt et al, 2020). Eutrophication by atmospherically deposited reactive nitrogen can have detrimental ef fects on some taxa (e.g. heathland herb and shrub species) while benefitting nitrophilous competitors, leading to declines in biodiversity. UK Centre for Ecology and Hydrology, Lancaster Environment Centre, Library Avenue, Bailrigg, Lancaster, LA1 4AP, UK
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