Improving pollutant deposition estimates to upland ecosystems (UplandDep)

Atmospheric deposition provides input of air pollutants into ecosystems that can damage plants or biodiversity, such as reactive nitrogen, ozone, sulfur, and heavy metals. It also provides a major removal pathway for chemical compounds from the atmosphere and therefore influences the atmospheric lifetime and transport distance of conventional man-made pollutants and unintended releases from volcanos, forest fires, and radioactive materials. Total deposition is composed of dry, wet, and occult deposition, referring to the deposition of gases and particles by atmospheric turbulence, washout and via fog droplets, respectively. These processes can only typically be measured over flat homogeneous surfaces, but parameterisations are also applied or extrapolated to upland areas. However, complex topography impacts deposition processes, e.g. by increasing rainfall (orographic enhancement), washout of lower, more polluted, clouds by precipitation from higher clouds (seeder-feeder effect), increased fog presence and complex dry deposition processes, e.g. on the up and down-wind side of ridges. All are highly uncertain at present and, whilst it is known that these processes significantly increase deposition, they are either not or poorly represented in deposition models. Upland areas contain many plant communities that are particularly sensitive to pollutants such as excess nitrogen, and they provide important catchment inputs into rivers, lakes and drinking water supplies. This project aims to make a significant step forward in our understanding of the processes that alter the deposition in complex upland terrain, and in our measurement and modelling tools to quantify this deposition, through the following objectives:

Develop and apply novel measurement approaches to constrain pollutant deposition enhancement to the uplands.
Assess the impact of altitude and topography on nitrogen concentration and deposition through high spatial resolution assessments of nitrogen content in mosses.
Quantify nitrogen deposition to two contrasting UK areas of complex topography through a combination of seasonal and intensive measurements, and high-resolution modelling.
Compare and improve precipitation maps for the UK, including interpolated gauge maps, radar products, and model estimates.
Integrate improved representations of the processes of seeder-feeder washout, dry deposition enhancements, and fog deposition, into modelling frameworks used for UK and European underpinning of environmental policies and emergency response (EMEP, NAME) suitable for sub 1 km to 25 km model applications.
Assess the impacts of complex terrain impacts on national nitrogen deposition budgets and ecosystem protection metrics.

The project outcomes will be incorporated into two key atmospheric chemistry and transport models (ACTMs): first, the EMEP model is used to assess exceedances of nitrogen Critical Loads across Europe and to underpin emission reduction targets under the EU Directives and UNECE Convention for long-range transboundary pollution (CLRTAP), whilst a downscaled version underpins UK Air Pollution Control Strategies. Second, the NAME model is used by the MetOffice within its emergency response to assess the impacts of releases such as nuclear accidents and volcanic eruptions. Project outcomes will also benefit the CBED (Concentration Based Estimates of Deposition) inferential modelling system used to derive the UK’s official estimates of deposition and Critical Loads exceedances of nitrogen and acidity for Defra, based on measured concentrations. This currently contains a very simplified seeder-feeder correction based on measurements of the early 1990s which results in significantly higher upland deposition than current ACTMs. The project will work with partners (MetOffice and EMEP MSC-W in Oslo/Gothenburg, and the USDA Forest Service) to reduce the uncertainties of upland deposition globally.