Abstract
Abstract. The Fine Resolution Atmospheric Multi-pollutant Exchange model (FRAME) was applied to model the spatial distribution of reactive nitrogen deposition and air concentration over the United Kingdom at a 1 km spatial resolution. The modelled deposition and concentration data were gridded at resolutions of 1 km, 5 km and 50 km to test the sensitivity of calculations of the exceedance of critical loads for nitrogen deposition to the deposition data resolution. The modelled concentrations of NO2 were validated by comparison with measurements from the rural sites in the national monitoring network and were found to achieve better agreement with the high resolution 1 km data. High resolution plots were found to represent a more physically realistic distribution of reactive nitrogen air concentrations and deposition resulting from use of 1 km resolution precipitation and emissions data as compared to 5 km resolution data. Summary statistics for national scale exceedance of the critical load for nitrogen deposition were not highly sensitive to the grid resolution of the deposition data but did show greater area exceedance with coarser grid resolution due to spatial averaging of high nitrogen deposition hot spots. Local scale deposition at individual Sites of Special Scientific Interest and high precipitation upland sites was sensitive to choice of grid resolution of deposition data. Use of high resolution data tended to generate lower deposition values in sink areas for nitrogen dry deposition (Sites of Scientific Interest) and higher values in high precipitation upland areas. In areas with generally low exceedance (Scotland) and for certain vegetation types (montane), the exceedance statistics were more sensitive to model data resolution.
Highlights
Emissions of reactive nitrogen to the atmosphere occur in the form of reduced nitrogen and oxidised nitrogen (NO and NO2)
In this study we focus on nitrogen deposition in the UK as a whole as well as a region of highly variable precipitation in Snowdonia in North Wales
The concentrations of reactive nitrogen compounds in air as well as nitrogen deposition to the surface were calculated with Fine Resolution Atmospheric Multi-pollutant Exchange model (FRAME) at a 1 km resolution for the UK for the year 2007
Summary
Emissions of reactive nitrogen to the atmosphere occur in the form of reduced nitrogen (ammonia gas) and oxidised nitrogen (NO and NO2). The former is emitted predominantly from agricultural activities including emissions from farm animal wastes whilst the latter is generated from fuel combustion, notably power generation and road transport, as shown in the UK national atmospheric emissions inventory (http://naei.defra.gov.uk). Atmospheric oxidation of NOx and chemical reaction can lead to the formation of nitric acid as well as ammonium and nitrate aerosols (Seinfield and Pandis, 1998). The efficiency of nitrate and ammonium aerosols to act as cloud condensation nuclei in the formation of rain clouds results in nitrogen being washed out of the atmosphere and wet deposited to the surface. Whilst the input of nitrogen to agricultural land is generally considered to be a desirable stimulus to productivity, the cycle of emissions and deposition of nitrogen to natural ecosystems has important consequences for changes to biodiversity and can lead to a decreasing incidence of nitrogen intolerant plant species which become replaced by nitrogen-loving plants
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