Modelling the atmospheric oxidised and reduced nitrogen budgets for the UKwith a Lagrangian multi-layer long-range transport model

The United Kingdom budgets of oxidised and reduced nitrogen (N) species, ma jor components of acidic deposition, are analysed with a long-range transpo rt model. In order to describe adequately the surface sources of trace spec ies, a simple description of vertical diffusion has been incorporated, alon g with a multiple resistance dry deposition velocity scheme. Also, a fully diurnal chemical scheme is utilised which speciates oxidised N species into nitrate and nitrite aerosols, nitrous and nitric acids, and peroxyacetyl n itrate. The model was validated against measurements of various species and was found to perform well for oxidised N species, although concentrations and subsequent dry deposition of peroxyacetyl nitrate is over-estimated. Ho wever, the over-estimation of peroxyacetyl nitrate affects the partitioning of dry deposited oxidised N species only. Dry deposition of reduced N was simulated well, according to other estimates, but wet deposition of reduced N was under-estimated by a factor of approximately 2. It is suggested that the cause of this is that the UK emissions inventory of NH3 is too small. In attribution studies, it was found that modelled results differed signifi cantly from those of EMEP. The modelled contribution of non-UK sources of n itrogen oxides was approximately 50% which may be compared with the EMEP es timation for the same year's emissions of 37%. The model showed a non-linea r response to changes in emissions which is explicable and in accord with c urrent understanding of atmospheric processes. Minor sources of nitrogen ox ides (lightning, aircraft and soils), not usually considered in long-range transport modelling were investigated: aircraft and lightning contribute tr ivial amounts of deposited N to the UK, but soil sources resulted in an add itional 5% oxidised N deposition.