Dispersion, deposition and impacts of atmospheric ammonia: quantifying local budgets and spatial variability

Ammonia is a reactive pollutant emitted primarily by agricultural sources n ear ground level in the rural environment. The consequence of these factors is that, in addition to the effects of long-range pollutant transport, amm onia has major effects at a local scale, with emission and receptor areas o ften closely located in the rural landscape. There is a substantial local s patial variability that needs to be considered in effects assessments, whil e variations in local deposition may affect the amount of ammonia available for impacts further afield. The wide-ranging UK programme ADEPT (Ammonia D istribution and Effects ProjecT) has addressed these issues through a combi nation of measurement and modelling activities concerning the distribution of emissions, atmospheric transport, deposition and effects assessment. The results are illustrated here by summarizing the findings of a joint experi ment at Burrington Moor, Devon, and wider modelling contrasting the variabi lity at a field scale with 5 km resolution estimates for the UK. The fracti on of emitted NH3 deposited locally is shown to depend critically on the do wnwind land-cover, with fluxes being dependent on interactions with the amm onia compensation point. This will restrict deposition back to agricultural land, but may mean that non-conservation woodlands could be of benefit to recapture a significant fraction of emissions. The generalized models demon strate the high spatial variability of ammonia impacts, with a case study b eing used to show the consequences at a field scale. In source regions subs tantial variability occurs at sub-1 km levels and this will have major cons equences for the emission reduction targets needed to protect ecosystems.