Monitoring and modelling trace-gas changes following the 2001 outbreak of Foot and Mouth Disease to reduce the uncertainties in agricultural emissions abatement

Abstract

Abstract The outbreak of Foot and Mouth Disease (FMD) in the UK during 2001 provided a case study to test the link between changing NH3 emissions and NH3 concentrations in air. Previous studies have shown an “ammonia gap” between modelled and observed changes, which might be due to low effectiveness of abatement measures, interactions with changing atmospheric chemistry, ammonia compensation points and inter-annual variability. This study therefore aimed to support policy development by assessing whether possible future reductions in ammonia emissions would achieve the desired outcome of reduced air concentrations. Two networks for monthly measurement of atmospheric NH3 were established, centred on Cumbria and Devon, these being two of the areas worst-affected by FMD. Measurements of CH4 were made in parallel to represent an inert tracer emitted by ruminants, for which near-source atmospheric concentration enhancements would be unaffected by atmospheric reactions. Measurements commenced immediately after the end of the FMD outbreak in February 2002, followed the period of restocking and ceased in January 2004. The spatio-temporal patterns of monthly NH3 and CH4 emissions and concentrations at 5 km resolution were modelled for the UK, providing a reference to interpret the measurements. Overall trends in monthly modelled and measured NH3 concentrations were not significant, these being masked by seasonal and inter-year variability. However, comparison of sampling locations in FMD-affected areas with those in areas not directly affected by FMD showed relative depletions in modelled NH3 in FMD-affected areas of ∼25% for Cumbria and ∼8% for Devon. These reductions were matched by measured depletions in NH3 in FMD-affected areas of ∼35% for Cumbria and ∼20% for Devon, relative to unaffected areas. The modelled recovery in NH3 emissions was slower than for CH4, due to the lag-time associated with NH3 emissions from manure storage and spreading. For CH4 concentrations, modelled reductions in affected areas relative to unaffected areas were 17% for Cumbria and 5% for Devon, but significant relative changes in the measured CH4 concentrations were not detectable. The results show that sites need to be more representative and/or sampling precision needs to be improved to detect such short-term CH4 signals. Conversely, this study has demonstrated that atmospheric NH3 concentrations responded to changes in regional emissions, but that such short-term changes are only detectable by a multi-site assessment of contrasting areas.