Application of Tracer Ratio and Inverse Dispersion Methods with Boat-Based Plume Measurements to Estimate Ammonia Emissions from Seabird Colonies

Abstract

Ammonia emissions from two contrasting seabird colonies in Scotland were measured, based on the determination of atmospheric concentrations downwind of the colonies. Atmospheric concentrations of ammonia (NH3) across the downwind plume were compared with the inverse application of a Gaussian dispersion model (ID) to calculate the modelled NH3 emission that would generate the measured cross-wind-integrated plume concentration. In parallel, a tracer gas (sulphur hexafluoride, SF6) was released from the colonies with air samples taken to allow determination of SF6 concentrations. On the basis of the known emission rate of SF6, the magnitude of ammonia emissions was estimated by the cross-wind-integrated tracer ratio (TR) of NH3/SF6 concentrations. Coupled with data on annual bird attendance, the measurements indicate annual emissions from the Isle of May and the Bass Rock of 18 and 132 tonnes NH3-N year−1, respectively. The measured NH3 emissions were compared with estimates of seabird nitrogen excretion to estimate the proportion of excreted N that is volatilised as NH3 (FNr). The emission estimates of the two methods compared favourably, giving 4 and 6 kg NH3-N h−1 (FNr = 15%) for the Isle of May for the ID and TR methods, respectively, and 21 and 25 kg NH3-N h−1 (FNr = 50%) for the Bass Rock for the ID and TR methods, respectively. The results provide the first measurement-based estimates to allow regional up scaling of ammonia emissions from seabirds.