Executive summary

The Kyoto Protocol requires that countries establish a national accounting system and quantification of their sources and sinks of greenhouse gases before the commitment period 2008–2010.

Generally, national inventories report greenhouse gas emissions by sectors or activities, using emission factors and statistics. In certain cases, inventories have been shown to be inaccurate due to regional and temporal variations of emissions factors or due to the omission of important sources.

The scope of this work is to analyse the Mace Head atmospheric record to provide an estimate of the fluxes of CO2, CH4 and N2O for Ireland over the period 1995–2000. The method is independent of statistical inventories, and therefore constitutes a ‘top–down’ verification of Irish greenhouse gas emissions inventories.

Radon-222 (Rn- 222), a radioactive noble gas emitted by soils is used to infer fluxes of the major European greenhouse species from the Mace Head data. The method uses correlation between synoptic changes in atmospheric Rn-222 taken as a reference tracer of continental (non-oceanic) sources, with changes in other species also measured at the site.

The measurements of Lead-212 (Pb-212) provide a fingerprint of regional air masses of recent origin (half a day) while Rn-222 acts as a medium-range continental tracer at synoptic time scales (about 4 days). Therefore, Rn-222 and Pb-212 are used to indicate continental sources in order to restrict source estimates to Ireland and avoid inclusion of remote European emissions.

Regression between gas species x and Rn-222 flux is used to infer the flux of x. Due to the uncertainty of Rn-222 fluxes over Ireland, Rn-222 flux measurements have been made during two intensive campaigns (October 2000 and July 2001) to determine the spatial variability of Rn-222 efflux from Irish soils by carrying out measurements over different soil types. Seasonally averaged emission fluxes for CH4, N2O and CO2 over Ireland were determined for the 1995–2000 period.

Mean flux densities are of the order of 220 × 103 kg C km–2 year–1 and 900 × 103 kg C km–2 year–1 for CO2 during wintertime and summertime, respectively. The annual averaged emission flux for CO2 for the period 1995–2000 is estimated to range between about 560 and 595 × 103 kg C km–2 year–1.

Using the total area for Ireland of 85,055 km2, this converts to 4.76–5.06 × 109 kg C year–1, which can be compared to the net CO2 emissions for Ireland (EPA, McGettigan – private communication). The average net emissions of CO2 for the period 1995–2000, inclusive, are 3.9 × 109 kg CO2 equivalent.

This is within 22–30% of the atmospheric flux derived emission carbon values. This methodology for inferring greenhouse gas emission fluxes is quite promising, but requires further studies of the spatial and seasonal variation of Rn-222 efflux from the Irish soil.

The use of air mass back trajectories, which show their origin within selected Irish regions, will be a useful tool in the estimation of greenhouse gas emissions on a regional basis within Ireland.