δ13C of atmospheric methane: An integrated technique for constraining emission sources in urban and background air

Spot samples of air from Mace Head (Ireland) and Egham (UK), have been analysed for methane concentration and δ13C since early 1995. These data are combined with 96 h back trajectories for air arriving at 12:00 GMT each day and with local area meteorological data for both sites (a) to monitor the isotopic changes in background air at 50-55 deg. N and assess methane source contributions from the North Atlantic and western European regions and (b) to improve our understanding of contributions of methane from large urban conurbations in the global emissions budget. The seasonal range in δ13C at Mace Head during 1995 and 1996 was 0.8 per mille in clean sector air (-47.1 to -47.9 per mille). Continental air arriving at Mace Head contains up to 250 ppb more methane than Atlantic air (1 ppb = 1 part per 109) and shows depletion in 13C by up to 0.8 per mille. Trajectories from the eastern seaboard of Canada contain 10-30 ppb more methane and show depletion in 13C by up to 0.15 per mille with respect to trajectories starting in the central North Atlantic Ocean. Source signatures of -62 per mille for eastern Canada, -65 per mille for western Ireland and -54 per mille for Continental Europe have been calculated for trajectories arriving at Mace Head. Diurnal sampling experiments in stable air at the Egham site (on the western fringe of London) have been used to estimate emissions for western London and surrounding rural areas. Concentration peaks representing sources within 2-3 km indicate dominant natural gas (-41 to -43 per mille) or landfill/livestock (-52 to -53 per mille) emissions. Sources from further afield combine to give a regional isotopic source signature of -48.8% for western London in 1996. (author)