Usefulness of Long-term Urban Greenhouse Gas monitoring: the London record

Long-term monitoring of CH4 at Egham, SW London, shows reduction in the source input since the mid 1990s. There is a distinct seasonal cycle, in part reflecting background variation, tracked by comparison with the Mace Head Atlantic record. Local emissions, led by higher fossil fuel consumption in the winter months, also contribute to the cycle. Inter-annual variability is in part meteorological. The urban increment can be estimated by comparing specific wind sectors (e.g. incoming SSW Atlantic air compared to easterly urban air). Ratios of CH4 to CO2, calculated from the continuous records, allow relative emissions of CH4 and CO2 to be quantified, providing immediate tests of inventories. Ratios of excess over background of CH4 to CO2 for periods of 7 consecutive days of easterly air flow to the Egham site (from London) indicate a reduction in CH4 emissions of 11% relative to CO2 over the period 1999-2007. Isotopes discriminate sharply between methane sources. Diurnal (Keeling plot) δ13CCH4 campaigns identify source mixes. For London, both CO2 and CH4 annual emissions are cited to 0.1 ton (i.e. to 9 significant figures of CO2, and to 6 figures for methane). However, it can be difficult to reconcile isotopic measurements of local methane increments with declared emissions budgets (Lowry et al., 2001). Rapid, inexpensive, small-sample isotopic techniques (Fisher et al., 2006) allow simple tests of the veracity of emissions declarations. Local emissions can be detected, by geographic location, given known background patterns. Moreover, seasonal variation can be tracked. This makes it potentially possible cheaply to audit emissions in populated areas. Lowry, D., et al. (2001) J. Geophys. Res., 106, 7427-48 Fisher, R., et al. (2006) Rapid Comm. Mass Spectrometry, 20, 200-208.