Emissions of greenhouse gases from a North American megacity

Atmospheric column abundances of carbon dioxide (CO 2 ), carbon monoxide (CO), methane (CH 4 ) and nitrous oxide (N 2 O) have been measured above the South Coast air basin (SCB), a densely populated urban region of Southern California, USA, which includes Los Angeles and the surrounding suburbs. Large diurnal variations in CO and CH 4 are observed which correlate well with those in CO 2 . Weaker correlations are seen between N 2 O and CO 2 , with large uncertainties. We compute yearly SCB emissions of CO and CH 4 to be 1.4 ± 0.3 Tg CO and 0.6 ± 0.1 Tg CH 4 . We compare our calculated emissions to the California Air Resources Board (CARB) and the Emission Database for Global Atmospheric Research (EDGAR) estimates. Our measurements confirm that urban emissions are a significant source of CH 4 and in fact may be substantially higher than currently estimated. If our emissions are typical of other urban centers, these findings suggest that urban emissions could contribute 7–15% to the global anthropogenic budget of methane.