Atmospheric column abundances of carbon dioxide (CO2), carbon monoxide (CO), methane (CH4) and nitrous oxide (N2O) 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 CH4 are observed which correlate well with those in CO2. Weaker correlations are seen between N2O and CO2, with large uncertainties. We compute yearly SCB emissions of CO and CH4 to be 1.4 ± 0.3 Tg CO and 0.6 ± 0.1 Tg CH4. 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 CH4 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.