Seasonal and Diurnal Variations in Anthropogenic Sources of CO2 in the Los Angeles Megacity

Greenhouse gas emissions in urban regions comprise the bulk of anthropogenic contributions to the atmosphere. Governments compile bottom-up inventories from the different source sectors, but top-down measurements of the individual species, such as CO2, are totals. Top-down measurements are required to verify bottom-up inventories based on economic data. We must use other methods, such as analysis of ratios between species and ratios between isotopologues, in addition to model calculations, in order to determine the contributions of the various sectors that are actually observed in the atmosphere. Because of the very large signal in megacities, due to large anthropogenic emissions, these are the ideal regions to study to disentangle the signals from such sources as fossil fuel combustion, including identifying gasoline and natural gas signatures, and the biosphere. In this study, we extend our isotopic study of mid-afternoon flask samples in Pasadena, CA to continuous measurements of CO2 mixing ratios and δ13C in order to understand how the relative contributions of different sources vary with time of day and with time of year. Summers are characterized by larger contributions from natural gas combustion during the afternoons and evenings than during the mornings.