Toward consistency between trends in bottom-up CO2 emissions and top-down atmospheric measurements in the Los Angeles megacity

Large urban emissions of greenhouse gases result in large atmospheric enhancements relative to background that are easily measured. Using CO_2 mole fractions and Δ^(14)C and δ^(13)C values of CO_2 in the Los Angeles megacity observed in inland Pasadena (2006–2013) and coastal Palos Verdes peninsula (autumn 2009–2013), we have determined time series for CO_2 contributions from fossil fuel combustion (C_(ff)) for both sites and broken those down into contributions from petroleum and/or gasoline and natural gas burning for Pasadena. We find a 10 % reduction in Pasadena C_(ff) during the Great Recession of 2008–2010, which is consistent with the bottom-up inventory determined by the California Air Resources Board. The isotopic variations and total atmospheric CO_2 from our observations are used to infer seasonality of natural gas and petroleum combustion. The trend of CO_2 contributions to the atmosphere from natural gas combustion is out of phase with the seasonal cycle of total natural gas combustion seasonal patterns in bottom-up inventories but is consistent with the seasonality of natural gas usage by the area's electricity generating power plants. For petroleum, the inferred seasonality of CO_2 contributions from burning petroleum is delayed by several months relative to usage indicated by statewide gasoline taxes. Using the high-resolution Hestia-LA data product to compare C_(ff) from parts of the basin sampled by winds at different times of year, we find that variations in observed fossil fuel CO_2 reflect seasonal variations in wind direction. The seasonality of the local CO_2 excess from fossil fuel combustion along the coast, on Palos Verdes peninsula, is higher in autumn and winter than spring and summer, almost completely out of phase with that from Pasadena, also because of the annual variations of winds in the region. Variations in fossil fuel CO_2 signals are consistent with sampling the bottom-up Hestia-LA fossil CO_2 emissions product for sub-city source regions in the LA megacity domain when wind directions are considered.