[1] We present an analysis of methane (CH 4 ; 68% confidence interval (CI), assuming uncorrelated errors between regions). Summing across all regions of California, optimized CH 4 emissions are only marginally consistent between CALGEM-and EDGAR42-based inversions (48.35 ± 6.47 versus 64.97 ± 11.85 Tg CO 2 eq), because emissions from coastal urban regions (where landfill and natural gas emissions are much higher in EDGAR than CALGEM) are not strongly constrained by the measurements. Combining our results with those from a recent study of the South Coast Air Basin narrows the range of estimates to 43-57 Tg CO 2 eq yr À1 (1.3-1.8 times higher than the current state inventory). These results suggest that the combination of rural and urban measurements will be necessary to verify future changes in California's total CH 4 emissions.

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Methane (CH 4) and carbon monoxide (CO) mixing ratios were measured at an air quality monitoring station near the Mt. Wilson (MW) Observatory in southern California starting in the spring of 2007. Diurnal variation and mixing ratio correlation (R 2 ¼ 0.81) were observed. The correlation results observed agree with previous aircraft measurements collected over the greater Los Angeles (LA) metropolitan area. The consistent agreement between CH 4 and CO indicates these gases are well-mixed before reaching the sampling site and the emission source contributions of both compounds are reasonably constant. Since CH 4 and CO are considered non-reactive on the time scale of dispersion within the LA urban area and their emission sources are likely to be similarly distributed (e.g., associated with human activities) they are subject to similar scales of atmospheric transport and dilution. This behavior allows the relationship of CH 4 and CO to be applied for estimation of CH 4 emissions using well-documented CO emissions. Applying this relationship a ''top-down'' CH 4 inventory was calculated for LA County based on the measurements observed at MW and compared with the California Air Resources Board (CARB) ''bottom-up'' CH 4 emissions inventory based on the Intergovernmental Panel on Climate Change recommended methodologies. The ''top-down'' CH 4 emissions inventory is approximately onethird greater than CARB's ''bottom-up'' inventory for LA County. Considering the uncertainties in both methodologies, the different CH 4 emissions inventory approaches are in good agreement, although some under and/or uninventoried CH 4 sources may exist.

Abstract. The Los Angeles megacity, which is home to more than 40 % of the population in California, is the second largest megacity in the United States and an intense source of anthropogenic greenhouse gases (GHGs). Quantifying GHG emissions from the megacity and monitoring their spatiotemporal trends are essential to be able to understand the effectiveness of emission control policies. Here we measure carbon dioxide (CO 2 ) and methane (CH 4 ) across the Los Angeles megacity using a novel approach -ground-based remote sensing from a mountaintop site. A Fourier transform spectrometer (FTS) with agile pointing optics, located on Mount Wilson at 1.67 km above sea level, measures reflected nearinfrared sunlight from 29 different surface targets on Mount Wilson and in the Los Angeles megacity to retrieve the slant column abundances of CO 2 , CH 4 and other trace gases above and below Mount Wilson. This technique provides persistent space-and time-resolved observations of path-averaged dry-air GHG concentrations, XGHG, in the Los Angeles megacity and simulates observations from a geostationary satellite. In this study, we combined high-sensitivity measurements from the FTS and the panorama from Mount Wilson to characterize anthropogenic CH 4 emissions in the megacity using tracer-tracer correlations. During the period between September 2011 and October 2013, the observed XCH 4 : XCO 2 excess ratio, assigned to anthropogenic activities, varied from 5.4 to 7.3 ppb CH 4 (ppm CO 2 ) −1 , with an average of 6.4 ± 0.5 ppb CH 4 (ppm CO 2 ) −1 compared to the value of 4.6 ± 0.9 ppb CH 4 (ppm CO 2 ) −1 expected from the California Air Resources Board (CARB) bottom-up emission inventory. Persistent elevated XCH 4 : XCO 2 excess ratios were observed in Pasadena and in the eastern Los Angeles megacity. Using the FTS observations on Mount Wilson and the bottom-up CO 2 emission inventory, we derived a topdown CH 4 emission of 0.39 ± 0.06 Tg CH 4 year −1 in the Los Angeles megacity. This is 18-61 % larger than the state government's bottom-up CH 4 emission inventory and consistent with previous studies.

Abstract. Large urban emissions of greenhouse gases result in large atmospheric enhancements relative to background that are easily measured. Using CO2 mole fractions and Δ14C and δ13C values of CO2 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 CO2 contributions from fossil fuel combustion (Cff) 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 Cff 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 CO2 from our observations are used to infer seasonality of natural gas and petroleum combustion. The trend of CO2 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 CO2 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 Cff from parts of the basin sampled by winds at different times of year, we find that variations in observed fossil fuel CO2 reflect seasonal variations in wind direction. The seasonality of the local CO2 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 CO2 signals are consistent with sampling the bottom-up Hestia-LA fossil CO2 emissions product for sub-city source regions in the LA megacity domain when wind directions are considered.