Abstract
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 (CO2) and methane (CH4) 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 near-infrared sunlight from 29 different surface targets on Mount Wilson and in the Los Angeles megacity to retrieve the slant column abundances of CO2, CH4 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 CH4 emissions in the megacity using tracer–tracer correlations. During the period between September 2011 and October 2013, the observed XCH4 : XCO2 excess ratio, assigned to anthropogenic activities, varied from 5.4 to 7.3 ppb CH4 (ppm CO2)−1, with an average of 6.4 ± 0.5 ppb CH4 (ppm CO2)−1 compared to the value of 4.6 ± 0.9 ppb CH4 (ppm CO2)−1 expected from the California Air Resources Board (CARB) bottom-up emission inventory. Persistent elevated XCH4 : XCO2 excess ratios were observed in Pasadena and in the eastern Los Angeles megacity. Using the FTS observations on Mount Wilson and the bottom-up CO2 emission inventory, we derived a top-down CH4 emission of 0.39 ± 0.06 Tg CH4 year−1 in the Los Angeles megacity. This is 18–61% larger than the state government's bottom-up CH4 emission inventory and consistent with previous studies.
Highlights
The Los Angeles megacity – a sprawling urban expanse of ∼ 100 km × 100 km and 15 million people – covers only ∼ 4 % of California’s land area but is home to more than 43 % of its population and dominates the state’s anthropogenic greenhouse gas (GHG) emissions
Variations in planetary boundary layer (PBL) height do not affect the diurnal profile of XCO2 and XCH4 as they would in in situ measurements, in which diurnal variation is often characterized by GHG concentration peaks in the morning and evening when the PBL is shallow and a minimum in midday when the PBL has grown (Newman et al, 2013)
This study has shown that spatially resolved CH4 : CO2 emission ratio measurements can be made over a megacity domain using a remote sensing method that simulates the observations from an imaging spectrometer such as geostationary satellite observations (GEO)-Fourier transform spectrometer (FTS) from geostationary orbit
Summary
The Los Angeles megacity – a sprawling urban expanse of ∼ 100 km × 100 km and 15 million people – covers only ∼ 4 % of California’s land area but is home to more than 43 % of its population and dominates the state’s anthropogenic greenhouse gas (GHG) emissions. Kort et al (2013) concluded that the size and complexity of the Los Angeles megacity urban dome require a network of at least eight strategically located continuous surface in situ observing sites to quantify and track GHG emissions over time with ∼ 10 % uncertainty. This minimum network would have limited capabilities to identify and isolate emissions from specific sectors and/or localized sources. We demonstrate that simultaneous XCH4 and XCO2 measurements are essential to quantify megacity GHG emissions and provide critical information from which to attribute emissions from different economic sectors
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