Trends in Monthly Methane Emissions in Los Angeles Inferred by Mountaintop Remote Sensing Measurements

Abstract:

Methane (CH₄) is the second most important greenhouse gas and a target of new emissions regulations in the United States. Despite its importance, there are large uncertainties in its emissions. In the megacity of Los Angeles, anthropogenic CH₄ is emitted from a variety of sources including wastewater treatment plants, landfills, oil wells, dairy farms and the natural gas infrastructure. To quantify the methane budget in the basin, it is necessary to understand the spatial and temporal variability of the emission patterns. To address this issue, since 2011, continuous daytime mountaintop remote sensing measurements of CH₄ and CO₂ have been acquired by a JPL-built Fourier Transform Spectrometer (FTS) at the California Laboratory of Atmospheric Remote Sensing (CLARS) on Mount Wilson. At an altitude of 1.67 km a.s.l., the CLARS-FTS samples the dry-air slant column abundances of CH₄ and CO₂ (XCH₄ and XCO₂) by targeting 29 reflection points in the L.A. basin, including a reference point at the observatory.

Using this unique dataset and tracer-to-tracer correlation analysis, we derive the monthly trends of top-down methane emissions over a period of nearly four years from 2011 to 2015. Consistent strong emission peaks in winter and weaker peaks in summer were observed during this period. We compare our top-down emissions and bottom-up emissions database to understand the drivers of the observed CH₄ monthly emission patterns in the basin.

Copyright 2015. California Institute of Technology. Government sponsorship acknowledged.