Monthly trends of methane emissions in Los Angeles from 2011 to 2015 inferred by CLARS-FTS observations

This paper presents an analysis of methane emissions from the Los Angeles Basin at monthly timescales across a 4-year time period - from September 2011 to August 2015. Using observations acquired by a ground-based near-infrared remote sensing instrument on Mount Wilson, California, combined with atmospheric CH4-CO2 tracer-tracer correlations, we observed -18 to +22% monthly variability in CH4 : CO2 from the annual mean in the Los Angeles Basin. Top-down estimates of methane emissions for the basin also exhibit significant monthly variability (-19 to +31% from annual mean and a maximum month-to-month change of 47 %). During this period, methane emissions consistently peaked in the late summer/early fall and winter. The estimated annual methane emissions did not show a statistically significant trend over the 2011 to 2015 time period.