Transcontinental methane measurements: Part 1. A mobile surface platform for source investigations

The potent greenhouse gas, methane, CH4, originates from a wide range of anthropogenic and natural sources. A ground-based, satellite-scale, transcontinental (Florida to California) survey was conducted to understand better emissions from key sources including wetlands, forest fire, and geologic sources, as well as to acquire desert background values and lower atmosphere vertical profiling in the San Bernardino Mountains. A total of 6600 measurements along 7020 km of roadways were made by flame ion detection, gas chromatography (GC) onboard a recreational vehicle in 2010, and during a second survey with a cavity ring-down spectrometer system in Southern California in 2012. Significant vibration reduction efforts allowed near continuous mobile GC measurements.Nocturnal CH4 measurements tended to be higher compared to daytime values, sometime significantly, for similar sources and were concluded due to day/night meteorological differences. The lowest GC observations were 1.80 ppm, observed in the California desert, ∼60 ppb less than minimum desert CH4 observed in 2012. Thanks to smoke visualization of a brush fire plume, the flux from the fire was estimated at 0.15 kiloton day−1. Geologic CH4 emissions from the La Brea tar pit and surrounding areas were surprisingly strong, with peak concentrations of nearly 50 ppm and highly elevated CH4 concentrations extending over at least ∼100 km2, and accounting potentially for a significant fraction of the LA basin CH4 emissions. Geologic CO2 emissions also were observed.

► First-ever cross-country continent scale methane measurements.
► First quantification of methane emissions from the La Brea tar pit/seepage area geologic source.
► Desert background methane measurements.
► Lower atmospheric vertical profiling using a mountain descending road.