Abstract
Airborne in situ and remote sensing measurements of methane were performed over the marine seeps in the Santa Barbara Channel close to the Coal Oil Point in California on two days in June and August 2014 with the aim to re-assess their methane emissions. During this period, methane column averaged dry air mole fractions derived from airborne remote sensing measurements in the short-wave infrared and airborne in situ measurements of methane indicate that emissions are 2–6 kt CH 4 y − 1 , significantly lower than expected from previous publications. This is also confirmed by the on ground in situ measurement time series recorded at the onshore West Campus Monitoring Station in Santa Barbara. Using a time series of methane data, a decline in methane concentrations between 2008 and 2015 of more than a factor of two was derived for air masses originating from the seep field direction.
Highlights
Methane, CH4, is the second most important greenhouse gas whose atmospheric concentrations are modified by anthropogenic activity, as well as by natural processes
Airborne measurements on 4 June 2014 over the Coal Oil Point seep field were performed between 11:35 and 14:30 local time
The exact boundary layer depth is difficult to assess from in situ observations as the aerosol profile indicates a strong change at 250 m, whereas potential temperature indicates a depth of about 150 m
Summary
CH4, is the second most important greenhouse gas whose atmospheric concentrations are modified by anthropogenic activity, as well as by natural processes. The seep field under investigation in this study is located in the northern Santa Barbara Channel, California, offshore of Coal Oil Point (COP), and is known to emit substantial quantities of CH4 along with other hydrocarbon gases, oil and tar (see, for example, Allen et al [3], Hornafius et al [4], Leifer et al [5]). These seeps have been intensively studied in the past. Airborne hyperspectral imaging spectroscopy has been used to map CH4 plumes from a range of the larger seeps [12,13,14]
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