  The twin Sentinel-3 satellites have multi-band radiometers which observe in
methane-sensitive shortwave infrared bands with daily global coverage and 500 m
ground pixel resolution. We investigate the methane observation capability of
Sentinel-3 and how its coverage-resolution combination fits between Sentinel-5p
and Sentinel-2 within a tiered observation approach for methane leak
monitoring. Sentinel-5p measures methane with high precision and daily global
coverage, allowing worldwide leak detection but with a coarse spatial
resolution of 7 km x 5.5 km. The Sentinel-2 twin satellites have multi-band
instruments that can identify source locations of major leaks (> 1 t/h) with
their methane observations of 20 m resolution under favorable observational
conditions, but these satellites lack daily global coverage We show that
methane enhancements can be retrieved from the shortwave infrared band
measurements of Sentinel-3. We report the lowest emission detections by
Sentinel-3 in the 8-20 t/h range, depending on location and wind conditions. We
demonstrate Sentinel-3's capability of identification and monitoring of methane
leaks using two case studies. Near Moscow, Sentinel-3 shows that two major
short-term leaks, separated by 30 km, occurred simultaneously at a gas pipeline
and appear as a single methane plume in Sentinel-5p data. For another
Sentinel-5p leak detection near the Hassi Messaoud oil/gas field in Algeria,
Sentinel-3 identifies the leaking facility emitting continuously for 6 days,
and Sentinel-2 pinpoints the source of the leak at an oil/gas well. Sentinel-2
and Sentinel-3 also show the 6-day leak was followed by a four-month period of
burning of the leaking gas, suggesting a gas well blowout to be the cause of
the leak.
