Remote Measurement of Natural Gas Methane Under Cloud with a BOMEM Fourier Transform Spectrometer (FTS) Instrument
W.F.J. Evans1, H. Buijs2 and R. LaChance2
1NorthWest Research Associates, Redmond, WA 98052; 425-898-4229, E-mail: firstname.lastname@example.org
2ABB Bomem, Quebec G1K 9H4, Canada
Remote measurement of tropospheric gases can be achieved by utilizing thermal emission under-cloud. The cloud deck acts as a cold optically thick blackbody at the cloud base temperature. Emission features from the atmospheric gases below the cloud are superimposed on the cloud blackbody emission. In the thermal spectral region are bands from CH4 , O3, CO2, H2O, N2O and CFCs. Methane is a major component of natural gas and a strong greenhouse gas. Spectral measurements under cloudy conditions determine the radiative flux from lower tropospheric CH4 since downward emission from the upper troposphere and stratosphere above is blocked by the cloud layer. The measurement of gases requires a calibrated high resolution measurement of spectral radiance in the infrared region from 700 to 2500 cm-1: these are conducted with the Atmospheric Radiation Measurement Atmospheric Emitted Radiance Interferometer (AERI) instruments. The AERI instrument is an FTS with internal blackbody calibration sources automated for continuous operation. In zenith spectra of the radiance from clouds above sites, features from most Greenhouse Gases are evident. The combination of the total column and the cloud base height yields a mean methane mixing ratio below the cloud base. Measurements of CH4, CO2, CO, O3, CFC11 and CFC12 are demonstrated. Methane mixing ratios are elevated near leaking gas wells or fields. Ethane may be measurable. Applications to the monitoring of gas leaks are discussed. These under-cloud measurements in the lower troposphere could augment current in situ measurements. The meteorological situation could be used to quantify the amount of methane leaking into the lower atmosphere in a gas field. AERI algorithms provide temperature and humidity profiles to compensate for boundary layer effects on vertical profiles of gases. The methodology to quantify gas leaks near gas fields is under development as is an affordable FTS system suitable for monitoring in gas fields.