Read gases through light

Ibuki, the first Greenhouse gases Observing SATellite, launched on January 23, 2009. The GOSAT program is a joint effort of Japan’s Ministry of the Environment, JAXA and the National Institute for Environmental Studies. Its scientific task is to observe atmospheric gases, especially carbon dioxide and methane, rather than photograph invisible emissions as colored smoke.

The launch photograph documents H-IIA Flight 15 carrying Ibuki. Its instruments remain enclosed. Once operating in orbit, the original TANSO Fourier transform spectrometer measured an interference signal that could be transformed into a spectrum: light intensity distributed across wavelengths. Gas absorption patterns provide evidence for estimating gas abundance.

Reflected sunlight passes through the atmosphere before reaching the satellite; thermal emission supplies another kind of infrared observation. The original reflected-light bands require daylight, while the thermal band can operate by day and night. The associated cloud and aerosol imager helps characterize conditions that affect the spectrum.

A column-averaged dry-air mole fraction compares the amount of a gas with the amount of dry air along a vertical atmospheric column. In an original arithmetic example, 400 gas molecules per million dry-air molecules corresponds to 400 parts per million, or ppm. This is an illustrative ratio, not a reported GOSAT measurement. It is neither a ground-level reading at one height nor a rate of emission from a particular facility.

GOSAT product documentation distinguishes usable retrievals from scenes without available data. Clouds and other observation conditions can prevent a suitable estimate. A blank on a concentration map therefore does not mean that the atmosphere contained no carbon dioxide or methane.

For an original quality-screening exercise, start with 100 attempted observations and retain 30 after applying stated checks. The retained fraction is 30 percent of those attempts. It is not the percentage of Earth covered by gas, nor proof that the other 70 scenes contained zero gas. Report the selection rule alongside the result so readers can judge what was sampled.