‘Atmospheric chemistry instruments’ is used here to describe a range of different types of instruments that use various techniques and different parts of the electromagnetic spectrum to undertake measurements of the atmosphere’s composition. Each atmospheric gas is characterised by its ‘absorption’ and ‘emission’ spectra, which describe how the molecules respond to different frequencies of radiation. Remote sensing instruments exploit these ‘signatures’ to provide information on atmospheric composition, using measurements over a range of wavelengths, between UV and microwave.
Atmospheric absorption tends to be dominated by water vapour, carbon dioxide, and ozone, with smaller contributions from methane and other trace gases. Relatively broadband instruments can be used for measurements of the dominant gases, but high spectral resolution sensors are needed to make measurements of other species, since they produce weaker signals, and these must be discriminated from the signals from more abundant gases.
The instruments are typically operated in either:
— nadir-viewing mode: looking directly down to measure the radiation emitted or scattered in a small solid angle centred around a measurement point on the Earth, with resulting high spatial resolution in the horizontal direction, but limited vertical resolution; or
— limb-viewing mode: scanning of positions beyond the horizon to observe paths through the atmosphere at a range of altitudes – providing high vertical resolution (a few km) but limited horizontal resolution (tens of km at best) and particularly useful for studying the middle atmosphere.
Emission or absorption spectra can be studied in limb-viewing mode. One approach – known as occultation – uses known astronomical bodies (such as the Sun and stars) as well characterised target sources, and measures the effect of the Earth’s atmosphere on the radiation reaching the satellite to determine atmospheric composition.