Atmospheric Chemistry instruments are typically operated in either nadir-viewing mode looking vertically directly down to measure the radiation emitted or scattered, or in a limb-viewing mode that scans positions beyond the horizon to observe paths through the atmosphere at different altitudes.  Nadir-viewing instruments provide high spatial resolution in the horizontal direction but limited vertical resolution, whereas limb-viewing instruments provide a high vertical resolution (a few km) but limited horizontal resolution (tens of km at best).

Limb sounding observations are made from the microwave and infrared – where thermal emission is observed – to the visible and ultraviolet, where observations are typically of sunlight scattered in the limb or of airglow.

Limb geometries are the natural choice for stratospheric measurements because the signal is not masked by the denser tropospheric signal, the long raypath through the atmosphere provides large sensitivity to species with low atmospheric concentrations, and the variation of the observation angle allows vertical scanning of the atmosphere.

For many aspects of tropospheric sounding, nadir sounders are advantageous, due to their small horizontal footprint.

ENIVISAT’s SCIAMACHY instrument was able to view in both nadir and limb views - It observed the same atmospheric volume first in limb and then after about 7 minutes in nadir geometry. By using this Limb/Nadir matching three-dimensional information about the atmosphere can be obtained.

This video is an extract from the ESA MOOC on 'Monitoring Climate from Space'.

Featured Educator

• Dr Angela Benedetti, Senior Scientist, ECMWF