print page

Earth Challenge: NO2 measurements from ultralight aircrafts

Earth Challenge is a 27000 km expedition between Australia and Belgium, onboard 4 ultralight aircrafts, which took place in April and November 2009. The objective of the 7 pilots team, in cooperation with the World Wildlife Fund (WWF), was to draw the public's attention to the environmental problems such as sea rising, pollution and climate change. The project has been supported by BIRA-IASB. The UV-VIS DOAS group together with the mechanical workshop team, built a new remote-sensing instrument, namely the ULM-DOAS, which was installed onboard one of the aircraft. This instrument is primarily aimed to NO2 monitoring, but other chemical species are detectable, such as formaldehyde and sulfur dioxide.

Flight tracks of the Earth Challenge expedition on a NO2 GOME-2 map. The numbers correspond to the places where the measurements were particularly interesting.
(Courtesy : Jeroen van Gent)

The Ultralight Motorized-DOAS (ULM-DOAS) instrument is based on a compact grating spectrometer (200-750 nm) controlled by a PC-104 embedded single board computer, both of them and the power converter fit in the 27 cm side box seen beside. An optical fiber is installed under the wing of the plane and transmits the diffuse sky-light to the spectrometer. This instrument has no focusing optics and thus the field of view is simply the one of the fiber (25°). Measurements and spectral analysis are fully automated and a gps system is used for the georeferencing of the data.

The ULM-DOAS instrument

Example of result: Riyadh surrounding, with ULM-DOAS and OMI NO2 data superimposed. The arrow indicates the wind direction.

The slant columns measured by the ULM-DOAS are converted to vertical columns assuming a well-mixed boundary layer. In addition to Riyadh (shown above) ULM-DOAS and satellite (OMI and GOME-2) tropospheric NO2 columns could also be compared over Rajasthan (India) and the Po Valley (Italy). The highest columns were detected over Karachi, Pakistan. In Bengladesh, a large NO2 signal was observed downwind of the ship cemetery of Chittagong.

Ship cemetery of Chittagong (Bengladesh).

Compared to satellite data, the ULM-DOAS instrument is able to detect higher spatial frequency patterns in the NO2 field (around 5 km compared with an OMI pixel width of 13 km). It is well suited for tropospheric NO2 column validation, offering a low-cost alternative to larger aircraft measurements. Such measurements are also well suited for flux measurements from extended sources, providing accurate wind data.


Merlaud, A., Van Roozendael, M., van Gent, J., Fayt, C., Maes, J., Toledo-Fuentes, X., Ronveaux, O., and De Maziere, M.: DOAS measurements of NO2 from an ultralight aircraft during the Earth Challenge expedition, Atmospheric Measurement Techniques, 5, 2057-2068, 2012,doi:10.5194/amt-5-2057-2012

Alexis Merlaud, Caroline Fayt, Francois Vermeer: Earth Challenge: une aventure scientifique au service de l environnement, Science Connection, janvier 2010 (in Dutch: Earth Challenge, een menselijk avontuur voor het milieu)


Visit also the Earth challenge web site


For more information, please contact : Alexis Merlaud, Caroline Fayt, or Michel Van Roozendael

Last update on 29 Mar 2013