DOAS measurements of NO₂ from an ultralight aircraft during the Earth Challenge expedition

Abstract. We report on airborne Differential Optical Absorption Spectroscopy (DOAS) measurements of NO₂ tropospheric columns above South Asia, the Arabic peninsula, North Africa, and Italy in November and December 2009. The DOAS instrument was installed on an ultralight aircraft involved in the Earth Challenge project, an expedition of seven pilots flying on four ultralight aircraft between Australia and Belgium. The instrument recorded spectra in limb geometry with a large field of view, a set-up which provides a high sensitivity to the boundary layer NO₂ while minimizing the uncertainties related to the attitude variations. We compare our measurements with OMI (Ozone Monitoring Instrument) and GOME-2 (Global Ozone Monitoring Experiment 2) tropospheric NO₂ products when the latter are available. Above Rajasthan and the Po Valley, two areas where the NO₂ field is homogeneous, data sets agree very well. Our measurements in these areas are 0.1 ± 0.1 to 3 ± 1 × 10¹⁵ molec cm⁻² and 2.6 ± 0.8 × 10¹⁶ molec cm⁻², respectively. Flying downwind of Riyadh, our NO² measurements show the structure of the megacity's exhaust plume with a higher spatial resolution than OMI. Moreover, our measurements are larger (up to 40%) than those seen by satellites. We also derived tropospheric columns when no satellite data were available if it was possible to get information on the visibility from satellite measurements of aerosol optical thickness. This experiment also provides a confirmation for the recent finding of a soil signature above desert.