The ULM-DOAS instrument. Inside the box are a compact UV-Vis spectrometer and a PC-104. Light is collected directly by the optical fiber and a GPS is used to geolocalize the measurements. The whole system is powered with 12 V. 

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... which provides a high sensitivity to the boundary layer NO 2 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 NO 2 products when the latter are available. Above Rajasthan and the Po Valley, two areas where the NO 2 field is homogeneous, data sets agree very well. Our measurements in these areas are 0 . 1 ± 0 . 1 to 3 ± 1 × 10 15 molec cm − 2 and 2 . 6 ± 0 . 8 × 10 16 molec cm − 2 , respectively. Flying downwind of Riyadh, our NO 2 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. Nitrogen dioxide (NO 2 ) is a key species both in atmospheric chemistry, through its role in the ozone cycle, and as an in- dicator of air quality. In the troposphere, its main sources are anthropogenic and related to fossil fuel combustion in car engines, thermal power stations and industries (Jacob, 1999). NO 2 contributes to the photochemical smog seen above many cities and its effects on health have motivated the definition of acceptable exposure thresholds. The World Health Organization (WHO, 2003) recommends a maximum 1-h exposure concentration of 200 μg m − 3 and an annual av- erage of 40 μg m − 3 . In this paper we present airborne Differential Optical Absorption Spectroscopy (DOAS) NO 2 measurements during an ultralight aircraft expedition from Thailand to Belgium during November 2009. The tropospheric NO 2 loading can be remotely retrieved using its absorption bands in the ultraviolet-visible and the DOAS technique (Platt and Stutz, 2008). This is achieved from space by nadir-looking satellite-borne sensors like OMI (Ozone Monitoring Instrument) (Levelt et al., 2006) or GOME-2 (Global Ozone Monitoring Experiment 2) (Munro et al., 2006). These measurements are particularly valuable since they offer a global picture of the NO 2 field. However, their spatial resolution is limited by the pixel size (13 × 24 km 2 for OMI, 80 × 40 km 2 for GOME-2), which does not resolve fine-scale patterns. Satellite data also suf- fer from instrument drifts and require validation involving mostly ground-based DOAS instruments (e.g. Kramer et al., 2008; Herman et al., 2009; Pinardi et al., 2010; Shaiganfar et al., 2011), airborne in-situ measurements (Bucsela et al., 2008; Boersma et al., 2008) or, less frequently, airborne DOAS instruments (Heue et al., 2005). An aircraft is able to cover the spatial extent of a pixel in a short time, but such an experiment is expensive and requires dedicated aircraft. Ultra-light aircraft are well suited for NO 2 studies. Their ceiling is relatively low, but, at least in polluted zones, most of the NO 2 is close to the surface. Aircraft modifications are much easier than on normal planes since they do not require certifications from the aeronautics authorities. Ultra-light aircraft have so far been used to study the actinic flux (Junkermann, 2001), the aerosol profiles (Chazette et al., 2007; Raut and Chazette, 2008) and formaldehyde distribution (Junkermann, 2009). The Earth Challenge expedition (De Maegd, 2010), which took place in 2009, involved four ultralight aircraft flying from Australia to Belgium. It provided an opportunity to develop and test a new compact DOAS instrument, namely the Ultralight Motorized-DOAS (ULM-DOAS). In comparison with previous airborne DOAS experiments (e.g. Bruns et al., 2006; Prados-Roman et al., 2011; Merlaud et al., 2011), the optical set-up is very simple. We just record the scattered light intensity at the horizon within a large field of view without any telescope or scanner. However, this measurement geometry optimizes the sensitivity to boundary layer NO 2 while it limits the errors due to aircraft attitude (pitch, roll, and yaw) instabilities. In the next section we describe the technical aspects of the ULM-DOAS instrument and the Earth Challenge expedition. The methods used for the data analysis, i.e. the DOAS settings, radiative transfer modeling, and inversion schemes, are presented in Sect. 3. In Sect. 4, we study the sensitivity of our measurements to geometrical and geophysical parameters and propagate uncertainties on these parameters in an error budget. The methods and error analysis are applied in Sect. 5 to derive tropospheric NO 2 above interesting areas for which few local measurements have been reported. We compare our measurements with OMI and GOME-2 data for the days where it is possible, and we investigate the presence of a soil signature recently reported in GOME-2 spectra. Figure 2 shows the ULM-DOAS, which was developed at the Belgian Institute for Space Aeronomy (BIRA-IASB) and first used during the Earth Challenge expedition. The light is collected by a 400 μm-diameter optical fiber, which, during operation, is attached under a wing of the aircraft, pointing forward to the horizon. There is no focusing element at the entrance of the fiber, hence the field of view is directly related to the numerical aperture of the fiber, which corresponds to 25 ◦ (Fig. 1). This choice is motivated in Sect. 4. A black plas- tic baffle (not shown) is added to limit the stray light. The other extremity of the fiber is screwed to the spectrometer, which lies inside a 27 × 27 cm 2 aluminum box together with a PC-104 that controls it. The spectrometer is an AvaSpec- 2048 with a 50 μm entrance slit and a 600 l mm − 1 grating, blazed at 300 nm. It covers the spectral range from 200– 750 nm at a resolution of approximately 1.2 nm Full Width at Half Maximum (FWHM) at 460 nm. Figure 4 shows the slit function in the NO 2 fitting window. The instrument sensitivity to polarization is under 4 per mil. The detector is not temperature-stabilized and the typical shift variation during a flight is 0.2 nm. Both the spectral resolution and the shift are characterized in the DOAS analysis (Sect. 3.1). A GPS antenna is connected to the PC-104 for georeferencing the measurements. The whole set-up is powered by the aircraft’s 12 V. While measuring, the instrument is recording spectra continuously at an integration time of 5 ms. The noise is reduced by averaging a series of 10 accumulations on the CCD (charge-coupled device) to produce a spectrum. These spectra are transferred to the computer and filtered by the ac- quisition program, removing those with too low or saturated signal. A second averaging is then applied to a spectra series of 5 s to produce a final measurement point, the process being repeated continuously. The dark current is estimated from the mean of the signal in the range of 280–300 nm, where the atmosphere is opaque due to ozone absorption. Preliminary DOAS analyses (see Sect. 3.1) with preconvo- luted cross-sections are done in real time and saved on a USB key attached to the aluminum box. This allows for easy monitoring of the behavior of the instrument, especially when no scientists are present, as was the case during the Earth Challenge expedition. Earth Challenge was a 27 000 km expedition between Australia and Belgium onboard four ultralight aircraft, which took place in April and November 2009 (De Maegd, 2010). The team left from Sydney (Australia) on 5 April 2009 and reached Bangkok (Thailand) on 30 April 2009 with 37 flights. The second stage started from Bangkok, after the monsoon season, on 30 October 2009 and ended after 21 flights in Charleroi (Belgium) on 5 December 2009. The objective of the 7 pilots team, beside reaching Belgium, was to draw the public’s attention to major environmental problems, such as sea level rising, pollution and climate change, in cooperation with the World Wildlife Fund (WWF). The project was supported by BIRA-IASB, which used this opportunity to develop and test the new instrument described in the previous section. The aircraft used were four Coyote RANS-S6. Their cruise speed is 180 km h − 1 and they can reach an altitude of 4.8 km with a payload (including pilots) of 300 kg. The range is around 700 km, but additional 50 l oil tanks were added for the longest flights of the expedition, e.g. the 874 km crossing of the Gulf of Oman between Gwadar (Pakistan) and Dubaı (United Arab Emirates). Figure 3 shows the second part of the expedition superimposed on a monthly-averaged map of GOME-2 NO 2 tropospheric measurements during November 2009. The circled numbers correspond to the areas further studied in this work. Except for India and the Po Valley, they correspond to places where few local NO x measurements have been reported in the literature. For some of them, e.g. megacities like Karachi and Riyadh, high pollution levels are expected to be found. During the first part, instrument problems prevented the mak- ing of measurements after Brisbane (Australia). This section describes the three steps of the data analysis: the DOAS fit, which retrieves integrated concentration along the photon path, the air mass factor calculation used to derive a geophysical interpretation from the DOAS fit, and finally the propagation of the different uncertainties in the error budget. Molecular absorption of NO 2 is commonly retrieved in UV- visible atmospheric spectra using the DOAS technique (Platt and Stutz, 2008). This method relies on the fact that, for cer- tain molecules including NO 2 , the absorption cross-sections vary much more rapidly with wavelength than the scattering effects (Rayleigh and Mie). In practice, a measured spectrum ( I (λ) ) is divided by a reference ( I ref (λ) ) to remove solar Fraunhofer structures and reduce instrument effects. The slow ...

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... expedition 2.1 Instrument description Figure 2 shows the ULM-DOAS, which was developed at the Belgian Institute for Space Aeronomy (BIRA-IASB) and first used during the Earth Challenge expedition. The light is collected by a 400 µm-diameter optical fiber, which, during operation, is attached under a wing of the aircraft, pointing forward to the horizon. ...