Mobile MAX-DOAS observations of tropospheric trace gases

From Multi-Axis- (MAX-) DOAS observations, information on tropospheric trace gases close to the surface and up to the free troposphere can be obtained. Usually MAX- DOAS observations are performed at fixed locations, which allows to retrieve the diurnal variation of tropospheric species at that location. Alternatively, MAX- DOAS observations can also be made on mobile platforms like cars, ships or aircrafts. Then, in addition to the vertical (and temporal) distribution, also the horizontal variation of tropospheric trace gases can be measured. Such information is important for the quantitative comparison with model simulations, study of transport processes, and for the validation of tropospheric trace gas products from satellite observations. However, for MAX- DOAS observations from mobile platforms, the standard analysis techniques for MAX- DOAS observations can usually not be applied, because the probed airmasses can change rapidly between successive measurements. In this study we introduce a new technique which overcomes these problems and allows the exploitation of the full information content of mobile MAX- DOAS observations. Our method can also be applied to MAX- DOAS observations made at fixed locations in order to improve the accuracy especially in cases of strong winds. We apply the new technique to MAX- DOAS observations made during an automobile trip from Brussels to Heidelberg.