NO2 Profile Retrieval using Airborne Multiaxis DifferentialOptical Absorption Spectrometer (AMAXDOAS) data

A recent development in ground-based remote sensing of atmospheric constituents by UV/visible absorption measurements of scattered light is the simultaneous use of several horizon viewing directions in addition to the traditional zenith-sky pointing. The different light paths through the atmosphere enable the vertical distribution of some atmospheric absorbers such as NO2, BrO or O3 to be retrieved. This approach has recently been implemented on an airborne platform. The novel instrument called Airborne MultiAXis Differential Optical Absorption Spectrometer, AMAXDOAS, has been flown for the first time. This investigation includes sensitivity studies regarding the choice of the optimum number and the optimum directions of the lines of sight (LOS) used. An important question to be answered is to what extend profile information can be retrieved from such measurements regarding the trace gas NO2. These sensitivity studies using synthetic data are performed for a variety of representative measurement conditions including two wavelength regions, one in the UV and one in the visible, two different surface spectral reflectance, various LOS, two different flight altitudes, different aerosol loadings in the lower troposphere, various solar zenith angles (SZA), and varying step sizes in the retrieval grid since the retrieved profile is a discrete rather than a continuous function.The results of the sensitivity studies demonstrate that the AMAXDOAS measurements contain useful profile information, mainly at flight altitude and below the aircraft. Depending on wavelength and LOS used, the vertical resolution of the retrieved profiles is as good as 2 km near flight altitude. Above 14 km the profile information content of AMAXDOAS measurements is sparse. Thus Airborne multiaxis measurements are a promising tool for atmospheric studies in the troposphere and the UTLS region.