Measurements of horizontal trace gas distributions using airborne imaging differential optical absorption spectroscopy

Abstract

Nitrogen oxides (NOx = NO NO2) are a key species in the atmosphere, impacting on human health, tropospheric ozone formation, acid rain and climate. The major source of NOx are combustion processes. The lifetime of NOx in the atmosphere is short. Hence, it is found close to its sources and can serve as a proxy to identify anthropogenic pollution. The distinct spectral features of NO2 enable to retrieve its abundance by remote sensing, using differential optical absorption spectroscopy (DOAS). Spaceborne measurements of trace gases using the DOAS method have been performed for more than two decades and provided new insights on the atmospheric composition on a global and regional scale. Within this work, the DOAS technique is applied to airborne imaging DOAS measurements of the AirMAP instrument, developed at the University of Bremen. As the instrument is operated on an aircraft, the measurements have a much higher spatial resolution than the ones from satellite, due to the lower flight altitude. The aircraft platform provides a tool to gaplessly map the horizontal NO2 distribution at the scale of a large city, such as Berlin, in a time window of about two hours. The derived high resolution trace gas maps reveal spatial patterns in the NO2 distribution that cannot be resolved by the satellite measurements. This thesis presents trace gas maps from research flights covering different target areas, including shipping lanes, power plants and cities at a resolution better than 100 meter. Special efforts were made to increase the accuracy of the measurements by accounting for parameters that affect the atmospheric radiative transfer, such as scattering in the atmosphere and reflection at the surface. The air mass factors, which are needed to convert the measured differential slant column densities (dSCDs) to vertical column densities (VCDs), have a strong dependence on the surface reflectance, which has to be accounted for in the retrieval. This is especially important for measurements above urban areas, where surface properties vary strongly. As the instrument is not radiometrically calibrated, a method was developed to derive the surface reflectance from relative intensities measured by AirMAP. This method is based on radiative transfer calculations with SCIATRAN and a reference area for which the surface reflectance is known. Furthermore, the influence of aerosols on the retrieval was investigated for a variety of aerosol profiles that were measured in the context of the AROMAT campaigns in which AirMAP participated. Validation with independent measurements shows in general good agreement. The measurement datasets created, as well as the methods developed within this work, provide the basis to validate other high resolution datasets, such as regional chemistry models or satellite measurements at high resolution, such as the ones performed by the Sentinel-5p, launched in October 2017.