Optimization and application of satellite observations for air quality monitoring in Central Europe

Nitrogen oxides are key precursors of both ozone and secondary aerosols, and they are harmful to humans and ecosystems. The Ozone Monitoring Instrument (OMI) is a recent UV/visible spectrometer on NASA’s Aura satellite with a comparatively small pixel size and daily global coverage which makes it particularly suitable for air quality monitoring. Information on vertical tropospheric columns (VTCs) of tropospheric trace gases is derived from the spectroscopic data by means of a retrieval algorithm. The retrieval depends on a number of so-called a priori assumptions which introduce important uncertainties in the derived quantity. Current operational retrievals are based on global a priori data sets at coarse spatial and temporal resolution, which are much coarser than the resolution of individual OMI pixels. In order to obtain a more accurate retrieval of vertical tropospheric columns of nitrogen dioxide (NO2) than currently available, we are developing new data sets of critical retrieval parameters at high temporal and spatial resolution for Europe, such as high resolution surface pressure maps, surface reflectance, and a-priori vertical NO2 profiles from a regional model. As a first step, we analyzed the sensitivity of retrieved NO2 to the surface pressure, and the results demonstrate the importance of an accurate (i.e. high resolution) treatment of this parameter, in particular in the vicinity of complex topography such as the Alps. As a second step, a new illumination and viewing geometry dependent surface reflectance data set has been developed based on high temporal and spatial resolution multiangular reflectance observations from NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS). Comparisons of this new surface reflectance data set with the coarse albedo data set of the operational OMI product will be shown and a sensitivity analysis of the impact on the retrieved NO2 will be presented. As a third step, a-priori vertical NO2 profiles used in operational product will be compared with those from a regional model and with in-situ measurements, the impact on the retrieved NO2 accuracy will be discussed. In this study, we will present our improved retrieval methodology and demonstrate its application to nitrogen dioxide (NO2) observations over Central Europe. We will investigate spatial and temporal patterns such as the weekly and seasonal cycles and the distribution of emission sources, and will study meteorological effects such as transport across country borders. Our data set will be compared with atmospheric model simulations allowing constraining emission inventories of nitrogen oxides (NOx).