"The aim of VAP-OMI is to validate and improve atmospheric parameters retrieved from multi-spectral solar irradiance observations from space, using the Dutch/Finnish Ozone Monitoring Instrument (OMI) on board of the AURA satellite. It focuses on aerosol properties (Aerosol optical depth (AOD) and single scattering albedo (SSA)) and spectral surface UV Irradiance (UVI), using synchronized and collocated high quality ground-based (GB) measurements, combined with radiative transfer model sensitivity studies. The proposed work will be carried out at the Finnish Meteorological Institute (FMI) which hosts the OMI co-PI and world-leading experts on aerosol properties and UVI, retrieved both from space and from the ground. Firstly, possible improvement on the OMI multi-wavelength aerosol retrieval algorithm (OMAERO) will be investigated. The main issues to be considered are the effects of clouds (cloud screening), surface reflectance and the standard aerosol models that OMAERO currently uses. The retrieved AOD from OMI will be compared with GB AOD measurements taken from existing global aerosol networks (AERONET) as well as measurements of related aerosol campaigns such as ARCTAS and SCOUT. Of particular interest are measurements with three AERONET instruments recently installed in high latitudes, which allow studies of effects of snow and ice. Methodologies of SSA retrieval from GB solar irradiance measurements will be applied to the widely deployed Brewer spectroradiometer network in order to enlarge the GB SSA database, especially in the UV part of the solar spectrum. The OMAERO algorithm outputs will be compared to the Brewer SSA retrievals and also with other satellite derived and selected AERONET SSA measurements. Finally, OMI retrieved UVI will be compared using results from selected UV monitoring stations from the European UV Data Base (that is hosted at FMI).The effect of the atmospheric aerosol properties to the results of this comparison will be investigated."