Ultraviolet radiation is the main factor driving the tropospheric photochemistry and is strongly modulated by clouds and aerosols. A quantitative understanding of the radiation field and its effect on photochemistry is thus only possible with a detailed knowledge of the interaction between clouds and radiation. This knowledge is particularly needed for regional and global chemistry models which at present rely on very simple parameterizations for clouds. INSPECTRO combines space-based, aircraft and ground-based measurements as well as simulations with one- and three-dimensional radiative transfer models to characterize the radiation field in the presence of broken clouds. The goal of the proposed project is to improve our understanding of the three-dimensional ultraviolet radiation field and its implications for photochemistry in the lower troposphere under conditions representative for Europe.