This paper presents the design and the control of a spatialized additive synthesizer aiming at simulating environmental sounds. First the synthesis engine, based on a combination of an additive signal model and spatialization processes, is presented. Then, the control of the synthesizer, based on a hierarchical organization of sounds, is discussed. Complex environmental sounds (such as a water flow or a fire) may then be designed thanks to an adequate combination of a limited number of basic sounds consisting in elementary signals (impacts, chirps, noises). The mapping between parameters describing these basic sounds and high-level descriptors describing an environmental auditory scene is finally presented in the case of a rainy sound ambiance.