Sonification refers to sonic expression of data or information. It is often thought of as an auditory complement, providing additional information about data which can reveal patterns and facilitate interpretation and understanding of the data. Hence, the listening space created by a sonifi cation is always a hybrid where auditory augmentation complements other information modalities and, in some cases, also spatial qualities. In this work, we focus on sonifi cation in public and semi-public spaces, and specifi cally on musical sonifi cation – the use of musical sounds to create a sonic environment, augmenting or complementing a physical shared space. We draw upon established approaches in interaction design to focus our work on the user experience of musical sonifi cation in public and semi-public spaces. Specifi cally, we fi rst identify the experiential qualities of sonic atmosphere and performativity as important aspects of sonifi cation in public and semi-public spaces, then use those experiential qualities generatively in the speculative design of a musical sonifi cation sketch. The design sketch comprises a dynamic musical sonifi cation of air quality data, intending to give citizens an awareness and an enhanced individual and interpersonal understanding of air quality in their city.