Gas bubbles bursting at the sea surface produce drops, which contribute to marine aerosols. The contamination or enrichment of water by surface-active agents, of biological or anthropogenic origin, has long been recognized as affecting the bubble bursting processes and the spray composition. However, despite an improved understanding of the physics of a single bursting event, a quantitative understanding of the role of the physico-chemical conditions on assemblies of bursting bubbles remains elusive. We present experiments on the drop production by millimetric, collective bursting bubbles, under varying surfactant concentration and bubble density. We demonstrate that the production of supermicron droplets (with radius larger than 35 μm) is non-monotonic as the surfactant concentration increases. The bursting efficiency is optimal for short-lived, sparsely distributed and non-coalescing bubbles. We identify the combined role of contamination on the surface bubble arrangement and the modification of the jet drop production process in the bursting efficiency.