Nitrous acid (HONO) plays a crucial role in the self-cleansing capacity of the atmosphere but its formation mechanisms and spatial distributions are not well understood. This paper presents spectroscopic measurements of HONO, NO₂ and aerosol measurements from Melbourne, Australia. HONO levels are at a maximum in the middle of the day, which is unprecedented for an urban area, and these measurements provide evidence for the existence of a strong ground-based, daytime nitrogen oxide source.