Atmospheric aerosols contain a large amount of organic compounds, whose oxidation affects their physical properties through a process known as ageing. We have simulated atmospheric ageing experimentally to elucidate the nature and behaviour of residual surface films. Our results show an increasing amount of residue at near-zero temperatures, demonstrating that an inert product film may build up during droplet ageing, even if only ordinarily short-lived reactive species are initially emitted.