Soot particles produced by incomplete combustion processes are one of the major components of urban air pollution. Chemistry at their surfaces lead to the heterogeneous conversion of several key trace gases; for example NO₂ interacts with soot and is converted into HONO, which rapidly photodissociates to form OH in the troposphere. In the dark, soot surfaces are rapidly deactivated under atmospheric conditions, leading to the current understanding that soot chemistry affects tropospheric chemical composition only in a minor way. We demonstrate here that the conversion of NO₂ to HONO on soot particles is drastically enhanced in the presence of artificial solar radiation, and leads to persistent reactivity over long periods. Soot photochemistry may therefore be a key player in urban air pollution.