Mechanisms regarding formation and loss of nitrous acid (HONO) in the rural atmosphere are not well understood. Results of field observations and laboratory experiments implicate the importance of dew in controlling the surface-atmosphere exchange of HONO. Dew water, abundantly available on ground surfaces, especially on canopy surfaces in forested regions, during summer and autumn nights, serves as a sink and a temporary reservoir of atmospheric HONO and as a source in the morning when the dew droplets evaporate.