Nitrous acid (HONO) plays important roles in tropospheric chemistry, but its source(s) are not completely understood. Here we analyze measurements of HONO, nitrogen dioxide (NO2), and related parameters at a coastal site in Hong Kong during September–December 2012. The nocturnal NO2‐to‐HONO conversion rates were estimated in air masses passing over land and sea surfaces. The conversion rates in the “sea cases” (3.17–3.36 × 10−2 h−1) were significantly higher than those in the “land cases” in our study (1.20–1.30 × 10−2 h−1) and in previous studies by others. These results suggest that air‐sea interactions may be a significant source of atmospheric HONO and need to be considered in chemical transport models. Key Points Strong HONO production in sea‐coming air massesAir‐sea interactions may significantly affect atmospheric chemistryChemical transport models may need to consider HONO production on the sea