High HONO level during daytime was observed at a rural site Xinken in southeastern China during the 2004 Program of Regional Integrated Experiments of Air Quality over Pear River Delta (PRIDE-PRD). The high HONO concentration led to a daytime (9:00–15:00) OH formation up to three times faster than that from O3 photolysis. An unexplained daytime HONO source, approximately four times faster than the reaction of OH with NO, has been derived from the HONO budget estimation. According to current understanding, this unknown source was attributed to heterogeneous reactions of NO2 on ground and aerosol surfaces. The good correlation between the unknown source and J values is consistent with the photolytic HONO source observed in laboratory studies.