Occurrence of atmospheric nitrous acid in the urban area of Beijing (China)

► The HONO levels have been measured in the winter and summer of 2007 at Beijing. ► The temporal and diurnal variations and the meteorological effects were examined. ► HONO (PSS) explained 83% and 48% of HONO levels in winter and summer, respectively. ► The HNO3 and fine particulate NO3− photolysis contributed weakly as HONO source ► Processes JNO2 dependent on aerosol and ground surfaces were HONO sources in summer.