Validating HONO as an Intermediate Tracer of the External Cycling of Reactive Nitrogen in the Background Atmosphere

In the urban atmosphere, nitrogen oxide (NO═NO + NO₂)-related reactions dominate the formation of nitrous acid (HONO). Here, we validated an external cycling route of HONO and NO, i.e., formation of HONO resulting from precursors other than NO, in the background atmosphere. A chemical budget closure experiment of HONO and NO was conducted at a background site on the Tibetan Plateau and provided direct evidence of the external cycling. An external daytime HONO source of 100 pptv h^-1 was determined. Both soil emissions and photolysis of nitrate on ambient surfaces constituted likely candidate mechanisms characterizing this external source. The external source dominated the chemical production of NO with HONO as an intermediate tracer. The OH production was doubled as a result of the external cycling. A high HONO/NO ratio (0.31 ± 0.06) during the daytime was deduced as a sufficient condition for the external cycling. Literature review suggested the prevalence of high HONO/NO ratios in various background environments, e.g., polar regions, pristine mountains, and forests. Our analysis validates the prevalence of external cycling in general background atmosphere and highlights the promotional role of external cycling regarding the atmospheric oxidative capacity.