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External cycling regenerating nitrogen oxides (NO_x ≡ NO + NO₂) from their oxidative reservoir, NO_z, is proposed to reshape the temporal–spatial distribution of NO_xand consequently hydroxyl radical (OH), the most important oxidant in the atmosphere. Here we verify the in situ external cycling of NO_xin various environments with nitrous acid (HONO) as an intermediate based on synthesized field evidence collected onboard aircraft platform at daytime. External cycling helps to reconcile stubborn underestimation on observed ratios of HONO/NO₂and NO₂/NO_zby current chemical model schemes and rationalize atypical diurnal concentration profiles of HONO and NO₂lacking noontime valleys specially observed in low-NO_xatmospheres. Perturbation on the budget of HONO and NO_xby external cycling is also found to increase as NO_xconcentration decreases. Consequently, model underestimation of OH observations by up to 41% in low NO_xatmospheres is attributed to the omission of external cycling in models.