Synthesizing evidence for the external cycling of NO_x in high- to low-NO_x atmospheres

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_x and consequently hydroxyl radical (OH), the most important oxidant in the atmosphere. Here we verify the in situ external cycling of NO_x in 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/NO₂ and NO₂/NO_z by current chemical model schemes and rationalize atypical diurnal concentration profiles of HONO and NO₂ lacking noontime valleys specially observed in low-NO_x atmospheres. Perturbation on the budget of HONO and NO_x by external cycling is also found to increase as NO_x concentration decreases. Consequently, model underestimation of OH observations by up to 41% in low NO_x atmospheres is attributed to the omission of external cycling in models.