Synthesizing evidence for the external cycling of NO x in high- to low-NO x atmospheres.

External cycling regenerating nitrogen oxides (NO x  ≡ NO + NO 2 ) 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/NO 2 and NO 2 /NO z by current chemical model schemes and rationalize atypical diurnal concentration profiles of HONO and NO 2 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.