External cycling of NOx promotes the oxidative capacity of the atmosphere

Abstract External cycling routes regenerating NO x from its oxidative reservoir, NO z , might reshape the temporal–spatial distribution of NO x and consequently OH, the most important atmospheric oxidant. However, the kinetics and mechanisms of the external cycling are still in dispute. Here, we present two critical pieces of evidence from aircraft observations in various atmospheric chemical regimes to verify the external cycling, i.e., unexpectedly high HONO/NO 2 and NO 2 /NO y (≡ NO x + NO z ) ratios and atypical HONO and NO 2 diurnal profiles, both of which cannot be captured by GEOS-Chem modeling. Additionally, two features of the external cycling were portrayed, i.e., HONO as an intermediate for NO x production in the external cycling and promotion of the atmospheric photochemistry by external cycling in the low-NO x troposphere. Chemical model omitting the external cycling yields OH underestimation by 41% in low-NO x atmospheres. Our data provide a global view of the external cycling and its perturbations on the atmospheric oxidative capacity, which challenges the traditional view involving the continuous oxidative decay of NO x that leads to an extremely low abundance of NO x and thus suppressed OH in the troposphere.