External cycling of NOx promotes the oxidative capacity of the atmosphere

Abstract

Abstract External cycling routes regenerating NOx from its oxidative reservoir, NOz, might reshape the temporal–spatial distribution of NOx 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/NO2 and NO2/NOy (≡ NOx + NOz) ratios and atypical HONO and NO2 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 NOx production in the external cycling and promotion of the atmospheric photochemistry by external cycling in the low-NOx troposphere. Chemical model omitting the external cycling yields OH underestimation by 41% in low-NOx 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 NOx that leads to an extremely low abundance of NOx and thus suppressed OH in the troposphere.