Fig. 1: Schematic graph of internal cycling and external cycling of NO and HONO.
From: Synthesizing evidence for the external cycling of NOx in high- to low-NOx atmospheres

The left reaction scheme describes internal cycling between nitrogen oxides (NO = NO + NO) and nitrous acid (HONO) in high- NO atmospheres. HONO photolysis is the major route producing OH radical (OH). The right reaction scheme describes external cycling among oxidative reservior species of NO (NO), NO, and HONO in low-NO atmospheres. NO -catalysed radical chain propagration takes over as the major route perturbing OH and ozone (O) chemical production.