Heterogeneous chemistry of nitrous acid (HONO) on liquid sulfuric acid (H2SO4) was investigated at conditions that prevail in the stratosphere. The measured uptake coefficient (γ) of HONO on H2SO4 increased with increasing acid content, ranging from 0.03 for 65 wt % to about 0.1 for 74 wt %. In the aqueous phase, HONO underwent irreversible reaction with H2SO4 to form nitrosylsulfuric acid (NO+HSO4-). At temperatures below 230 K, NO+HSO4- was observed to be stable and accumulated in concentrated solutions (>70 wt % H2SO4) but was unstable and quickly regenerated HONO in dilute solutions (<70 wt %). HCl reacted with HONO dissolved in sulfuric acid, releasing gaseous nitrosyl chloride (ClNO). The reaction probability between HCl and HONO varied from 0.01 to 0.02 for 60-72 wt % H2SO4. In the stratosphere, ClNO photodissociates rapidly to yield atomic chlorine, which catalytically destroys ozone. Analysis of the laboratory data reveals that the reaction of HCl with HONO on sulfate aerosols can affect stratospheric ozone balance during elevated sulfuric acid loadings after volcanic eruptions or due to emissions from the projected high-speed civil transport (HSCT). The present results may have important implications on the assessment of environmental acceptability of HSCT. © 1996 American Chemical Society.