Measured and simulated vertical profiles of nitrous acid—Part II. Model simulations and indications for a photolytic source

Numerical simulations with a comprehensive mesoscale-γ model system were carried out to study the relative importance of individual sources for nitrous acid (HONO) on the daily cycle of the HONO concentration during the course of the day. The sources are the gas phase production, direct emissions of HONO, and heterogeneous reactions at aerosol surfaces and at the ground. The results of the simulations were compared to observations made during 18–19 October 2001 at the 200 m research tower at the Forschungszentrum Karlsruhe, Germany. We found that currently used chemical transport models are using a rate constant for the gas-phase reaction of NO with OH which is a factor of two lower than the recently published IUPAC data. The old rate constant gives 50% lower daytime HONO concentrations than the new one. The simulations showed that the direct emissions and the heterogeneous reactions at the ground are the most important sources of HONO during the night. While the observed nocturnal HONO concentrations close to the surface are reproduced by the simulations the daytime HONO concentrations are underestimated by 30–50%. We introduced an artificial photolytic HONO source which is proportional to the photolysis rate constant of NO2. This leads to a much better agreement between observed and simulated HONO concentrations during the day. In addition, we compared the production of the hydroxyl radical by the photolysis of HONO with its production via the photolysis of ozone. During 18 October 2001 when the ozone concentrations were low the photolysis of HONO is the dominant source of the OH radical while during a typical summer day the photolysis of HONO contributes only 20% of the photolysis of ozone.