HONO formation has been proposed as an important OH radical source insimulation chambers for more than two decades. Besides the heterogeneousHONO formation by the dark reaction of NO and adsorbed water, aphotolytic source has been proposed to explain the elevated reactivity insimulation chamber experiments. However, the mechanism of the photolyticprocess is not well understood so far.As expected, production of HONO and NO was also observed inside thenew atmospheric simulation chamber SAPHIR under solar irradiation. Thisphotolytic HONO and NO formation was studied with a sensitive HONOinstrument under reproducible controlled conditions at atmosphericconcentrations of other trace gases. It is shown that the photolytic HONOsource in the SAPHIR chamber is not caused by NO reactions and that itis the only direct NO source under illuminated conditions. Inaddition, the photolysis of nitrate which was recently postulated for theobserved photolytic HONO formation on snow, ground, and glass surfaces, canbe excluded in the chamber. A photolytic HONO source at the surface of thechamber is proposed which is strongly dependent on humidity, on lightintensity, and on temperature. An empirical function describes thesedependencies and reproduces the observed HONO formation rates to within 10%. It is shown that the photolysis of HONO represents the dominantradical source in the SAPHIR chamber for typical troposphericO/HO concentrations. For these conditions, the HONOconcentrations inside SAPHIR are similar to recent observations in ambientair.
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