The reaction of NO2 with solid anthrarobin (1,2,10-trihydroxy-anthracene)
Reactions of atmospheric oxidants with condensed organic materials are implicated in secondary processes of relevance to the radiation budget of the atmosphere and to its oxidation capacity. A solid film of anthrarobin (1,2,10-trihydroxyanthracene) was exposed to gaseous NO2 at concentrations in the ppb range at atmospheric pressure, temperatures between 283 and 313 K and relative humidity between 20 and 80%. Gaseous HONO was the main product evolving from the anthrarobin surface. The reaction probability based on gas-kinetic collisions ranging between 2 × 10−6 and 7 × 10−7 decreased with time due to consumption of reactive surface molecules. It also decreased as a function of the NO2 concentration, which is in agreement with a Langmuir–Hinshelwood type surface reaction. A Langmuir constant of 5 × 10−3 ppb−1 for the reversible adsorption and a surface reaction rate constant of about 10−20 cm2 s−1 was derived. The reaction rate increased with increasing temperature, and the corresponding activation energy for the overall process was +39 kJ mol−1. The effect of humidity was to increase the reaction rate but also to increase the apparent HONO output, which was clearly not due to a bulk effect. The results suggest that the primary reaction is an electron transfer process between a deprotonated hydroxy group (equivalent to a phenoxide ion) hydrolyzed on the surface in the presence of humidity and NO2 leading to a surface nitrite. The results provide evidence that the system investigated may be a model system for the reaction of NO2 with soot.