Photolytic degradation of molecular iodine adsorbed on model SiO2 particles.

A molecular derivatization method followed by gas chromatographic separation coupled with mass spectrometric detection was used to study photolytic degradation of I2 adsorbed on solid SiO2 particles. This heterogeneous photodegradation of I2 is studied at ambient temperature in synthetic air to better understand I2 atmospheric dispersion and environmental fate. The obtained laboratory results show a considerably enhanced atmospheric lifetime of molecular iodine adsorbed on solid media. The heterogeneous atmospheric residence time (τ) of I2 is calculated to be τ ≈ 187 min, i.e., τ ≈ 3 h. The obtained heterogeneous lifetime of I2 is shown to be considerably longer than its destruction by its principal atmospheric sink, namely, photolysis. The observed enhanced atmospheric lifetime of I2 on heterogeneous media will likely have direct consequences on the atmospheric transport of I2 that influences the toxicity or the oxidative capacity of the atmosphere.

Photoenhanced uptake of gaseous NO2 on solid organic compounds: a photochemical source of HONO?

In several recent field campaigns the existence of a strong daytime source of nitrous acid was demonstrated. The mechanism of this source remains unclear. Accordingly, in the present laboratory study, the effect of light (in the range 300-500 nm) on the uptake kinetics of NO2 on various surfaces taken as proxies for organic surfaces encountered in the troposphere (as organic aerosol but also ground surfaces) was investigated. In this collaborative study, the uptake kinetics and product formation rate were measured by different flow tube reactors in combination with a sensitive HONO instrument. Uptake on light absorbing aromatic compounds was significantly enhanced when irradiated with light of 300-420 nm, and HONO was formed with high yield when the gas was humidified. Especially organic substrates containing a combination of electron donors, such as phenols, and of compounds yielding excited triplet states, such as aromatic ketones, showed a high reactivity towards NO2. Based on the results reported a mechanism is suggested, in which photosensitised electron transfer is occurring. The results show that HONO can be efficiently formed during the day in the atmosphere at much longer wavelengths compared to the recently proposed nitrate photolysis.