Photodegradation of Molecular Iodine on SiO2 Particles: Influence of Temperature and Relative Humidity

Abstract

A molecular derivatization method followed by gas chromatographic separation coupled with mass spectrometric detection was used to study photodegradation of molecular I2 adsorbed on solid SiO2 particles. The heterogeneous photodegradation of I2 was studied as a function of temperature and relative humidity in synthetic air to better understand its environmental fate. Two sets of experiments were carried out. In the first set of experiments, the temperature was T = (298 ± 1) K and relative humidity was varied from ≤ 2% to 75%RH under given experimental conditions. In the second set of experiments, the relative humidity within the Pyrex bulb was 40%RH and the temperature was varied from 283 ± 1 ≤ T (K) ≤ 323 ± 1. The obtained results show a considerably enhanced atmospheric lifetime of molecular iodine adsorbed on solid media that does not depend on relative humidity of the environment. The obtained results show that the rate constant for the photolysis of molecular iodine adsorbed on model SiO2 particles depends on temperature and is reported to be J (T)=(1.24 ± 1.4)×10−2×exp[(1482±345)/T]/s over the measured temperature range. The heterogeneous atmospheric residence time () of I2 adsorbed on solid media is calculated to range from 2 to 4.1 h. The experimentally obtained heterogeneous lifetime of I2 is shown to be considerably longer than its destruction by its principal atmospheric sink, 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.