mineral dust: investiganting voc’s oxidation kinetics and product formation
Résumé
Mineral dust influences radiative transfer through direct absorption and scattering of solar and terrestrial radiation. Surfaces of uplifted minerals also initiate a series of chemical conversion, of both inorganic and organic species, thereby altering the atmospheric composition. In addition, these particles also carry photochemical properties so far mostly unconsidered. It is only recently that the photocatalytic nature of mineral dusts has been suggested and discussed (Chen et al. 2012 and George et al., 2015), especially their reactivity towards organic compounds. For instance, Styler and Donaldson (2012) reported CO2 formation when sand and volcanic particles coated with oxalic acid were iluminated.
In order to assess the importance of photocatalytic transformations of volatile organic compounds (VOC) on mineral dusts, we now initiated an investigation focussing on the associated kinetics and reaction mechanisms on Arizona test dust (a widely used proxy for mineral dust). We therefore report here on the conversion of butanol, a proxy for oxygenated organic compounds, under simulated atmospheric conditions with respect to light irradiation intensity, humidity and gas phase concentration. Arizona test dust contains 2-5% of Fe2O3 and 0.5-1% of TiO2, both important semiconductors that can lead to photocatalytic oxidation of organic compounds.
The experimental apparatus include a coated-wall flow tube reactor coupled to a High Resolution Proton Transfer Reaction Time of Flight Mass Spectrometer (PTR-ToF MS), providing reaction kinetics and product formation information. Seven actinic lamps surround the reactor and temperature is maintained constant by circulating thermostatically controlled water throughout an outer jacket. Experiments were carried out in atmospheric pressure.
Our results clearly show that light drastically affect the uptake kinetics, by promoting fast reactive uptake of the selected VOCs. For instance, in the case of butanol, light changes this system form unreactive to very reactive, leading to the production of more oxygenated products, such as aldehydes i.e., methanal, ethanal and propanal, which are important sources of atmospheric radicals. Therefore, we will present how light could trigger new particle phase chemistry and how this feeds back into atmospheric chemistry.