Heterogeneous Interaction of Various Natural Dust Samples with Isopropyl Alcohol as a Probe VOC

The adsorption properties of mineral dust toward organic molecules are poorly characterized so far. Heterogeneous processes between trace gases and mineral particles can affect the oxidative capacity of the atmosphere as well as constitute additional sources or sinks for these species. The current study investigates the adsorption efficiencies of natural dust samples collected from North and West Africa, Saudi Arabia, and Arizona desert regions toward isopropyl alcohol (IPA), a common organic pollutant released in significant amounts in the atmosphere, which is used here as a probe molecule. Experiments are performed under atmospheric pressure, room temperature 296 K, over the concentration range (0.15-615) × 10¹³ molecules cm^-3, and in the relative humidity (RH) range (0.01-85)%. The kinetic measurements are conducted inside a U-shaped flow reactor using zero air as bath gas and a chemical ionization mass spectrometer for real-time gas-phase monitoring. Kinetic and surface parameters such as initial uptake coefficients (γ₀) and adsorption equilibrium constants are measured. γ₀ is found to be independent of the IPA gas-phase concentration. However, concerning RH, γ is independent up to ca. 20%, but a dramatic decrease is observed above that threshold implying a competition between water molecules and IPA after the formation of a water monolayer on the dust sample. These results are simulated using an empirical expression of the form γ_RH = γ_dry - aRH ^b that allows the extrapolation of the uptake coefficient under any tropospheric RH conditions. Our uptake coefficient values show a linear correlation with the elemental Al/Si and Fe/Si ratios of the natural dusts studied. This was confirmed when comparing with data on inorganic species gathered from a comprehensive literature review (no such data exist for organics). To the best of our knowledge, this work is the first to demonstrate that initial uptakes are linearly correlated with the Al/Si ratio for both organic and inorganic species.