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A: Kinetics, Dynamics, Photochemistry, and Excited States
Heterogeneous Interaction of Various Natural Dust Samples with Isopropanol as a Probe VOC Mohamad Nour Zeineddine, Manolis N. Romanias, Véronique Riffault, and Frederic Thevenet J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02034 • Publication Date (Web): 14 May 2018 Downloaded from http://pubs.acs.org on May 15, 2018
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The Journal of Physical Chemistry
Heterogeneous Interaction of Various Natural Dust Samples with Isopropanol as a Probe VOC Mohamad N. Zeineddine*, Manolis N. Romanias*, Véronique Riffault, Frédéric Thévenet
IMT Lille Douai, Univ. Lille, SAGE, F-59000 Lille, France
*
Corresponding authors:
Mohamad N. Zeineddine, Tel.: +33 (0)3 27 71 22 68; Fax: +33 (0)3 27 71 29 14; E-mail:
[email protected] Manolis N. Romanias, Tel.: +33 (0)3 27 71 26 33; Fax: +33 (0)3 27 71 29 14; E-mail:
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Abstract 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 isopropanol (IPA), a common organic pollutant released in significant amounts in the atmosphere, and used here as a probe molecule. Experiments are performed under atmospheric pressure, room temperature 296K, 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 − aRHb 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.
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Introduction It is estimated that 1600 Tg of mineral dust are released into the atmosphere on an annual
basis with the ability to undergo long-range transportation.1 The role of heterogeneous interactions of atmospheric trace gases on mineral dust particles remains an important question in tropospheric chemistry. These interactions have been a subject of great interest in the past decade, due to their momentous capability in altering the atmosphere chemical balance and modifying dust particle properties.2, 3 Recent studies evidenced that mineral dust may significantly influence the local and regional abundances of trace gases in the troposphere.1, 4-6 The mineralogy of dust particles is complex and diverse; it depends on the primary emission sources.7-11 Mineral particles mostly originate from eroded soils, thus their chemical composition is similar to that of top soil material. Several studies on the elemental content of windblown dusts originating from various locations show that the primary elements are Silicon (Si) and Aluminum (Al).12,
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Si is mainly present in SiO2-based phases (e.g. quartz, cristobalite) and in various
aluminosilicate minerals. Aluminosilicates such as feldspars and clays (illite, kaolinite, montmorillonite, etc.) are the major aluminum-bearing minerals.12-14 Al is also present in the crystalline polymorphic phases of aluminum oxide Al2O3. Calcium (Ca) is also an important element of mineral dust mainly present in the forms of calcite, calcium oxide, aragonite, and dolomite. Besides the abovementioned elements, others such as K (K-feldspar, white mica, illite), Na (albite feldspar, smectite clay minerals), Fe (in mineral oxides e.g. hematite, magnetite), Ti (TiO2 phases of anatase and rutile) and Mg (dolomite) are also present in mineral dust and source sediments.8, 10, 13-16 Moreover, it should be noted that the mineralogical composition of airborne dust particles also depends on the size fractionation during emission and transport.10 To be able to account for the fractionation effect, Journet et al.10 specified the chemical composition of the finer textural classes of soils in the silt (2-63 µm) and clay (< 2 µm) fractions. The silt fraction contains primarily minerals
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such as quartz, feldspars and mica while other minerals such as clays and iron oxides are mostly found in the clay fraction.10 Literature studies on the heterogeneous chemistry of mineral dusts mainly focused on the uptake and transformation of inorganic trace gases.17 Nevertheless, only few studies have been conducted on natural mineral dust samples, while even less attempted to correlate the effect of chemical composition on mineral dust reactivity.17 Besides inorganic species, heterogeneous reactions of volatile organic compound (VOCs) are also of interest in the atmosphere since they may impact the transformation and transportation of VOC, and thus their atmospheric concentration as well as the mixing ratios of atmospheric oxidants and other related species at the local or even regional scales6,
18-20
In addition, heterogeneous reactions of VOCs can produce species such as
organic acids leading to secondary organic aerosol formation,11, 18 and can alter the physicochemical properties of atmospheric particles, such as size or hygroscopic properties.21, 22 Although the body of literature on heterogeneous reactions of VOCs has been recently expanded and reviewed,
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its
impact on the Earth’s atmosphere has yet to be asserted. There is still a strong lack of kinetic and product data for VOC reactions on natural mineral dusts, and particularly, how mineral dust composition can influence the uptake process. These pieces of information are of significant importance since current atmospheric models use elemental ratios as input data to simulate the impact of mineral dust reactivity to the tropospheric chemistry.24 Therefore, the accuracy of the predictions would be significantly improved if laboratory experiments could correlate the uptake efficiency with the elemental composition.
The objective of the present work is to evaluate the effect of chemical composition on the interaction of volatile organic compounds with natural dust samples. Natural dust samples from different regions of the world are used as surrogate materials. Isopropanol (IPA) is used as a model
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VOC; IPA is a common anthropogenic pollutant released in the atmosphere mainly by industrial activities. It is highly reactive and may easily lead to acetone and other lighter compounds thus being a good probe to evaluate the effect of the chemical composition.25-27 Kinetic measurements are conducted in a U-shaped flow reactor coupled with a SIFT-MS for the detection of the gas phase species. The adsorption isotherms and the uptake coefficients are determined over a wide range of concentrations (0.15 – 615) × 1013 molecules cm-3 in the relative humidity (RH) range (0.01–85)% and at room temperature (RT), 296 K. To the best of our knowledge this is the first experimental study reporting the effect of the chemical composition on VOC interaction with natural dust samples.
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Materials and methods
2.1 Materials Dust samples The four natural mineral dust samples used in this study originate from various regions in order to encompass a large diversity of chemical compositions (Figure S1). The natural top soil samples used were collected (i) close to the oasis of Nefta (Tunisia) and the city of Mbour (Senegal), corresponding to two different regions along North and West Africa and (ii) from Saudi Arabia (Rawdat arid region). The experiments are performed using only the smallest sieved size fraction of the natural dust samples (< 100 µm) that can be suspended in air. In particular, a mechanical sieve shaker is used to fractionate the soil samples into different classes. Then, the finest collected fraction (
