Article pubs.acs.org/JPCA
Investigating the Heterogeneous Interaction of VOCs with Natural Atmospheric Particles: Adsorption of Limonene and Toluene on Saharan Mineral Dusts Manolis N. Romanías,* Habib Ourrad, Frédéric Thévenet,* and Véronique Riffault Mines Douai, SAGE, 941 rue Charles Bourseul, F-59508 Douai, France Université de Lille, F-59000 Lille, France S Supporting Information *
ABSTRACT: The heterogeneous interaction of limonene and toluene with Saharan dusts was investigated under dark conditions, pressure of 1 atm, and temperature 293 K. The mineral dust samples were collected from six different regions along the Sahara desert, extending from Tunisia to the western Atlantic coastal areas of Morocco, and experiments were carried out with the smallest sieved fractions, that is, inferior to 100 μm. N2 sorption measurements, granulometric analysis, and X-ray fluorescence and diffraction (XRF and XRD) measurements were conducted to determine the physicochemical properties of the particles. The chemical characterization showed that dust originating from mideastern Sahara has a significantly higher SiO2 content (∼82%) than dust collected from the western coastal regions where the SiO2 relative abundance was ∼50%. A novel experimental setup combining diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), selected-ion flow-tube mass spectrometry (SIFT−MS), and long path transmission Fourier transform infrared spectroscopy (FTIR) allowed us to follow both the adsorbed and gas phases. The kinetic adsorption/desorption measurements were performed using purified dry air as bath gas, exposing each dust surface to 10 ppm of the selective volatile organic compound (VOC). The adsorption of limonene was independent of the SiO2 content, given the experimental uncertainties, and the coverage measurements ranged between (10 and 18) × 1013 molecules cm−2. Experimental results suggest that other metal oxides that could possibly influence dust acidity may enhance the adsorption of limonene. On the contrary, in the case of toluene, the adsorption capacities of the Saharan samples increased with decreasing SiO2 content; however, the coverage measurements were significantly lower than those of limonene and ranged between (2 and 12) × 1013 molecules cm−2. Flushing the surface with purified dry air showed that VOC desorption is not a completely reversible process at room temperature. The reversibly adsorbed fraction and the rate coefficients of desorption, kdes, depended inversely on the SiO2 relative abundance for both VOCs.
1. INTRODUCTION
have been mainly focused on the reaction of dust surfaces with inorganic species. In particular, the reactions of HNO3,6−12 NO 2,11−17 HONO,18−21 N 2 O5 ,22−24 O 3,6,25−27 SO 2 ,28,29 H2O230−36 and those of NO3,22,37 •OH,38 and HO2•39,40 radicals with different mineral oxide and dust surfaces have been investigated under various simulated environmental conditions (gas-phase concentration, relative humidity, temperature, irradiance intensity).41−43 The initial and steady-state uptake coefficients have been determined, and in a few studies the yield of the products formed has been reported;6,17,18,38 however, there is a significant lack of laboratory data regarding the heterogeneous interaction of volatile organic compounds (VOCs) with dust surfaces (whereas these species are as
Mineral dust is a key component of atmospheric aerosols. According to recent estimations, every year, 1600 Tg of mineral dust is released into the atmosphere, representing the largest mass emission rate of aerosol particles on a global scale.1,2 Although the primary sources are arid regions, because of global air circulation, mineral particles undergo long-range transportation to remote areas.3 The major source of mineral dust is the African continent mainly because of the presence of the Sahara desert, contributing to ∼62% of the total global emissions.4,5 Dust surfaces may provide the seedbed for specific interactions with trace gas molecules and therefore could play a key role in the transformation and environmental fate of many atmospheric species. The interaction of mineral dusts and their components with atmospheric trace gases has generated a great deal of interest over the past 15 years. A careful literature review shows that studies © XXXX American Chemical Society
Received: October 21, 2015 Revised: February 4, 2016
A
DOI: 10.1021/acs.jpca.5b10323 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
simultaneously monitored by selected-ion flow-tube mass spectrometry (SIFT−MS) and long path transmission Fourier transform infrared spectroscopy (FTIR). The total number of molecules adsorbed, Ns (molecules cm−2), the sample surface coverage, θ, the number of molecules desorbed upon flushing the surface, Ndes (molecules cm−2), and the desorption rate coefficients, kdes (min−1), were determined for each sample. To the best of our knowledge this is the first quantitative laboratory study reporting the heterogeneous interactions of VOCs with atmospheric Saharan dust samples.
significant as inorganic species in the atmospheric chemistry), as highlighted in the latest IUPAC evaluation where no recommended values were reported for VOC interactions with mineral oxides or dust surfaces. VOCs in the atmosphere come from both anthropogenic and natural sources.44 They are important precursors of tropospheric ozone (O3) and play a key role in the photochemical oxidation cycles in the atmosphere, therefore impacting air quality and global climate. Li et al.45 and Carlos-Cuellar et al.46 investigated the interaction of acetone, acetaldehyde, and propanol and formaldehyde, methanol, and acetic acid, respectively, on mineral oxides (i.e., SiO2, Fe2O3, and Al2O3) using a Knudsen cell reactor. The authors determined the initial uptake coefficients and the surface coverage of the VOCs on each mineral oxide surface. In addition, they observed that carbonyl compounds are weakly and reversibly adsorbed on SiO2, while on the more basic and acidic investigated oxides, these carbonyl compounds adsorb irreversibly and can partially react on the surface to form larger molecular weight compounds. In addition, Styler and Donaldson have investigated the photooxidation of isopropanol and n-propanol on TiO2−SiO2 mixed films, and they reported the formation of gas-phase acetone and propionaldehyde.47 These authors have also reported the photooxidation of oxalic acid to CO2 on Mauritanian dust.48 Quite recently, the heterogeneous interaction of formic acid with SiO2, α and γ-Al2O3, kaolinite, and irradiated TiO2 surfaces was studied.49,50 We used infrared spectroscopic techniques to analyze the adsorbed phase to (i) evaluate the role of adsorbed water in surface coordination, (ii) determine the adsorption kinetics, and (iii) identify the photoproducts formed.50 They observed that adsorbed water can increase the irreversible uptake of formic acid and they directly detected the formation of formate on the surface.49 As of today many questions remain unanswered regarding heterogeneous processes in the atmosphere, in particular: What is the impact of mineral aerosols on tropospheric chemistry? May the heterogeneous reaction of VOCs on dust surfaces influence the oxidation cycles of the troposphere? But more fundamentally are VOCs really interacting, that is, adsorbing, onto natural mineral dust surfaces? And if so, are we able to qualify and quantify that interaction? Within that framework, the aim of this study was to investigate the heterogeneous interactions of limonene (C10H16) and toluene (C7H8), used as model VOCs, with natural environmental samples. Limonene is representative of the monoterpene family, released in large amounts to the atmosphere mainly due to biogenic activities, and appears to have significant atmospheric interest.51,52 Measured environmental concentrations of limonene are typically around 0.1 to 2 ppb.53 Regarding toluene, this monosubstituted benzene derivative originates from significant anthropogenic and biogenic emissions. 54,55 The average concentrations of toluene vary considerably depending on the sampling location. Field campaigns performed in the mid-90s indicated that toluene concentration ranges between 0.9 and 2800 ppb.56 Quite recently, Hazrati et al. monitored the concentration of toluene in refueling stations of Ardabil city (Iran) to be around 500−600 ppb.57 In the present work, the heterogeneous interactions of both compounds with natural Saharan dust samples were investigated under atmospheric pressure and room temperature with a novel experimental setup, allowing monitoring of both the adsorbed and gas phases in real time. The time evolution of the adsorbed phase was recorded using diffuse reflectance infrared Fourier transform spectroscopy (DRIFT), while the gas phase was
2. EXPERIMENTAL SECTION 2.1. Dust Sample Collection. The mineral dust samples were collected in August 2013 from six different regions along the Sahara desert. The objective was to identify a potential variation of the dust chemical composition with location points extending from Tunisia to Morocco (Figure S1). In particular, the first dust sample was collected 5 km west of Nefta, an oasis town in the southwest of Tunisia about 30 km from the Algerian border. The second and third sampling points were 2 km east of Touggourt and 10 km east of N′Goussa cities, respectively, in Algeria. A fourth sample was collected 2 km south of Bordj Mokhtar city, which is located south of Algeria near the Malian border. Finally, two samples originating from Morocco were collected 2 km east of Tarfaya and 3 km north of Laayoune cities, respectively, on the coast of the Atlantic Ocean. Sample numbering is based on the geographic longitude of the collection points rather than on the collection dates. Moreover, in the remainder of the paper, for clarity, the dust samples will be referred to by the town/city names previously given. Furthermore, it is important to note that the physicochemical characterization of the samples and the kinetic experiments with the selected VOCs were performed using the smallest sieved size fraction of the Saharan dusts (
