Ozone Uptake by Clay Dusts under Environmental Conditions

Clay is the most emitted type of dust in the atmosphere and offers a large surface for heterogeneous interactions with atmospheric compounds. In this study, we focus on the uptake of ozone (O₃) by montmorillonite and kaolinite dust samples at atmospheric pressure in a coated-wall flow tube reactor. The influence of relevant environmental parameters, such as O₃ concentration, relative humidity, and temperature, is determined. A mechanism for O₃ interaction with the surface sites of clays is described, and atmospheric implications are discussed. Although the impact of O₃ uptake by fresh clay dust seems limited in the atmosphere, this work highlights the need to consider relevant and thoroughly defined uptake coefficients in models. Moreover, the presence of a pressure-dependence in the mechanism of O₃ uptake by dust suggested in this work helps reconciling measurements made at atmospheric pressure with those performed at low pressure in Knudsen cells

Heterogeneous Interaction of H₂O₂ with TiO₂ Surface under Dark and UV Light Irradiation Conditions

The heterogeneous interaction of H2O2 with TiO2 surface was investigated under dark conditions and in the presence of UV light using a low pressure flow tube reactor coupled with a quadrupole mass spectrometer. The uptake coefficients were measured as a function of the initial concentration of gaseous H2O2 ([H2O2]0 = (0.17–120) × 1012 molecules cm–3), irradiance intensity (JNO2 = 0.002–0.012 s–1), relative humidity (RH = 0.003–82%), and temperature (T = 275–320 K). Under dark conditions, a deactivation of TiO2 surface upon exposure to H2O2 was observed, and only initial uptake coefficient of H2O2 was measured, given by the following expression: γ0(dark) = 4.1 × 10–3/(1 + RH0.65) (calculated using BET surface area, estimated conservative uncertainty of 30%) at T = 300 K. The steady-state uptake coefficient measured on UV irradiated TiO2 surface, γss(UV), was found to be independent of RH and showed a strong inverse dependence on [H2O2] and linear dependence on photon flux. In addition, slight negative temperature dependence, γss(UV) = 7.2 × 10–4 exp[(460 ± 80)/T], was observed in the temperature range (275–320) K (with [H2O2] ≈ 5 × 1011 molecules cm–3 and JNO2 = 0.012 s–1). Experiments with NO addition into the reactive system provided indirect evidence for HO2 radical formation upon H2O2 uptake, and the possible reaction mechanism is proposed. Finally, the atmospheric lifetime of H2O2 with respect to the heterogeneous loss on mineral dust was estimated (using the uptake data for TiO2) to be in the range of hours during daytime, i.e., comparable to H2O2 photolysis lifetime (∼1 day), which is the major removal process of hydrogen peroxide in the atmosphere. These data indicate a strong potential impact of H2O2 uptake on mineral aerosol on the HOx chemistry in the troposphere

Ozone Chemistry and Photochemistry at the Surface of Icelandic Volcanic Dust: Insights from Elemental Speciation Analysis

Volcanic particulate matter (PM), whether emitted directly as ash or indirectly via suspension of glaciogenic sediments, comprises a large fraction of atmospheric PM in Iceland, a major high-latitude dust source area. This PM leads to direct reductions in air quality and health; in addition, because it provides a surface for reactions with trace pollutant gases, it also has the potential to indirectly influence the chemical composition of the troposphere. Here, we investigate the reaction of gas-phase ozone with a volcanic dust sample obtained from the Mýrdalssandur source region in southern Iceland. We find that the steady-state surface area-scaled ozone uptake coefficient (γBET) for this sample decreases with increasing ozone mixing ratio and relative humidity, which implies that the reaction proceeds via a Langmuir–Hinshelwood mechanism with water vapor as competitive adsorbate. Using the γBET values we obtain here, we conclude that the ozone flux to volcanic PM would be <10% of its flux to the ground surface under typical Icelandic weather conditions, even during major dust events. Interestingly, although the Mýrdalssandur dust sample is high in elemental Ti, which in its anatase and rutile forms is a powerful semiconductor photocatalyst, its photochemistry is relatively modest. We use electron microprobe analysis to help resolve this apparent contradiction: in particular, we show that the bulk of the Ti in this sample is present in its glass fraction, with the remainder present not as anatase or rutile but rather in other predicted mineral phases (pyroxene, plagioclase, ilmenite, titanomagnetite, and olivine). These results highlight the advantages of using elemental speciation analysis to understand the atmospheric reactivity of volcanic PM

Ozone Chemistry and Photochemistry at the Surface of Icelandic Volcanic Dust: Insights from Elemental Speciation Analysis

Volcanic particulate matter (PM), whether emitted directly as ash or indirectly via suspension of glaciogenic sediments, comprises a large fraction of atmospheric PM in Iceland, a major high-latitude dust source area. This PM leads to direct reductions in air quality and health; in addition, because it provides a surface for reactions with trace pollutant gases, it also has the potential to indirectly influence the chemical composition of the troposphere. Here, we investigate the reaction of gas-phase ozone with a volcanic dust sample obtained from the Mýrdalssandur source region in southern Iceland. We find that the steady-state surface area-scaled ozone uptake coefficient (γBET) for this sample decreases with increasing ozone mixing ratio and relative humidity, which implies that the reaction proceeds via a Langmuir–Hinshelwood mechanism with water vapor as competitive adsorbate. Using the γBET values we obtain here, we conclude that the ozone flux to volcanic PM would be <10% of its flux to the ground surface under typical Icelandic weather conditions, even during major dust events. Interestingly, although the Mýrdalssandur dust sample is high in elemental Ti, which in its anatase and rutile forms is a powerful semiconductor photocatalyst, its photochemistry is relatively modest. We use electron microprobe analysis to help resolve this apparent contradiction: in particular, we show that the bulk of the Ti in this sample is present in its glass fraction, with the remainder present not as anatase or rutile but rather in other predicted mineral phases (pyroxene, plagioclase, ilmenite, titanomagnetite, and olivine). These results highlight the advantages of using elemental speciation analysis to understand the atmospheric reactivity of volcanic PM

Heterogeneous Interaction of Isopropanol with Natural Gobi Dust

The adsorption of isopropanol on Gobi dust was investigated in the temperature (T) and relative humidity (RH) ranges of 273–348 K and <0.01–70%, respectively, using zero air as bath gas. The kinetic measurements were performed using a novel experimental setup combining Fourier-Transform InfraRed spectroscopy (FTIR) and selected-ion flow-tube mass spectrometry (SIFT-MS) for gas-phase monitoring. The initial uptake coefficient, γ₀, of isopropanol was measured as a function of several parameters (concentration, temperature, relative humidity, dust mass). γ₀ was found independent of temperature while it was inversely dependent on relative humidity according to the empirical expression: γ₀ = 5.37 × 10^–7/(0.77+RH^0.6). Furthermore, the adsorption isotherms of isopropanol were determined and the results were simulated with the Langmuir adsorption model to obtain the partitioning constant, <i>K</i>_Lin, as a function of temperature and relative humidity according to the expressions: <i>K</i>_Lin = (1.1 ± 0.3) × 10^–2 exp [(1764 ± 132)/<i>T</i>] and <i>K</i>_Lin = 15.75/(3.21+RH^1.77). Beside the kinetics, a detailed product study was conducted under UV irradiation conditions (350–420 nm) in a photochemical reactor. Acetone, formaldehyde, acetic acid, acetaldehyde, carbon dioxide, and water were identified as gas-phase products. Besides, the surface products were extracted and analyzed employing HPLC; Hydroxyacetone, formaldehyde, acetaldehyde, acetone, and methylglyoxal were identified as surface products while the formation of several other compounds were observed but were not identified. Moreover, the photoactivation of the surface was verified employing diffuse reflectance infrared fourier transform spectroscopy (DRIFTs)

Rate Coefficients for the Gas-Phase Reactions of Nitrate Radicals with a Series of Furan Compounds

The atmospheric reaction of a series of furan compounds (furan (F), 2-methylfuran (2-MF), 3-methylfuran (3-MF), 2,5-dimethylfuran (2,5-DMF), and 2,3,5-trimethylfuran (2,3,5-TMF)) with nitrate radical (NO3) has been investigated using the relative rate kinetic method in the CHamber for the Atmospheric Reactivity and the Metrology of the Environment (CHARME) simulation chamber at the laboratoire de Physico-Chimie de l’Atmosphere (LPCA) laboratory (Dunkerque, France). The experiments were performed at (294 ± 2) K atmospheric pressure and under dry conditions (relative humidity, RH < 2%) with proton transfer mass reaction–time of flight–mass spectrometer (PTR-ToF-MS) for the chemical analysis. The following rate coefficients (in units cm3 molecule–1 s–1) were determined: furan, k(F) = (1.51 ± 0.38) × 10–12, 2-methylfuran, k(2‑MF) = (1.91 ± 0.32) × 10–11, 3-methylfuran, k(3‑MF) = (1.49 ± 0.33) × 10–11, 2,5-dimethylfuran, k(2,5‑DMF) = (5.82 ± 1.21) × 10–11, and 2,3,5-trimethylfuran, k(2,3,5‑TMF) = (1.66 ± 0.69) × 10–10. The uncertainty on the measured rate coefficient (ΔkFC) includes both the uncertainty on the measurement and that on the rate coefficient of the reference molecule. To our knowledge, this work represents the first determination for the rate coefficient of the 2,3,5-TMF reaction with NO3. This work shows that the reaction between furan and methylated furan compounds with nitrate radical (NO3) is the dominant removal pathway during the night with lifetimes between 0.5 and 55 min for the studied molecules

Mineral Oxides Change the Atmospheric Reactivity of Soot: NO₂ Uptake under Dark and UV Irradiation Conditions

The heterogeneous reactions between trace gases and aerosol surfaces have been widely studied over the past decades, revealing the crucial role of these reactions in atmospheric chemistry. However, existing knowledge on the reactivity of mixed aerosols is limited, even though they have been observed in field measurements. In the current study, the heterogeneous interaction of NO₂ with solid surfaces of Al₂O₃ covered with kerosene soot was investigated under dark conditions and in the presence of UV light. Experiments were performed at 293 K using a low-pressure flow-tube reactor coupled with a quadrupole mass spectrometer. The steady-state uptake coefficient, γ_ss, and the distribution of the gas-phase products were determined as functions of the Al₂O₃ mass; soot mass; NO₂ concentration, varied in the range of (0.2–10) × 10¹² molecules cm^–3; photon flux; and relative humidity, ranging from 0.0032% to 32%. On Al₂O₃/soot surfaces, the reaction rate was substantially increased, and the formation of HONO was favored compared with that on individual pure soot and pure Al₂O₃ surfaces. Uptake of NO₂ was enhanced in the presence of H₂O under both dark and UV irradiation conditions, and the following empirical expressions were obtained: γ_ss,BET,dark = (7.3 ± 0.9) × 10^–7 + (3.2 ± 0.5) × 10^–8 × RH and γ_ss,BET,UV = (1.4 ± 0.2) × 10^–6 + (4.0 ± 0.9) × 10^–8 × RH. Specific experiments, with solid sample preheating and doping with polycyclic aromatic hydrocarbons (PAHs), showed that UV-absorbing organic compounds significantly affect the chemical reactivity of the mixed mineral/soot surfaces. A mechanistic scheme is proposed, in which Al₂O₃ can either collect electrons, initiating a sequence of redox reactions, or prevent the charge-recombination process, extending the lifetime of the excited state and enhancing the reactivity of the organics. Finally, the atmospheric implications of the observed results are briefly discussed

Water Interaction with Mineral Dust Aerosol: Particle Size and Hygroscopic Properties of Dust

For many years, the interaction between dust particles and water molecules has been a subject of interest for the atmospheric sciences community. However, the influence of the particle size on the hygroscopicity of mineral particles is poorly evaluated. In the current study, diffused reflectance infrared Fourier transform (DRIFT) spectroscopy is used to evaluate the <i>in situ</i> water adsorption on natural Arizona test dust (ATD) particles. Five different ATD size fractions, 0–3, 5–10, 10–20, 20–40, and 40–80 μm, are used, corresponding to the entire range of uplifted mineral particles in the atmosphere (<100 μm). N₂ sorption measurement, particle size distribution, and elemental analyses are performed to determine the physicochemical properties of the samples. The water adsorption experiments are conducted in an optical cell under flow conditions at room temperature and under the relative humidity (RH) range of 2–90%. Experimental results are simulated with a modified three-parameter Brunauer–Emmett–Teller (BET) equation. Water monolayers are found to be formed at 13 ± 1, 17 ± 1, 22 ± 2, 25 ± 2, and 28 ± 2% RH for ATD of 0–3, 5–10, 10–20, 20–40, and 40–80 μm, respectively. Additional water layers are formed at higher RH conditions. Thorough comparisons point that smaller particles adsorb water more efficiently. To better assess the impact of size on water uptake, for the first time, the desorption kinetics of water are determined. It is found that water desorption follows second-order kinetics, and results are fitted to determine the desorption rate coefficients for each dust grade. As a conclusion, results provide evidence that the size distribution is a key factor influencing water uptake onto mineral dust that could impact mineral particle scattering ability, adsorption, and photoreactivity properties

Photodegradation of Pyrene on Al₂O₃ Surfaces: A Detailed Kinetic and Product Study

In the current study, the photochemistry of pyrene on solid Al₂O₃ surface was studied under simulated atmospheric conditions (pressure, 1 atm; temperature, 293 K; photon flux, <i>J</i>_NO₂ = 0.002–0.012 s^–1). Experiments were performed using synthetic air or N₂ as bath gas to evaluate the impact of O₂ to the reaction system. The rate of pyrene photodegradation followed first order kinetics and was enhanced in the presence of O₂, <i>k</i>_d(synthetic air) = 7.8 ± 0.78 × 10^–2 h^–1 and <i>k</i>_d(N₂) = 1.2 ± 0.12 × 10^–2 h^–1 respectively, due to the formation of the highly reactive O₂^•– and HO^• radical species. In addition, <i>k</i>_d was found to increase linearly with photon flux. A detailed product study was realized and for the first time the gas/solid phase products of pyrene oxidation were identified using off-line GC-MS and HPLC analysis. In the gas phase, acetone, benzene, and various benzene-ring compounds were determined. In the solid phase, more than 20 photoproducts were identified and their kinetics was followed. Simulation of the concentration profiles of 1- and 2-hydroxypyrene provided an estimation of their yields, 33% and 5.8%, respectively, with respect to consumed pyrene, and their degradation rates were extracted. Finally, the mechanism of heterogeneous photodegradation of pyrene is discussed

OH Radical and Chlorine Atom Kinetics of Substituted Aromatic Compounds: 4‑Chlorobenzotrifluoride (<i>p</i>‑ClC₆H₄CF₃)

The mechanisms for the OH radical and Cl atom gas-phase reaction kinetics of substituted aromatic compounds remain a topic of atmospheric and combustion chemistry research. 4-Chlorobenzotrifluoride (p-chlorobenzotrifluoride, p-ClC6H4CF3, PCBTF) is a commonly used substituted aromatic volatile organic compound (VOC) in solvent-based coatings. As such, PCBTF is classified as a volatile chemical product (VCP) whose release into the atmosphere potentially impacts air quality. In this study, rate coefficients, k1, for the OH + PCBTF reaction were measured over the temperature ranges 275–340 and 385–940 K using low-pressure discharge flow-tube reactors coupled with a mass spectrometer detector in the ICARE/CNRS (Orléans, France) laboratory. k1(298–353 K) was also measured using a relative rate method in the thermally regulated atmospheric simulation chamber (THALAMOS; Douai, France). k1(T) displayed a non-Arrhenius temperature dependence with a negative temperature dependence between 275 and 385 K given by k1(275–385 K) = (1.50 ± 0.15) × 10–14 exp((705 ± 30)/T) cm3 molecule–1 s–1, where k1(298 K) = (1.63 ± 0.03) × 10–13 cm3 molecule–1 s–1 and a positive temperature dependence at elevated temperatures given by k1(470–950 K) = (5.42 ± 0.40) × 10–12 exp(−(2507 ± 45) /T) cm3 molecule–1 s–1. The present k1(298 K) results are in reasonable agreement with two previous 296 K (760 Torr, syn. air) relative rate measurements. The rate coefficient for the Cl-atom + PCBTF reaction, k2, was also measured in THALAMOS using a relative rate technique that yielded k2(298 K) = (7.8 ± 2) × 10–16 cm3 molecule–1 s–1. As part of this work, the UV and infrared absorption spectra of PCBTF were measured (NOAA; Boulder, CO, USA). On the basis of the UV absorption spectrum, the atmospheric instantaneous UV photolysis lifetime of PCBTF (ground level, midlatitude, Summer) was estimated to be 3–4 days, assuming a unit photolysis quantum yield. The non-Arrhenius behavior of the OH + PCBTF reaction over the temperature range 275 to 950 K is interpreted using a mechanism for the formation of an OH–PCBTF adduct and its thermochemical stability. The results from this study are included in a discussion of the OH radical and Cl atom kinetics of halogen substituted aromatic compounds for which only limited temperature-dependent kinetic data are available