Reactividad atmosférica y astroquímica de compuestos oxigenados y potenciales sustitutos de gases de efectos invernadero.

This doctoral thesis is divided into two parts. Part 1 concerns the gas-phase reactivity of fluorinated and oxygenated compounds under physical conditions of the Earth's atmosphere and Part 2 focuses on the gas-phase reactivity of oxygenated compounds at temperatures of the interstellar medium (ISM). Although these environments appear to be very different (and they are, in terms of temperature and pressure), they have in common that the first oxidation step of organic molecules could occur by means of their reaction with hydroxyl radicals (OH), which are present in both environments.
\nPART 1. Atmospheric reactivity of fluorinated and oxygenated compounds
\nThe emission of a pollutant coming from anthropogenic and/or natural sources has an important effect on the oxidizing capacity of our atmosphere. Fluorinated compounds are found among the most important anthropogenic pollutants. Thus, the use of chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), significant (stratospheric) ozone depleting substances, were banned in the Montreal Protocol (1989). Moreover, the C-F bonds strongly absorb the infrared (IR) radiation contributing to the global warming of the atmosphere, in a greater or lesser extent depending on its atmospheric lifetime (?). Since then, several replacements for CFCs and HCFCs have been proposed, e.g. hydrofluorocarbons (HFCs), which generally react faster, although they still had a high impact. In the past decades, hydrofluoethers (HFEs) have been proposed as good replacements for HFCs since the ether group confers a higher reactivity towards atmospheric oxidants, reducing the effect on the global warming despite the large absorption in the IR region (Blowers et al., 2008). For that reason, a good HFC substitute is one that is quicky depleted in the atmosphere, despite the large absorption of IR radiation.
\nThe fluorinated compound that has the greatest impact on the global warming of the atmosphere is SF6. In 2011, the global annual mean atmospheric concentration of SF6 was 7.3 ppt, increasing up to 10 ppt in 2019 (IPCC, 2021). The global warming potential (GWP) of SF6 relative to CO2 is 26700 at a horizon time of 100 years (Hodnebrog et al., 2020). Nowadays, perfluoronitriles (PFNs) have been proposed as SF6 substitutes (Hyrenbach et al., 2015; Kieffel et al., 2015). Even though PFNs have no H atom in their molecular structure, they are expected to react faster than SF6 (3200 years), reducing GWP.
\nFor that reason, in the present PhD thesis the atmospheric chemistry of the following FLUORINATED COMPOUNDS has been investigated: CF3CH2OCH3, CHF2CF2CH2OCH3, CF3CF2CH2OCH3, CH2=CHCH2OCF2CHF2, and (CF3)2CFCN.
\nAnother class of pollutants emitted into the atmosphere both from anthropogenic and natural sources are the ALDEHYDES, such as the ones studied in this PhD thesis: CH3CH2CH(CH3)CHO (2-methylbutanal, 2MB hereinafter) and CH3CH2CH2CH(CH3)CHO (2-methylpentanal, 2MP hereinafter). Some sources of 2MB are the fermentation and drying processes of cocoa beans (Utrilla-Vázquez et al., 2020), the manufacturing process of tea leaves (Flaig et al., 2020) or emissions as a consequence of the stress suffered by grapevine leaves due to drought (Griesser et al., 2015). 2MP is a volatile component of some foods (Oser y Ford, 1973; Mazza et al., 1980; Ahn et al., 1999; Aisala et al., 2019) and water (Bao et al., 1998). The atmospheric degradation of these aldehydes may contribute to the formation of secondary pollutants that may have a significant impact on tropospheric chemistry and air pollution at a local or regional scale.
\nOnce emitted, the overall ? of a pollutant R can be estimated from the individual lifetimes due to its reaction with an atmospheric oxidant (Ox = OH radicals, chlorine atoms, nitrate radicals, or ozone) as follows:
\n1??=?1??Ox Eq. 1
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\n?Ox is estimated from the rate coefficient for the Ox + R reaction at 298 K (kOx) and the average concentration of Ox in the atmosphere ([Ox]) according to the following expression:
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\n??Ox=1??Ox[Ox] Eq. 2
\nIn this PhD thesis, the kinetic measurements of the gas-phase rate coefficients for the OH-reactions with the pollutants mentioned above are presented as a function of temperature and total pressure, k(T,P), in a range simulating the T and P conditions of the troposphere.
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\nKinetic measurements of the gas-phase OH-reactions
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\nThe rate coefficients for the OH-reactions with HFEs, a PFN, and aldehydes as a function of temperature and total pressure, k(T,P), were determined using the absolute method based on the Pulsed Laser Photolysis-Laser Induced Fluorescence (PLP-LIF) technique. The experimental system employed has been already described elsewhere (Albaladejo et al., 2002; Antiñolo et al., 2011; Blázquez et al., 2022).
\nA slow flow Pyrex reaction cell (~200 cm3) was used to introduce a gas mixture containing the pollutant diluted in helium, the small flow passing through a liquid solution of the OH-precursor (H2O2 or HNO3), and a main flow of helium (bath gas). The total pressure in the reactor P was adjusted by means of a needle valve at the gas exit of the reaction cell. The reactor temperature T was varied by circulating a heating/cooling liquid through an external jacket. The liquid used in the thermostatic bath was water for T > 273 K or ethanol for T ? 273 K.
\nThe OH radicals were generated in situ by the PLP at 248 nm of the OH precursor by using a KrF excimer laser. At reaction times t 0, OH were excited at 282 nm by means of the doubling output of a dye laser pumped by a Nd-YAG laser. The excited OH radicals emit the laser induced fluorescence (LIF) at ~308 nm, which was detected by a photomultiplier tube and converted into an electrical signal (ILIF). The temporal evolution of ILIF was obtained by the combination of two pulse delay generators: one of them starts the pulse sequence and determines the initial time (t=0, photolysis) and the other one varies the delay between the photolysis and the excitation lasers and the LIF detection (t variable). Under pseudo-first order conditions ([OH]0<<[R]), ILIF decays exponentially:
\n??LIF=??LIF,0 ???????? Eq. 3 where ??LIF,0 is the LIF intensity at time zero and k' is the pseudo-first order rate coefficient, which depends on the concentration of the reagent (HFE, PFN, or aldehyde), [R], and the loss rate of OH in the absence of compound, mainly due to the reaction with its precursor (k'0):
\nk' = k(T,P) [R] + k'0 Eq. 4
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\nNo pressure dependence of k(T,P) was found in the investigated total pressure range of 50 - 600 Torr for any investigated reaction. Therefore, the rate coefficient is designated simply as k(T). The temperature dependence of k(T) in the temperature range of 263 - 358 K is well-described by the Arrhenius expression:
\n??(??)=???????a???? Eq. 5
\nwhere A is the pre-exponential factor, Ea is the activation energy, and R is the ideal gas constant. A summary of the kinetic results obtained in the present PhD thesis is given in Table 1. This summary includes the rate coefficient at room temperature, k(298 K), and the Arrhenius parameters, A and Ea.
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\nTable 1. Summary of OH-rate coefficients at room temperature and Arrhenius parameters obtained in this work.
\n
\nCompound\tk(298 K) /
\ncm3 molecule-1 s-1\tT-range
\n/ K\tA / 10-12 cm3 molecule-1 s-1\tEa / kJ mol-1
\nCF3CH2OCH3\t(6.88±0.21)×10-13\t263-353\t3.88±0.89\t4.22±0.57
\nCHF2CF2CH2OCH3\t(9.91±0.24)×10-13\t263-353\t2.81±0.33\t2.60±0.29
\nCF3CF2CH2OCH3\t(8.88±0.20)×10-13\t263-353\t2.60±0.31\t2.65±0.15
\nCH2=CHCH2OCF2CHF2\t(1.40±0.05)×10-11\t263-353\t2.30±0.35\t-4.53±0.38
\n(CF3)2CFCN\t(1.51±0.25)×10-15\t278-358\t0.59±0.32\t15.4±1.3
\nCH3CH2CH(CH3)CHO\t(2.68±0.07)×10-11\t263-353\t8.88±0.81\t-2.75±0.23
\nCH3CH2CH2CH(CH3)CHO\t(3.28±0.08)×10-11\t263-353\t6.18±0.94\t-4.23±0.38
\nUncertainties are two times the standard deviation.
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\nAs it can be seen in Table 1, Ea is positive for the OH-reaction with saturated HFEs and the PFN, while it is negative for CH2=CHCH2OCF2CHF2 and aldehydes. A positive activation energy may imply that the H-abstraction (for example in HFEs) by OH is an elementary reaction or, if it is a complex mechanism, the sum of the activation energies of all steps is positive. For unsaturated molecules with H atoms in their structure, the OH-addition to the double/triple bond is another possible pathway. Thus, the reaction of CH2=CHCH2OCF2CHF2 with OH may take place both by H-abstraction and OH- addition to the double bond. However, for unsaturated molecules without H atoms, like (CF3)2CFN, the OH-addition to the triple bond is the most probable reaction pathway. In the aldehydes, the reaction is likely to occur through the H-abstraction by the OH radical via a complex mechanism, resulting in a global negative activation energy.
\nKinetic measurements of the gas-phase Cl-reaction with CH2=CHCH2OCF2CHF2
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\nDuring a three-months stay in the Department of Chemistry of the University of Crete, Heraklion (Greece), under the supervision of Dr. Vassileios C. Papadimitriou, the rate coefficient for the Cl-reaction with CH2=CHCH2OCF2CHF2 was measured as a func-tion of temperature (260 - 363 K) and pressure (34 - 721 Torr). The measurements were made using a relative rate method using a FTIR (Fourier Transform InfraRed) spectrometer with a DLaTGS (Deuterated Lanthanum ?-Alanine doped TriGlycine Sulphate) detector and a 15.1-cm single path cell. Ethane, propane, and iso-butane were used as reference compounds. CH2=CHCH2OCF2CHF2 was introduced into a pho-tolysis cell together with the reference compound and the precursor of Cl atoms ((COCl)2), which were diluted until the desired total pressure (~700 Torr of synthetic air for high pressure measurements or ~35 Torr of oxygen for low pressure measure-ments) was reache

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