UV Absorption Cross Sections Research Articles

Atmospheric impact of 2-methylpentanal emissions: kinetics, photochemistry, and formation of secondary pollutants

Abstract. The tropospheric fate of 2-methylpentanal (2MP) has been investigated in this work. First, the photochemistry of 2MP under simulated solar conditions was investigated by determining the UV absorption cross sections (220–360 nm) and the effective photolysis quantum yield in the UV solar actinic region (λ > 290 nm). The photolysis rate coefficient in that region was estimated using a radiative transfer model. Photolysis products were identified by Fourier transform infrared (FTIR) spectroscopy. Secondly, a kinetic study of the chlorine (Cl) and hydroxyl (OH) reactions of 2MP was also performed at 298 K and as a function of temperature (263–353 K), respectively. For the Cl reaction, a relative kinetic method was used in a smog chamber coupled to FTIR spectroscopy, whereas for the OH reaction, the pulsed laser photolysis (PLP) with laser-induced fluorescence (LIF) technique was employed. The estimated lifetime of 2MP depends on the location, the season, and the time of the day. Under mild–strong irradiation conditions, UV photolysis of 2MP may compete with its OH reaction in a mid-latitude inland urban atmosphere, while Cl reaction dominates in mid-latitude coastal urban areas at dawn. Finally, the gaseous product distribution of the Cl and OH reactions was measured in a smog chamber as well as the formation of secondary organic aerosols (SOAs) in the Cl reaction and its size distribution (diameter between 5.6 and 560 nm). The implications for air quality are discussed based on the observed products.

Quantifying BrO and SO2 distributions in volcanic plumes—Recent advances in imaging Fabry-Pérot interferometer correlation spectroscopy

Bromine monoxide (BrO) and sulphur dioxide (SO2) are two gases frequently observed in volcanic plumes by spectroscopic techniques capable of continuous gas monitoring like, e.g., Differential Optical Absorption Spectroscopy (DOAS). The spatio-temporal resolution of DOAS measurements, however, only allows to determine average gas fluxes (minutes to hours resolution). In particular, it is insufficient to record two-dimensional images of SO2 and BrO in real-time (seconds time resolution). Thus, it is impossible to resolve details of chemical conversions of reactive plume constituents. However, these details are vital for further understanding reactive halogen chemistry in volcanic plumes. Therefore, instruments that combine high spatio-temporal resolution and high gas sensitivity and selectivity are required. In addition, these instruments must be robust and compact to be suitable for measurements in harsh and remote volcanic environments. Imaging Fabry-Pérot interferometer (FPI) correlation spectroscopy (IFPICS) is a novel technique for atmospheric trace gas imaging. It allows measuring atmospheric gas column density (CD) distributions with a high spatial and temporal resolution, while at the same time providing selectivity and sensitivity comparable to DOAS measurements. IFPICS uses the periodic transmission spectrum of an FPI, that is matched to the periodic narrowband (vibrational) absorption features of the target trace gas. Recently, IFPICS has been successfully applied to volcanic SO2. Here we demonstrate the applicability of IFPICS to much weaker (about two orders of magnitude) trace gas optical densities, such as that of BrO in volcanic plumes. Due to its high reactivity, BrO is extremely difficult to handle in the laboratory. Thus, based on the similarity of the UV absorption cross sections, we used formaldehyde (HCHO) as a spectral proxy for BrO in instrument characterization measurements. Furthermore, we present recent advances in SO2 IFPICS measurements and simultaneous measurements of SO2 and BrO from a field campaign at Mt Etna in July 2021. We find photon shot-noise limited detection limits of 4.7 × 1017 molec s0.5 cm−2 for SO2 and of 8.9 × 1014 molec s0.5 cm−2 for BrO at a spatial resolution of 512 × 512 pixels and 200 × 200 pixels, respectively. Furthermore, an estimate for the BrO to SO2 ratio (around 10–4) in the volcanic plume is given. The prototype instrument presented here provides spatially resolved measurements of the reactive volcanic plume component BrO. The temporal resolution of our approach allows studies of chemical conversions inside volcanic plumes on their intrinsic timescale.

Signal decay and recovery in matrix-assisted laser desorption/ionization and the thermal model

In matrix-assisted laser desorption/ionization (MALDI), as a UV laser continuously irradiates the same spot on a sample and before the sample is completely depleted, the ion signals decrease as the number of laser shots increases. This prevents the complete usage of the sample and decreases the detection sensitivity of MALDI. In this study, we investigated this phenomenon by conducting experiments involving four commonly used matrices: α-cyano-4-hydroxycinnamic acid (CHCA), sinapinic acid (SA), ferulic acid (FA), and 2,5-dihydroxybenzoic acid (2,5-DHB). As the number of laser shots increased, the numbers of ions desorbed from CHCA, SA, and FA decreased much faster than that desorbed from 2,5-DHB. No decrease in photon absorption was observed, but the decrease in the number of desorbed ions was accompanied by a decrease in the number of desorbed neutrals, an increase in the ratio of analyte to matrix ions, a decrease in surface temperature, and changes in the UV fluorescence spectra. These results suggest photochemical products were generated on the surfaces of the samples. These photochemical products have smaller UV absorption cross sections than do the original molecules, resulting in the aforementioned decreases in surface temperature and the numbers of desorbed ions and neutrals. These changes can be described in terms of the thermal model of MALDI. The photochemical products can be removed by a single high-fluence laser shot, and the ion signals can be partially recovered. The possible photochemical products of the reactions involved in MALDI are discussed herein.

Shock wave and modelling study of the unimolecular dissociation of Si(CH3)2F2: an access to spectroscopic and kinetic properties of SiF2.

The thermal dissociation of Si(CH3)2F2 was studied in shock waves between 1400 and 1900 K. UV absorption-time profiles of its dissociation products SiF2 and CH3 were monitored. The reaction proceeds as a unimolecular process not far from the high-pressure limit. Comparing modelled and experimental results, an asymmetric representation of the falloff curves was shown to be most realistic. Modelled limiting high-pressure rate constants agreed well with the experimental data. The UV absorption spectrum of SiF2 was shown to be quasi-continuous, with a maximum near 222 nm and a wavelength-integrated absorption cross section of 4.3 (±1) × 10-23 cm3 (between 195 and 255 nm, base e), the latter being consistent with radiative lifetimes from the literature. Experiments over the range 1900-3200 K showed that SiF2 was not consumed by a simple bond fission SiF2 →SiF + F, but by a bimolecular reaction SiF2 + SiF2 → SiF + SiF3 (rate constant in the range 1011-1012 cm3 mol-1 s-1), followed by the unimolecular dissociation SiF3 → SiF2 + F such that the reaction becomes catalyzed by the reactant SiF2. The analogy to a pathway CF2 + CF2 → CF + CF3, followed by CF3 → CF2 + F, in high-temperature fluorocarbon chemistry is stressed. Besides the high-temperature absorption cross sections of SiF2, analogous data for SiF are also reported.

UV absorption cross sections of vaporized 1-methylnaphthalene at elevated temperatures

In this work, the UV absorption cross sections of the fluorescence tracer 1-methylnaphthalene are determined in the range of 230–330 nm. The experiments are performed in a continuously scavenged gas flow cell, which allows for defined homogeneous conditions regarding temperature, pressure, and tracer/fuel composition. A LSDS (laser driven light source) is used for irradiation, which enables high spectral emission intensities in the UV range studied. For detection, a spectrograph in combination with an intensified camera is applied. Absorption cross sections at temperatures up to 850 K are determined and compared to sparsely available published data. Possible uncertainties caused by the optical setup and the flow cell, respectively, are considered.

Photolysis and oxidation by OH radicals of two carbonyl nitrates: 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone

Abstract. Multifunctional organic nitrates, including carbonyl nitrates, are important species formed in NOx-rich atmospheres by the degradation of volatile organic compounds (VOCs). These compounds have been shown to play a key role in the transport of reactive nitrogen and, consequently, in the ozone budget; they are also known to be important components of the total organic aerosol. However, very little is known about their reactivity in both the gas and condensed phases. Following a previous study that we published on the gas-phase reactivity of α-nitrooxy ketones, the photolysis and reaction with OH radicals of 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone (which are a β-nitrooxy ketone and γ-nitrooxy ketone, respectively) were investigated for the first time in simulation chambers. The photolysis frequencies were directly measured in the CESAM chamber, which is equipped with a very realistic irradiation system. The jnitrate/jNO2 ratios were found to be (5.9±0.9)×10-3 for 4-nitrooxy-2-butanone and (3.2±0.9)×10-3 for 5-nitrooxy-2-pentanone under our experimental conditions. From these results, it was estimated that ambient photolysis frequencies calculated for typical tropospheric irradiation conditions corresponding to the 1 July at noon at 40∘ N (overhead ozone column of 300 and albedo of 0.1) are (6.1±0.9)×10-5 s−1 and (3.3±0.9)×10-5 s−1 for 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone, respectively. These results demonstrate that photolysis is a very efficient sink for these compounds with atmospheric lifetimes of few hours. They also suggest that, similarly to α-nitrooxy ketones, β-nitrooxy ketones have enhanced UV absorption cross sections and quantum yields equal to or close to unity and that γ-nitrooxy ketones have a lower enhancement of cross sections, which can easily be explained by the larger distance between the two chromophore groups. Thanks to a product study, the branching ratio between the two possible photodissociation pathways is also proposed. Rate constants for the reaction with OH radicals were found to be (2.9±1.0)×10-12 and (3.3±0.9)×10-12 cm3 molecule−1 s−1, respectively. These experimental data are in good agreement with rate constants estimated by the structure–activity relationship (SAR) of Kwok and Atkinson (1995) when using the parametrization proposed by Suarez-Bertoa et al. (2012) for carbonyl nitrates. Comparison with photolysis rates suggests that the OH-initiated oxidation of carbonyl nitrates is a less efficient sink than photodissociation but is not negligible in polluted areas.

CH2OO Criegee intermediate UV absorption cross-sections and kinetics of CH2OO + CH2OO and CH2OO + I as a function of pressure.

The UV absorption cross-sections of the Criegee intermediate CH2OO, and kinetics of the CH2OO self-reaction and the reaction of CH2OO with I are reported as a function of pressure at 298 K. Measurements were made using pulsed laser flash photolysis of CH2I2/O2/N2 gas mixtures coupled with time-resolved broadband UV absorption spectroscopy at pressures between 6 and 300 Torr. Results give a peak absorption cross-section of (1.37 ± 0.29) × 10-17 cm2 at ∼340 nm and a rate coefficient for the CH2OO self-reaction of (8.0 ± 1.1) × 10-11 cm3 s-1, with no significant pressure dependence of the absorption cross-sections or the self-reaction kinetics over the range investigated. The rate coefficient for the reaction between CH2OO and I demonstrates pressure dependence over the range investigated, with a Lindemann fit giving k0 = (4.4 ± 1.0) × 10-29 cm6 s-1 and k∞ = (6.7 ± 0.6) × 10-11 cm3 s-1. The origins of IO in the system have been investigated, the implications of which are discussed.

UV attenuation by Martian brines

UV radiation on the surface of Mars is an important factor affecting the survivability of microorganisms on Mars; a habitable niche on Mars would be one that provides protection from harmful UV radiation. The possibility of Martian brines providing a habitable niche on Mars via attenuation of UV radiation is investigated. Minerals could have leached into bodies of water on early Mars and may remain present in the polar ice caps. Aqueous solutions of nine different salts found on the surface of Mars were prepared and UV absorption cross sections were derived in the spectral range 230–400 nm. The salts investigated were Fe(NO3)3, Fe2(SO4)3, FeSO4, FeCl2, MnSO4, CaSO4, MgSO4, MgCl2, and NaCl. UV absorption cross sections found ranged from 10−24 to 10−17 cm2molecule−1, with the cation primarily affecting absorption strength. In particular, salts containing Fe3+ were found to absorb significant amounts of UV radiation. Combining the derived absorption cross sections with the solar radiation at Mars, transmitted UV radiation was simulated for different combinations of depth and salt concentration. Greater than 95% UV absorption from 230 to 400 nm is possible with iron salts at depths and concentrations expected to be found on past and present-day Mars. The results of this work are promising for investigating the habitability of Mars and other rocky planets that lack an atmosphere capable of attenuating UV radiation.

On the Photodesorption of CO2 Ice Analogs: The Formation of Atomic C in the Ice and the Effect of the VUV Emission Spectrum

Abstract CO2 ice has a phase transition at 30 K when its structure changes from amorphous to crystalline. Using reflection absorption infrared spectroscopy, Öberg et al. observed that the photodesorption yield of CO2 ice deposited at 60 K and irradiated at 18 K is 40% lower than that of CO2 ice deposited and irradiated at 18 K. In this work, CO2 ices were deposited at 16–60 K and UV-irradiated at 16 K to rule out the temperature effect and figure out the relationship between photodesorption yield and ice structure. IR spectroscopy is a common method used for measurement of the photodesorption yield in ices. We found that undetectable C atoms produced in irradiated CO2 ice can account for 33% of the amount of depleted CO2 molecules in the ice. A quantitative calibration of QMS was therefore performed to convert the measured ion current into photodesorption yield. During various irradiation periods, the dominant photodesorbing species were CO, O2, and CO2, and their photodesorption yields in CO2 ices deposited at different temperature configurations were almost the same, indicating that ice morphology has no effect on the photodesorption yield of CO2 ice. In addition, we found that the lower desorption yield reported by Martín-Doménech et al. is due to a linear relationship between the photodesorption yield and the combination of the energy distribution of the Microwave-Discharge Hydrogen-flow Lamp and UV absorption cross section of ices.

UV absorption spectra of trans-2-pentenal, trans-2-hexenal and 2-methyl-2-pentenal

C5 and C6 unsaturated aldehydes are a category of BVOCs released by plants in response to their wounds and stress. In this work, the gas phase UV absorption spectra of trans-2-pentenal (t-2P), trans-2-hexenal (t-2H) and 2-methyl-2-pentenal (2M2P) are determined between 240 and 400 nm, at room temperature and at pressures (1–9) torr. The reported UV absorption cross sections are (cm2molecule−1): σt-2P, 330 = (4.86 ± 0.53) × 10−20, σt-2H, 330 = (5.43 ± 0.57) × 10−20 and σ2M2P, 320 = (6.62 ± 0.50) × 10−20. Their photolysis constants as well as their atmospheric lifetimes toward photolysis were then estimated.

UV absorption of Criegee intermediates: quantitative cross sections from high-level ab initio theory.

Criegee intermediates (CIs) are increasingly recognized as important intermediates in atmospheric and combustion chemistry. Modelling in these fields requires reliable characterization of the CI's UV absorption cross sections. Different experimental approaches provided seemingly contradictory results both for the simplest CI as well as for higher analogues. To resolve these discrepancies, we model simultaneously the positions, widths, shapes and absolute intensities of the first absorption bands of CIs using the semiclassical reflection principle approach based on the path integral molecular dynamics. The UV spectra were efficiently processed via the kernel density estimation method. We show that the multireference character complicates the description of the system and the appropriate treatment of the electron correlation is vital. Multi-reference methods with dynamical correlation are problematic due to high sensitivity of the results with respect to the active space. Single-reference based methods become reliable once the triple excitations components are included. Interestingly, the very similar CC2 and ADC(2) methods provide highly conflicting results which are, however, reconciled at the CC3 and ADC(3) levels. To calculate the spectra of higher CIs, we introduce composite EOM-CCSD/T and CC2/3 methods providing excellent absorption cross sections at an acceptable computational cost. We provide the first absolute intensities for individual conformers of the CH3CHOO molecule.

Atmospheric Chemistry of E- and Z-CF3CH═CHF (HFO-1234ze): OH Reaction Kinetics as a Function of Temperature and UV and IR Absorption Cross Sections.

We report here the rate coefficients for the OH reactions (kOH) with E-CF3CH═CHF and Z-CF3CH═CHF, potential substitutes of HFC-134a, as a function of temperature (263-358 K) and pressure (45-300 Torr) by pulsed laser photolysis coupled to laser-induced fluorescence techniques. For the E-isomer, the existing discrepancy among previous results on the T dependence of kOH needs to be elucidated. For the Z-isomer, this work constitutes the first absolute determination of kOH. No pressure dependence of kOH was observed, while kOH exhibits a non-Arrhenius behavior: kOH(E) = [Formula: see text] and kOH(Z) = [Formula: see text] cm3 molecule-1 s-1, where uncertainties are 2σ. UV absorption cross sections, σλ, are reported for the first time. From σλ and considering a photolysis quantum yield of 1, an upper limit for the photolysis rate coefficients and lifetimes due to this process in the troposphere are estimated: 3 × 10-8 s-1 and >1 year for the E-isomer and 2 × 10-7 s-1 and >2 months for Z-CF3CH═CHF, respectively. Under these conditions, the overall estimated tropospheric lifetimes are 15 days (for the E-isomer) and 8 days (for the Z-isomer), the major degradation pathway being the OH reaction, with a contribution of the photolytic pathway of less than 3% (for E) and 13% (for Z). IR absorption cross sections were determined both experimentally (500-4000 cm-1) and theoretically (0-2000 cm-1). From the theoretical IR measurements, it is concluded that the contribution of the 0-500 cm-1 region to the total integrated cross sections is appreciable for the E-isomer (9%) but almost negligible for the Z-isomer (0.5%). Nevertheless, the impact on their radiative efficiency and global warming potential is negligible.

Gas-phase UV absorption cross-sections and photolysis kinetics of 4-hydroxy-3-hexanone: Atmospheric implications

The UV absorption spectrum and the photolysis reaction kinetics of 4-hydroxy-3-hexanone (4H3H) in the 275–330nm region have been investigated for the first time. The photolysis frequency of this compound in the atmosphere was evaluated using actinometry and found to be J4H3H,atm=(5.9±3.1)×10−3h−1. An atmospheric effective quantum yield was estimated to about 10% and a photolysis lifetime of about 7days was determined. The comparison with the reactivity of other hydroxyketones suggests that the kinetics of hydroxyketones photolysis depends on their structure, mainly the branching and the proximity of the two functions.

UV absorption and fluorescence properties of gas-phase p-difluorobenzene

1,4-Difluorobenzene (p-DFB) is a promising aromatic tracer for determining concentration, temperature, and O2 partial pressure in mixing gas flows based on laser-induced fluorescence (LIF). Signal quantification requires the knowledge of absorption and fluorescence properties as a function of environmental conditions. We report absorption and fluorescence spectra as well as fluorescence lifetimes of p-DFB in the temperature, pressure, and oxygen partial pressure range that is relevant for many applications including internal combustion engines. The UV absorption cross section, investigated between 296 and 675 K, has a peak value close to 266 nm and decreases with temperature, while still exceeding other single-ring aromatics. Time-resolved fluorescence spectra were recorded after picosecond laser excitation at 266 nm as a function of temperature (296–1180 K), pressure (1–10 bar), and O2 partial pressure (0–210 mbar) using a streak camera (temporal resolution 50 ps) coupled to a spectrometer. The fluorescence spectra red-shift (~2 nm/100 K) and broaden (increase in full width at half maximum by 58% in the investigated temperature range) with temperature. In N2 as bath gas (1 bar), the fluorescence lifetime τ eff decreases with temperature by a factor of about 20 (from 7 ns at 298 K down to 0.32 ns at 1180 K), while at 8 bar the shortest lifetime at 975 K is 0.4 ns. A noticeable pressure dependence (i.e., reduced τ eff) is only visible at 675 K and above. Quenching of p-DFB LIF by O2 (for partial pressures up to 210 mbar) shortens the fluorescence lifetime significantly at room temperature (by a factor of 8), but much less at higher temperatures (by a factor of 1.8 at 970 K). For fixed O2 partial pressures (52 mbar and above), τ eff shows a plateau region with temperature which shifts toward higher temperatures at the higher O2 partial pressures. O2 quenching is less prominent for p-DFB compared to other aromatic compounds investigated so far. The temperature dependence of O2 quenching can be approximately expressed by an exponential function. The influence of temperature, total pressure, and O2 partial pressure on absorption cross sections and fluorescence quantum yields are given as empirical functions that allow for interpolation. For typical applications, p-DFB LIF provides up to three orders of magnitude stronger signal compared to toluene LIF.

Atmospheric Chemistry of 1-Methoxy 2-Propyl Acetate: UV Absorption Cross Sections, Rate Coefficients, and Products of Its Reactions with OH Radicals and Cl Atoms.

The rate coefficients for the reactions of OH and Cl with 1-methoxy 2-propyl acetate (MPA) in the gas phase were measured using absolute and relative methods. The kinetic study on the OH reaction was conducted in the temperature (263-373) K and pressure (1-760) Torr ranges using the pulsed laser photolysis-laser-induced fluorescence technique, a low pressure fast flow tube reactor-quadrupole mass spectrometer, and an atmospheric simulation chamber/GC-FID. The derived Arrhenius expression is kMPA+OH(T) = (2.01 ± 0.02) × 10-12 exp[(588 ± 123/T)] cm3 molecule-1 s-1. The absolute and relative rate coefficients for the reaction of Cl with MPA were measured at room temperature in the flow reactor and the atmospheric simulation chamber, which led to k(Cl+MPA) = (1.98 ± 0.31) × 10-10 cm3 molecule-1 s-1. GC-FID, GC-MS, and FT-IR techniques were used to investigate the reaction mechanism in the presence of NO. The products formed from the reaction of MPA with OH and their yields were methyl formate (80 ± 7.3%), acetic acid (50 ± 4.8%), and acetic anhydride (22 ± 2.4%), while for Cl reaction, the obtained yields were 60 ± 5.4, 41 ± 3.8, and 11 ± 1.2%, respectively, for the same products. The UV absorption cross section spectrum of MPA was determined in the wavelength range 210-370 nm. The study has shown no photolysis of MPA under atmospheric conditions. The obtained results are used to derive the atmospheric implication.

Atmospheric fate of hydrofluoroolefins, CxF2x+1CH[dbnd]CH2 (x = 1,2,3,4 and 6): Kinetics with Cl atoms and products

Rate coefficients for the gas-phase reactions of CxF2x+1CHCH2 (x = 1, 2, 3, 4 and 6) with Cl atoms were determined at (298 ± 2) K and (710 ± 5) Torr of air using a relative rate technique. Two experimental setups with simulation chambers were employed with Fourier Transform Infrared (FTIR) spectroscopy and Gas Chromatography coupled to Mass Spectrometry (GC-MS) as detection techniques. The Cl-rate coefficients obtained were (in 10−10 cm3 molecule−1 s−1): (0.85 ± 0.11) for CF3CHCH2, (1.11 ± 0.08) for C2F5CHCH2, (1.12 ± 0.18) for C3F7CHCH2, (0.97 ± 0.09) for C4F9CHCH2, and (0.99 ± 0.08) for C6F13CHCH2. Additionally, the gas-phase products were identified and quantified, when possible, by FTIR spectroscopy or GC-MS. The main reaction product was reported to be CxF2x+1C(O)CH2Cl. The fluorinated species, CxF2x+1CHO and CxF2x+1C(O)CH2Cl, were identified. CF3C(O)CH2Cl and CF3CHO were found to be formed with molar yield of (69 ± 5)% and (9 ± 1)%, respectively. The global lifetime of the investigated CxF2x+1CHCH2 due to their Cl-reaction is more than 100 days so this route does not compete with the removal by OH radicals. This lifetime is long enough for CxF2x+1CHCH2 to be transported to remote areas where they can be degraded. However, at a local scale, in marine regions at dawn the removal of CxF2x+1CHCH2 is expected to occur in ca. 1 day. The atmospheric degradation of these hydrofluoroolefins by Cl atoms is not expected to be a source of bioaccumulative perfluorinated carboxylic acids, CxF2x+1C(O)OH. Additionally, the UV absorption cross sections of CF3C(O)CH2Cl were determined together with the rate coefficient of the OH reaction by an absolute kinetic method at room temperature.

Atmospheric degradation of pyridine: UV absorption spectrum and reaction with OH radicals and O3

The UV absorption spectrum of pyridine and its gas phase reactions with OH radicals and O3 were investigated. UV absorption cross-sections were determined by using a D2-lamp system in the range 200–350nm. The kinetic studies were carried out at room temperature and atmospheric pressure of purified air. The rate coefficient for the reaction of pyridine with OH was determined relative to that with acetone while that with O3 was measured under pseudo first order conditions. The rate coefficients obtained are (in cm3molecule−1s−1): k(OH+pyridine)=(5.40±0.80)×10−13andk(O3+pyridine)=(3.28±1.70)×10−20.

Photochemical degradation of isoprene-derived 4,1-nitrooxy enal

Abstract. In isoprene-impacted environments, carbonyl nitrates are produced from NO3-initiated isoprene oxidation, which constitutes a potentially important NOx reservoir. To better understand the fate of isoprene carbonyl nitrates, we synthesized a model compound, trans-2-methyl-4-nitrooxy-2-buten-1-al (4,1-isoprene carbonyl nitrate, or 4,1-isoprene nitrooxy enal), and investigated its photochemical degradation process. The measured OH and O3 oxidation rate constants (298 K) for this nitrooxy enal are 4.1(±0.7) × 10−11 cm3 molecules−1 s−1 and 4.4(±0.3) × 10−18 cm3 molecules−1 s−1, respectively. Its UV absorption spectrum was determined, and the result is consistent with TDDFT calculations. Based on its UV absorption cross section and photolysis frequency in a reaction chamber, we estimate that the ambient photolysis frequency for this compound is 3.1(±0.8) × 10−4 s−1 for a solar zenith angle of 45°. The fast photolysis rate and high reactivity toward OH lead to a lifetime of less than 1 h for the isoprene nitrooxy enal, with photolysis being a dominant daytime sink. The nitrate products derived from the OH oxidation and the photolysis of the nitrooxy enal were identified with an iodide-based chemical ionization mass spectrometer. For the OH oxidation reaction, we quantified the yields of two nitrate products, methyl vinyl ketone nitrate and ethanal nitrate, which together contributed to 36(±5) % of the first-generation products.

Absolute UV absorption cross sections of dimethyl substituted Criegee intermediate (CH3)2COO

The absolute absorption cross sections of (CH3)2COO under a jet-cooled condition were measured via laser depletion to be (1.32±0.10)×10−17cm2molecule−1 at 308nm and (9.6±0.8)×10−18cm2molecule−1 at 352nm. The peak UV cross section is estimated to be (1.75±0.14)×10−17cm2molecule−1 at 330nm, according to the UV spectrum of (CH3)2COO (Huang et al., 2015) scaled to the absolute cross section at 308nm.

Assessment of the atmospheric loss processes initiated by OH radicals and sunlight, and the radiative efficiency for a series of hydrofluoroolefins, CF3(CF2)x=1,3,5CH[dbnd]CH2

Hydrofluoroolefins (HFOs) of the type CF3(CF2)x≥0CHCH2, are currently being suggested as substitutes of some hydrofluorocarbons (HFCs). In this work, an assessment of the atmospheric removal of CF3(CF2)x=1,3,5CHCH2, initiated by reaction with hydroxyl (OH) radicals and UV solar radiation is addressed. For that purpose, the rate coefficients for the OH + CF3(CF2)x=1,3,5CHCH2 reaction, kOH(T = 263–358 K), were determined by the pulsed laser photolysis-laser induced fluorescence technique. A slightly negative temperature dependence of kOH was observed, obtaining Ea/R (in K) values of −124 ± 15, −128 ± 6 and −160 ± 10, for CF3CF2CHCH2, CF3(CF2)3CHCH2 and CF3(CF2)5CHCH2, respectively. The estimated atmospheric lifetimes are around 8 days, considering that HFOs are well-mixed in the troposphere. Furthermore, an evaluation of the long-wave and short-wave absorption process of these HFOs have been carried out by determining the UV (191–367 nm) and IR (4000-500 cm−1) absorption cross sections at 298 K. Based on the obtained UV absorption cross sections, no photolysis of CF3(CF2)x=1,3,5CHCH2 is expected in the troposphere (λ > 290 nm). These species strongly absorb IR radiation in the atmospheric IR window. Despite the strong absorption in the IR region, the lifetime corrected radiative efficiencies are low (0.033 W m−2 ppb−1 for CF3(CF2)3CHCH2 and 0.039 Wm−2 ppb−1 for CF3(CF2)5CHCH2). Calculation of GWPs for these species has been performed as a function of the horizon time, providing values higher than unity for a short-period term, decreasing dramatically for longer periods. Therefore, it is concluded that emissions of these species do not affect the radiative forcing of climate, making them suitable replacements of large-GWP HFCs.