Peroxy radicals, RO2, are key species in the atmosphere. They are formed from a reaction of OH radi-cals with hydrocarbons: RH + OH + O2 -> RO2 + H2OIn polluted environments, RO2 radicals react predominantly with NO, leading to formation of NO2, and eventually through photolysis of NO2 to formation of O3.At low NOx concentrations such as in the marine boundary layer or the background troposphere, the life-time of RO2 radicals increases and other reaction pathways such as self- and cross reaction with other RO2 or with HO2 radicals become competitive.To study the reactivity of peroxy radicals, UV absorption spectroscopy has been employed in the past: this technique gives good sensitivity for peroxy radicals, but poor selectivity as these radicals have broad absorption features in the UV. We have established a technique allowing to follow peroxy radicals with a better selectivity compared to UV, but with still good sensitivity by coupling laser photolysis to cw-Cavity Ring Down Spectroscopy in the near IR. Two identical cw-CRDS paths are installed in a recently constructed temperature-controlled photolysis reactor in a small angle with respect to the Excimer photolysis beam, leading to an overlap of around 35 cm between the photolyzed volume and the detection volume. A third detection path for UV absorption measurements is installed in a slightly larger angle, leading to an overlap of around 20 cm between photolysis and absorption volume.Here, we will present the first results obtained in the new reactor: the reaction between RO2 radicals and NO2. This reaction leads in an equilibrium reaction to the formation of RO2NO2 species. If the lifetime of these RO2NO2 are long enough, they will be transported and become a NOx source in remote environments. Therefore, determination of rate- and equilibrium constants of such reactions is important. In this work, two RO2 radicals have been generated simultaneously by 248nm laser photolysis of acetone, leading to roughly 1/3 CH3C(O)O2 radicals and 2/3 CH3O2 radicals. Time-resolved decays have then been observed for both radicals in the presence of different NO2 concentrations. The detection of both RO2 radicals is done simultaneously by high sensitivity cw-CRDS. RO2 concentrations can be decreased to a level where self-reaction becomes negligible at still excellent S/N ratio, making the measurement of RO2 + NO2 reaction straightforward. NO2 is quantified by UV-multipass absorption spectroscopy at 532nm in the photolysis reactor, and concentrations are compared with the calculated ones from the use of calibrated flowmeters.

This study reports the first measurements of ozone-seeded diethyl ether (DEE) cool flame propagation velocities using a stagnation plate burner and particle image velocimetry (PIV) at atmospheric pressure. Four flame conditions were investigated, with equivalence ratios ranging from 0.3 to 0.5, ozone mole fraction in the mixture from 1.4 % to 1.6 %, and inlet gas velocities between 0.8 and 1 m.s . The axial velocity profiles were derived using PIV-1 for each flame condition by seeding the reactive mixture with silicon oil particles (d(p) = 0.8 mu m, T = 50 degrees C). These data are used to validate various kinetic mechanisms: the Serinyel et al. mechanism is found to accurately predict the axial velocity profiles, while the Tran et al. mechanism underestimates the reference velocities. From the experimental and numerical data, the unstrained cool flame velocity was S-u(0) determined using a numerically-assisted non-linear extrapolation method.

A novel coupling of a stagnation plate burner and the SAPHIRS instrument located at synchrotron SOLEIL has been performed to enable the observation of labile species and radicals within stabilized cool flames. Gases were extracted from a dimethyl ether/O2/O3/N2 cool flame with a capillary probe and expanded through two consecutive skimmers to reach the synchrotron vacuum ultraviolet-photoelectron photoion coincidence spectrometer. threshold photoelectron spectra (TPES) and total ion yields (TIY) were recorded as a function of the photon energy, and led to the identification of species that have so far never been observed in cool flames, including the only expected keto- or aldo-hydroperoxide (hydroperoxymethylformate), hydrogen peroxide, methyl and hydroperoxyl radicals. The quantification of the detected species has been performed and is compared with the predictions of a kinetic model, demonstrating the validity of the experimental approach and suggesting possible ways for improvement.

This study aims to investigate the feasibility of experimental determination of DME/O2/O3 cool flame propagation speeds using Particle Image Velocimetry (PIV) in a stagnation plate burner operated at atmospheric pressure. A specific PIV data analysis procedure was developed in order to improve the accuracy of the measurements in this particular configuration. Five flame conditions, with equivalence ratio varying from 0.3 to 0.5 and ozone mole fraction varying from 1.5 to 2% were investigated to compare experimental results with kinetic modeling. Three ozone-submechanisms, respectively from Jian et al. (Jian et al., 2022), Halter et al. (Halter et al., 2011) and Zhao et al. (Zhao et al., 2016), were coupled with our previously developed DME mechanism (Panaget et al., 2021) and used to compare experimental and simulated axial velocity profiles. Results show that a thoughtful choice of the ozone-submechanism is of particular importance in predicting an accurate cool flame velocity in these conditions. A numerically assisted non-linear extrapolation method is proposed for the determination of the unstrained cool flame speed Su,0. Additionally, simulations for which the plate temperature reaches the maximal flame temperature (adiabatic conditions) were performed, demonstrating a negligible effect of the plate temperature on the determined Su,0. A kinetic analysis is also presented to highlight the most sensitive chemical reactions influencing the reference cool flame speed Su,ref, showing the preponderant role of the fuel low temperature chemistry.

Cool flames of diethyl ether were stabilized and extensively studied in a stagnation plate burner. Two conditions were selected, enabling the assessment of the impact of ozone-seeding on the cool flame: In the first case, ozone was seeded to stabilize a lean cool flame with an equivalence ratio of 0.5, and in a second case a stoichiometric flame was stabilized without O-3 addition. Excited formaldehyde chemiluminescence (CH2O*) was used to measure the cool flame position in the burner. Detailed temperature and mole fraction profiles of stable intermediates of the oxidation of diethyl ether were measured for both conditions. Relevant detailed kinetic models issued from the literature, respectively developed by Tran et al. [Tran et al., Proc. Comb. Inst. 2019, 37, 511-519] and Serinyel et al. [Serinyel et al., Combust. Flame 2018, 193, 453-462] were used in order to improve the understanding of the low temperature kinetics of this ether, as well as the effect of ozone on the species distribution after the cool flame. The Tran et al. model is able to correctly predict the cool flame position as well as its heat release in the lean case, but shows high discrepancy in the prediction of products formation, while the Serinyel et al. model shows a fair prediction of the cool flame products distribution. A numerical comparison between a stoichiometric flame, with and without ozone-seeding, was also performed in order to gain some insight into the ozone influence on the low temperature products distribution. Main results show that the R<(O)over dot> radical decomposition is of particular importance in our conditions, which formation is directly linked to R<(O)over dot>(2) bimolecular reactions, reinforcing the link between atmospheric chemistry and low temperature combustion. (c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

This work reports the implementation and the validation of the extrapolation method for thermocouple temperature measurements corrected from the radiation losses in sooting flames. This simple method relies on the use of thermocouples having different size diameters and enables a fast and direct determination of the flame temperature by extrapolation to zero diameter. We propose here a detailed study of the possibilities and limitations offered by this method based on experimental measurements and comparison with well-established methods carried out in a laminar diffusion sooting flame. In details, a specific fast insertion setup using four different sized thermocouples has been implemented to record temperature values at different heights in the flame. From these data, we highlight that a linear calibration curve correlates the raw measured temperatures to the flame temperatures corrected of the radiation losses can be easily and rapidly obtained. The impact of soot deposition on the thermocouple on the temperature measurement is also discussed. To assess the reported thermocouple methodology, a direct comparison is made between the temperature profile determined along the vertical central axis of the flame by the extrapolation method with OH and NO LIF thermometry measurements as well as numerical simulation. Finally, we also report the comparison of experimental and simulated radial temperature profiles highlighting the adequate dynamic of the method for temperature profile determination in high temperature gradient conditions (500 K/mm). This work demonstrates that the extrapolation method is an efficient and fast method to determine accurate temperature profiles in flames, even in presence of soot particles up to a few hundred ppb, which can be useful for the development of fast and cheap sensors for either laboratory or larger-scale applications.

&lt;p&gt;&lt;strong&gt;Atmospheric Chemistry linked to HO&lt;sub&gt;x&lt;/sub&gt; radicals of a Suburban Forest during the ACROSS summer Field Campaign&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Nesrine. Shamas&lt;sup&gt;1&lt;/sup&gt;, Sebastien. Batut&lt;sup&gt;1&lt;/sup&gt;, Amaury. Lahccen&lt;sup&gt;1&lt;/sup&gt;, Vincent Michoud&lt;sup&gt;2&lt;/sup&gt;, Christopher Cantrell&lt;sup&gt;3&lt;/sup&gt;, S&amp;#233;bastien Dusanter&lt;sup&gt;4&lt;/sup&gt;, Joel Brito&lt;sup&gt;4&lt;/sup&gt;, Alexandre Tomas&lt;sup&gt;4&lt;/sup&gt;, Ahmad Lahib&lt;sup&gt;4&lt;/sup&gt;, Marina Jamar&lt;sup&gt;4&lt;/sup&gt;, Christa. Fittschen&lt;sup&gt;1&lt;/sup&gt;, Coralie. Schoemaecker&lt;sup&gt;1&lt;/sup&gt;&lt;/p&gt;&lt;p&gt;&lt;sup&gt;1&lt;/sup&gt; PC2A, CNRS &amp;#8211; University Lille, B&amp;#226;t. C11, Cit&amp;#233; Scientifique, Villeneuve d&amp;#8217;Ascq, France&lt;/p&gt;&lt;p&gt;&lt;sup&gt;2&lt;/sup&gt; LISA, CNRS &amp;#8211; Universit&amp;#233; Paris Cit&amp;#233; and Universit&amp;#233; Paris-Est Cr&amp;#233;teil, F-94010 Cr&amp;#233;teil, France&lt;/p&gt;&lt;p&gt;&lt;sup&gt;3&lt;/sup&gt; LISA, CNRS &amp;#8211; Universit&amp;#233; Paris-Est Cr&amp;#233;teil and Universit&amp;#233; Paris Cit&amp;#233;, F-94010 Cr&amp;#233;teil, France&lt;/p&gt;&lt;p&gt;&lt;sup&gt;4&lt;/sup&gt;1IMT Nord Europe, Institut Mines-T&amp;#233;l&amp;#233;com, Univ. Lille, Center for Energy and Environment, F-59000 Lille, France&lt;/p&gt;&lt;p&gt;Paris, one of the largest European megacities, transports pollution to different surrounding areas depending on the variation of the wind direction associated with specific meteorological conditions. The relatively unique situation of this isolated megacity from other urban areas make it a suitable location to study the impact of urban emissions on the chemistry of close biogenic environments such as forests and vice versa. In order to investigate this influence, the ACROSS (Atmospheric ChemistRy Of the Suburban foreSt) field campaign was performed during summer 2022, with a measurement site located in the Rambouillet forest. The combination of the data provided during this field campaign from different research groups (such as measurements of VOCs, inorganic species, particle concentration and composition, &amp;#8230;) will allow a better understanding of the influence of mixing anthropogenic urban or oceanic air masses, leading to different NO concentrations, with biogenic forestry emissions on the oxidation of tropospheric VOCs. This will ultimately help improving this chemistry within atmospheric models. The UL-FAGE instrument was deployed during the ACROSS campaign, where different types of measurements were performed: OH, HO&lt;sub&gt;2&lt;/sub&gt;, RO&lt;sub&gt;x &lt;/sub&gt;radical quantification at the ground level and OH reactivity. The OH reactivity was alternatively measured at two different levels: below (ground level) and above the forest canopy (top of a 40 m tower). Clear stratification was observed during the night with a higher OH reactivity at the ground level than above the canopy. Comparison between the measured and the calculated OH reactivity allows to identify the diurnal missing reactivity at both levels. Preliminary results of the OH reactivity and the radical quantification will be presented.&lt;/p&gt;&lt;p&gt;&lt;strong&gt;References&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;[1] Baklanov et al., Advances in Science and Research, 4, 115&amp;#8211;120, 2010&lt;/p&gt;&lt;p&gt;[2] MEGAPOLI campaign 2009-2010 special issue, https://acp.copernicus.org/articles/special_issue248.html page, accessed 24 May 2019&lt;/p&gt;

Soot particles released in the atmosphere have long been investigated for their ability to affect the radiative forcing. Although freshly emitted soot particles are generally considered to yield only positive contributions to the radiative forcing, atmospheric aging can activate them into efficient cloud condensation or ice nuclei, which can trigger the formation of persistent clouds and ultimately provide a negative contribution to the radiative forcing. Depending on their residence time in the atmosphere, soot particles can undergo several physical and chemical aging processes that affect their chemical composition, particle size distribution and morphology, and ultimately their optical and hygroscopic properties. The impact of the physical-chemical aging on the properties of soot particles is still difficult to quantify, as well as their effect on the radiative forcing of the atmosphere. This work investigates the hygroscopic properties of chemically aged soot particles obtained from the combustion of aviation fuel, and in particular the interplay between aging mechanisms initiated by two widespread atmospheric oxidizers (O3 and SO2). Activation is measured in water supersaturation conditions using a cloud condensation nuclei counter. Once particle morphology and size distribution are taken into account, the hygroscopicity parameter κ is derived using κ-Köhler theory and correlated to the change of the chemical composition of the particles aged in a simulation chamber. While fresh soot particles are poor cloud condensation nuclei (κ < 10-4) and are not significantly affected by either O3 or SO2 at the timescale of the experiments, rapid activation is observed when they are simultaneously exposed to both oxidizers. Activated particles become efficient cloud condensation nuclei, comparable to the highly hygroscopic particulate matter typically found in the atmosphere (κ = 0.2-0.6 at RH = 20 %). Statistical analysis reveals a correlation between the activation and sulfur-containing ions detected on the chemically aged particles that are absent from the fresh particles.

The low-temperature combustion kinetics of dimethyl ether (DME) were studied by means of stabilized cool flames in a heated stagnation plate burner configuration using ozone-seeded premixed flows of DME/O2. Direct imaging of CH2O* chemiluminescence and laser-induced fluorescence of CH2O were used to determine the flame front positions in a wide range of lean and ultra-lean equivalence ratios and ozone concentrations for two strain rates. The temperature and species mole fraction profiles along the flame were measured by coupling thermocouples, gas chromatography, micro-chromatography, and quadrupole mass spectrometry analysis. A new kinetic model was built on the basis of the Aramco 1.3 model, coupled with a validated submechanism of O3 chemistry, and was updated to improve the agreement with the obtained experimental results and experimental data available in the literature. The main results show the efficiency of the tested model to predict the flame front position and temperature in every tested condition, as well as the importance of reactions typical of atmospheric chemistry in the prediction of cool flame occurrence. The agreement on the fuel and major products is overall good, except for methanol, highlighting some missing kinetic pathways for the DME/O2/O3 system, possibly linked to the direct addition of atomic oxygen on the fuel radical, modifying the product distribution after the cool flame.

Trace gas measurements were performed during the LANDEX (the LANDes EXperiment) Episode 1 field campaign in the summer 2017, in one of the largest European maritime pine forests (> 95% Pinus pinaster) located in southwestern France. Efforts have been focused on obtaining a good speciation of 20 major biogenic volatile organic compounds (BVOCs, including pinenes, carenes, terpinenes, linalool, camphene, etc.). This was made possible by the development of a new and specific chromatographic method. In order to assess the role of BVOCs in the local gas phase chemistry budget, their reactivity with the main atmospheric oxidants (hydroxyl radicals (OH), ozone (O3) and nitrate radicals (NO3)) and the corresponding consumption rates were determined. When considering the OH reactivity with BVOCs, isoprene and linalool accounted for 10–47% of the OH depletion during daytime, and monoterpenes for 50–65%, whereas monoterpenes were the main contributors during the night (70–85%). Sesquiterpenes and monoterpenes were the main contributors to the ozone reactivity, especially β-caryophyllene (30–70%), with a maximum contribution during nighttime. Nighttime nitrate reactivity was predominantly due to monoterpenes (i.e. 90–95%). Five specific groups have been proposed to classify the 19 BVOCs measured in the forest, according to their reactivity with atmospheric oxidants and their concentrations. The total amount of BVOCs consumed under and above the forest canopy was evaluated for 7 BVOCs (i.e. isoprene, α-pinene, β-pinene, myrcene, limonene + cis-ocimene and Δ3-carene). The reactivity of atmospheric oxidants and BVOCs at a local level are discussed in order to highlight the compounds (BVOCs, other VOCs), the atmospheric oxidants and the main associated reactive processes observed under the canopy of a maritime pine forest.

Total hydroxyl radical (OH) reactivity measurements were conducted during the LANDEX intensive field campaign in a coniferous temperate forest located in the Landes area, southwestern France, during July 2017. In order to investigate inter-canopy and intra-canopy variability, measurements were performed inside (6 m) and above the canopy level (12 m), as well as at two different locations within the canopy, using a comparative reactivity method (CRM) and a laser photolysis–laser-induced fluorescence (LP-LIF) instrument. The two techniques were intercompared at the end of the campaign by performing measurements at the same location. Volatile organic compounds were also monitored at both levels with a proton transfer time-of-flight mass spectrometer and online gas chromatography instruments to evaluate their contribution to total OH reactivity, with monoterpenes being the main reactive species emitted in this forest dominated by Pinus pinaster Aiton. Total OH reactivity varied diurnally, following the trend of biogenic volatile organic compounds (BVOCs), the emissions and concentrations of which were dependent on meteorological parameters. Average OH reactivity was around 19.2 and 16.5 s−1 inside and above the canopy, respectively. The highest levels of total OH reactivity were observed during nights with a low turbulence (u*≤0.2 m s−1), leading to lower mixing of emitted species within the canopy and thus an important vertical stratification characterized by a strong concentration gradient. Comparing the measured and the calculated OH reactivity highlighted an average missing OH reactivity of 22 % and 33 % inside and above the canopy, respectively. A day–night variability was observed on missing OH reactivity at both heights. Investigations showed that during daytime, missing OH sinks could be due to primary emissions and secondary products linked to a temperature-enhanced photochemistry. Regarding nighttime missing OH reactivity, higher levels were seen for the stable and warm night of 4–5 July, showing that these conditions could have been favorable for the accumulation of long-lived species (primary and secondary species) during the transport of the air mass from nearby forests.

Combustion and other high-temperature processes frequently result in the emission of aerosols in the form of polydisperse fractal-like aggregates made of condensed-phase nanoparticles (soot for instance). If certain conditions are met, the emitted aerosol particles are known to evolve into important cloud condensation nuclei (CCN) in the atmosphere. In this work, the hygroscopic parameter κ of complex morphology aggregates is calculated from the supersaturation-dependent activated fraction Fa=Fa(SS) in the frame of κ-Köhler theory. The particle size distribution is approximated with the morphology-corrected volume equivalent diameter calculated from the electrical mobility diameter by taking into account the diameter of the primary particle and the fractal dimension of the aggregate experimentally obtained from transmission electron microscopy measurements. Activation experiments are performed in water supersaturation conditions using a commercial CCN-100 condensation nuclei counter. The model is tested in close-to-ideal conditions of size-selected, isolated spherical particles (ammonium sulfate nanoparticles dispersed in nitrogen), then with complex polydisperse fractal-like aggregates (soot particles activated by exposure to ozone with κ as low as 5×10-5) that represent realistic anthropogenic emissions in the atmosphere.

Soot particles are considered as important aerosols in the atmosphere due to their potential role as cloud condensation nuclei (CCNs). Freshly released soot is generally considered as hydrophobic. However, the so-called aging process in the atmosphere can modify morphology and surface composition of soot particles and turn them into efficient CCNs. The hygroscopic properties of soot are commonly measured from their activated fraction F_a that is the ratio of the number of nucleated droples on the number of CCNs. For spherical and monodisperse aerosols, the κ-Kohler theory is generally used to obtain the parameter κ which quantifies the hygroscopic properties of aerosol. In this work, a model to obtain κ is proposed by taking into account the size distribution and the morphology of the aerosol particles. The model is first tested on dry ammonium sulfate that is well known to produce quasi-spherical particles. The results of the activation experiments are in good agreement with the predictions of the model. Then, the model is applied to soot particles that are characterized by complex morphology. Soot particles had to be activated by aging with ozone before activation experiments to increase their k into the validity range of the model (i.e. to turn fresh soot to hydrophilic aggregates). The parameters of the determination of κ are soot particle mobility diameter Dm, fractal dimension Df, the primary particle diameter Dpp and the geometric standard deviation of volume equivalent particles of soot.

Field campaigns have been carried out with the FAGE (fluorescence assay by gas expansion) technique in remote biogenic environments in the last decade to quantify the in situ concentrations of OH, the main oxidant in the atmosphere. These data have revealed concentrations of OH radicals up to a factor of 10 higher than predicted by models, whereby the disagreement increases with decreasing NO concentration. This was interpreted as a major lack in our understanding of the chemistry of biogenic VOCs (volatile organic compounds), particularly isoprene, which are dominant in remote pristine conditions. But interferences in these measurements of unknown origin have also been discovered for some FAGE instruments: using a pre-injector, all ambient OH is removed by fast reaction before entering the FAGE cell, and any remaining OH signal can be attributed to an interference. This technique is now systematically used for FAGE measurements, allowing the reliable quantification of ambient OH concentrations along with the signal due to interference OH. However, the disagreement between modelled and measured high OH concentrations of earlier field campaigns as well as the origin of the now-quantifiable background OH is still not understood. We present in this paper the compelling idea that this interference, and thus the disagreement between model and measurement in earlier field campaigns, might be at least partially due to the unexpected decomposition of a new class of molecule, ROOOH, within the FAGE instruments. This idea is based on experiments, obtained with the FAGE set-up of the University of Lille, and supported by a modelling study. Even though the occurrence of this interference will be highly dependent on the design and measurement conditions of different FAGE instruments, including ROOOH in atmospheric chemistry models might reflect a missing piece of the puzzle in our understanding of OH in clean atmospheres.

Recent reports [Jara-Toro etal., Angew. Chem. Int. Ed. 2017, 56, 2166 and PCCP2018, 20, 27885] suggest that the rate coefficient of OH reactions with alcohols would increase by up to two times in going from dry to high humidity. This finding would have an impact on the budget of alcohols in the atmosphere and it may explain differences in measured and modeled methanol concentrations. The results were based on a relative technique carried out in a small Teflon bag, which might suffer from wall reactions. The effect was reinvestigated using a direct fluorescence probe of OH radicals, and no catalytic effect of H2O could be found. Experiments in a Teflon bag were also carried out, but the results of Jara-Toro etal. were not reproducible. Further theoretical calculations show that the water-mediated reactions have negligible rates compared to the bare reaction and that even though water molecules can lower the barriers of reactions, they cannot make up for the entropy cost.