<p><strong>Abstract.</strong> It was recently shown by the CERN CLOUD experiment that biogenic highly oxygenated molecules (HOMs) form particles under atmospheric conditions in the absence of sulfuric acid where ions enhance the nucleation rate by one to two orders of magnitude. The biogenic HOMs were produced from ozonolysis of α-pinene at 5 °C. Here we extend this study to compare the molecular composition of positive and negative HOM clusters measured with atmospheric pressure interface time-of-flight mass spectrometers (APi-TOFs), at three different temperatures (25 °C, 5 °C and −25 °C). Most negative HOM clusters include a nitrate (NO<sub>3</sub><sup>−</sup>) ion and the spectra are similar to those seen in the nighttime boreal forest. On the other hand, most positive HOM clusters include an ammonium (NH<sub>4</sub><sup>+</sup>) ion and the spectra are characterized by mass bands that differ in their molecular weight by ~ 20 C atoms, corresponding to HOM dimers. At lower temperatures the average oxygen to carbon (O : C) ratio of the HOM clusters decreases for both polarities, reflecting an overall reduction of HOM formation with decreasing temperature. This indicates a decrease in the rate of autoxidation with temperature due to a rather high activation energy as has previously been determined by quantum chemical calculations. Furthermore, at the lowest temperature (−25 °C) the presence of C<sub>30</sub> clusters show that HOM monomers start to contribute to the nucleation of positive clusters. These experimental findings are supported by quantum chemical calculations of the binding energies of representative neutral and charged clusters.</p>

<p><strong>Abstract.</strong> Aromatic hydrocarbons make up a large fraction of anthropogenic volatile organic compounds and contribute significantly to the production of tropospheric ozone and secondary organic aerosol (SOA). A series of toluene and 1,2,4-trimethylbenzene (1,2,4-TMB) photooxidation experiments were performed in an environmental chamber under relevant polluted conditions (NO<sub>x</sub> ~ 10 ppb). An extensive suite of instrumentation including two Proton-Transfer Reaction Mass-Spectrometers (PTR-MS) and two Chemical Ionization Mass-Spectrometers (NH<sub>4</sub><sup>+</sup> CIMS and I<sup>-</sup> CIMS) allowed for quantification of reactive carbon in multiple generations of oxidation. Hydroxyl radical (OH)-initiated oxidation of both species produces ring-retaining products such as cresols, benzaldehydes, and bicyclic intermediate compounds, as well as ring scission products such as epoxides, and dicarbonyls. We show that the oxidation of bicyclic intermediate products leads to formation of compounds with high oxygen content (O:C ratio up to 1.1). These compounds, previously identified as highly oxygenated molecules (HOMs), are produced by more than one pathway with differing numbers of reaction steps with OH, including both autooxidation and phenolic pathways. We report the elemental composition of these compounds formed under relevant urban high-NO conditions. We show that ring-retaining products for these two precursors are more diverse and abundant than predicted by current mechanisms. We present speciated elemental composition of SOA for both precursors and confirm that highly oxygenated products make up a significant fraction of SOA. Ring scission products are also detected in both the gas and particle phases, and their yields and speciation overall agree with the kinetic model prediction.<p>

Abstract. In September 2017, we conducted the Proton-transfer-reaction mass-spectrometry (PTR-MS) Intercomparison campaign at CABauw (PICAB), a rural site in central Netherlands. Nine research groups deployed a total of eleven instruments covering a wide range of instrument types and performance. We applied a new calibration method based on fast injection of a gas standard through a sample loop. This approach allows calibrations on time scales of seconds and within a few minutes an automated sequence can be run allowing to retrieve diagnostic parameters that indicate the performance status. We developed a method to retrieve the mass dependent transmission from the fast calibrations, which is an essential characteristic of PTR-MS instruments, limiting the potential to calculate concentrations based on counting statistics and simple reaction kinetics in the reactor/drift tube. Our measurements show that PTR-MS instruments follow the simple reaction kinetics if operated in the standard range for pressures and temperature of the reaction chamber (i.e. 1–4 mbar, 30–120 ℃, respectively), and a reduced field strength E/N in the range of 100–160 Td. If artefacts can be ruled out, it becomes possible to quantify the signals of uncalibrated organics with accuracies better than ±30 %. The simple reaction kinetics approach produces less accurate results at E/N levels below 100 Td, because significant fractions of primary ions form water hydronium clusters. De-protonation through reactive collisions of protonated organics with water molecules need to be considered when the collision energy is a substantial fraction of the exoergicity of the proton transfer reaction, and/or if protonated organics undergo many collisions with water molecules.

<p><strong>Abstract.</strong> Sources and sinks of isoprene oxidation products from low NO<sub>x</sub> isoprene chemistry have been studied at the CERN CLOUD (Cosmics Leaving Outdoor Droplets) chamber with a custom-built selective reagent ion time of flight mass spectrometer (SRI-ToF-MS), which allows quantitative measurement of isoprene hydroxy hydroperoxides (ISOPOOH). <br><br> The measured concentrations of the main oxidation products were compared to chemical box model simulations based on the Leeds Master Chemical Mechanism (MCM) v3.3. The modelled ISOPOOH concentrations are by a factor of 20 higher than the observed and methyl vinyl ketone (MVK) and methacrolein (MACR) concentrations are by a factor of up to 2 lower compared to observations, despite the artifact-free detection method. <br><br> Addition of catalytic conversion of 1,2-ISOPOOH and 4,3-ISOPOOH to MVK and MACR on the stainless steel surface of the chamber to the chemical mechanism resolves the discrepancy between model predictions and observation. This suggests that isoprene chemistry in a metal chamber under low NO<sub>x</sub> conditions cannot be described by a pure gas phase model alone. Biases in the measurement of ISOPOOH, MVK and MACR can not only be caused intra-instrumentally but also by the general experimental setup. <br><br> The work described here extends the role of heterogeneous reactions affection gas phase composition and properties from instrumental surfaces, described previously, to general experimental setups. The role of such conversion reactions on real environmental surfaces is yet to be explored.</p>

<p><strong>Abstract.</strong> The photooxidation of methyl vinyl ketone (MVK) was investigated in the atmospheric simulation chamber SAPHIR for conditions at which organic peroxy radicals (RO<sub>2</sub>) mainly reacted with NO (<q>high NO</q> case) and for conditions at which other reaction channels could compete (<q>low NO</q> case). Measurements of trace gas concentrations are compared to calculated concentration time series applying the Master Chemical Mechanism (MCM version 3.3.1). Product yields of methylglyoxal and glycolaldehyde are determined from measurements. For the high NO case, the methylglyoxal yield is (19 ± 3) % and the glycolaldehyde yield is (65 ± 14) % consistent with recent literature studies. For the low NO case, the methylglyoxal yield reduced to (5 ± 2) % because other RO<sub>2</sub> reaction channels that do not form methylglyoxal become important. Consistent with literature data, the glycolaldehyde yield of (37 ± 9) % determined in the experiment is not reduced as much as implemented in the MCM suggesting additional reaction channels producing glycolaldehyde. At the same time, direct quantification of OH radicals in the experiments shows the need for an enhanced OH radical production at low NO conditions similar to previous studies investigating the oxidation of the parent VOC isoprene and methacrolein, the second major oxidation product of isoprene. For MVK the model-measurement discrepancy is up to a factor of 2. Product yields and OH observations are consistent with assumptions of additional RO<sub>2</sub> plus HO<sub>2</sub> reaction channels as proposed in literature for the major RO<sub>2</sub> species formed from the reaction of MVK with OH. This study, however, shows that also hydroxyperoxy radical concentrations are underestimated by the model, suggesting that additional OH is not directly produced from RO<sub>2</sub> radical reactions, but indirectly via increased HO<sub>2</sub>. Quantum chemical calculations show that HO<sub>2</sub> could be produced from a fast 1,4-H shift of the second most important MVK derived RO<sub>2</sub> species (reaction rate constant 0.003 s<sup>−1</sup>). However, additional HO<sub>2</sub> from this reaction is not sufficiently large to bring modelled HO<sub>2</sub> radical concentrations into agreement with measurements due to the small yield of this RO<sub>2</sub> species. An additional reaction channel of the major RO<sub>2</sub> species with a reaction rate constant of (0.006 ± 0.004) s<sup>−1</sup> would be required that produces concurrently HO<sub>2</sub> radicals and glycolaldehyde to achieve model-measurement agreement. A unimolecular reaction similar to the 1,5-H shift reaction that was proposed in literature for RO<sub>2</sub> radicals from MVK would not explain product yields for conditions of experiments in this study. A set of H-migration reactions for the main RO<sub>2</sub> radicals were investigated by quantum chemical and theoretical kinetic methodologies, but did not reveal a contributing route to HO<sub>2</sub> radicals or glycolaldehyde.</p>