@article{PangNovelliKaminskietal.2022, author = {Pang, Jacky Yat Sing and Novelli, Anna and Kaminski, Martin and Acir, Ismail-Hakki and Bohn, Birger and Carlsson, Philip T. M. and Cho, Changmin and Dorn, Hans-Peter and Hofzumahaus, Andreas and Li, Xin and Lutz, Anna and Nehr, Sascha and Reimer, David and Rohrer, Franz and Tillmann, Ralf and Wegener, Robert and Kiendler-Scharr, Astrid and Wahner, Andreas and Fuchs, Hendrik}, title = {Investigation of the limonene photooxidation by OH at different NO concentrations in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber)}, series = {Atmospheric Chemistry and Physics}, volume = {22}, journal = {Atmospheric Chemistry and Physics}, number = {13}, publisher = {Copernicus}, issn = {1680-7324}, doi = {10.5194/acp-22-8497-2022}, pages = {8497 -- 8527}, year = {2022}, abstract = {Abstract. The oxidation of limonene by the hydroxyl (OH) radical and ozone (O3) was investigated in the atmospheric simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a large Reaction Chamber) in experiments performed at different nitric oxide (NO) mixing ratios from nearly 0 up to 10 ppbv. For the experiments dominated by OH oxidation, the formaldehyde (HCHO) yield was experimentally determined and found to be (12 ± 3), (13 ± 3), and (32 ± 5) \% for experiments with low (∼ 0.1 ppbv), medium (∼ 0.3 ppbv), and high NO (5 to 10 ppbv), respectively. The yield in an ozonolysis-only experiment was (10 ± 1) \%, which agrees with previous laboratory studies. The experimental yield of the first-generation organic nitrates from limonene-OH oxidation is calculated as (34 ± 5) \%, about 11 \% higher than the value in the Master Chemical Mechanism (MCM), which is derived from structure-activity relationships (SARs). Time series of measured radicals, trace-gas concentrations, and OH reactivity are compared to results from zero-dimensional chemical box model calculations applying MCM v3.3.1. Modeled OH reactivity is 5 to 10 s-1 (25 \% to 33 \% of the OH reactivity at the start of the experiment) higher than measured values at the end of the experiments under all chemical conditions investigated, suggesting either that there are unaccounted loss processes of limonene oxidation products or that products are less reactive toward OH. In addition, model calculations underestimate measured hydroperoxyl radical (HO2) concentrations by 20 \% to 90 \% and overestimate organic peroxyl radical (RO2) concentrations by 50 \% to 300 \%. The largest deviations are found in low-NO experiments and in the ozonolysis experiment. An OH radical budget analysis, which uses only measured quantities, shows that the budget is closed in most of the experiments. A similar budget analysis for RO2 radicals suggests that an additional RO2 loss rate constant of about (1-6) × 10-2 s-1 for first-generation RO2 is required to match the measured RO2 concentrations in all experiments. Sensitivity model runs indicate that additional reactions converting RO2 to HO2 at a rate constant of about (1.7-3.0) × 10-2 s-1 would improve the model-measurement agreement of NOx, HO2, and RO2 concentrations and OH reactivity. Reaction pathways that could lead to the production of additional OH and HO2 are discussed, which include isomerization reactions of RO2 from the oxidation of limonene, different branching ratios for the reaction of RO2 with HO2, and a faster rate constant for RO2 recombination reactions. As the exact chemical mechanisms of the additional HO2 and OH sources could not be identified, further work needs to focus on quantifying organic product species and organic peroxy radicals from limonene oxidation.}, language = {en} } @article{TanHantschkeKaminskietal.2021, author = {Tan, Zhaofeng and Hantschke, Luisa and Kaminski, Martin and Acir, Ismail-Hakki and Bohn, Birger and Cho, Changmin and Dorn, Hans-Peter and Li, Xin and Novelli, Anna and Nehr, Sascha and Rohrer, Franz and Tillmann, Ralf and Wegener, Robert and Hofzumahaus, Andreas and Kiendler-Scharr, Astrid and Wahner, Andreas and Fuchs, Hendrik}, title = {Atmospheric photo-oxidation of myrcene: OH reaction rate constant, gas-phase oxidation products and radical budgets}, series = {Atmospheric Chemistry and Physics}, volume = {21}, journal = {Atmospheric Chemistry and Physics}, number = {20}, publisher = {Copernicus GmbH}, issn = {1680-7324}, doi = {10.5194/acp-21-16067-2021}, pages = {16067 -- 16091}, year = {2021}, abstract = {Abstract. The photo-oxidation of myrcene, a monoterpene species emitted by plants, was investigated at atmospheric conditions in the outdoor simulation chamber SAPHIR (Simulation of Atmospheric PHotochemistry In a Large Reaction Chamber). The chemical structure of myrcene consists of one moiety that is a conjugated π system (similar to isoprene) and another moiety that is a triple-substituted olefinic unit (similar to 2-methyl-2-butene). Hydrogen shift reactions of organic peroxy radicals (RO2) formed in the reaction of isoprene with atmospheric OH radicals are known to be of importance for the regeneration of OH. Structure-activity relationships (SARs) suggest that similar hydrogen shift reactions like in isoprene may apply to the isoprenyl part of RO2 radicals formed during the OH oxidation of myrcene. In addition, SAR predicts further isomerization reactions that would be competitive with bimolecular RO2 reactions for chemical conditions that are typical for forested environments with low concentrations of nitric oxide. Assuming that OH peroxy radicals can rapidly interconvert by addition and elimination of O2 like in isoprene, bulk isomerization rate constants of 0.21 and 0.097 s-1 (T=298 K) for the three isomers resulting from the 3′-OH and 1-OH addition, respectively, can be derived from SAR. Measurements of radicals and trace gases in the experiments allowed us to calculate radical production and destruction rates, which are expected to be balanced. The largest discrepancies between production and destruction rates were found for RO2. Additional loss of organic peroxy radicals due to isomerization reactions could explain the observed discrepancies. The uncertainty of the total radical (ROx=OH+HO2+RO2) production rates was high due to the uncertainty in the yield of radicals from myrcene ozonolysis. However, results indicate that radical production can only be balanced if the reaction rate constant of the reaction between hydroperoxy (HO2) and RO2 radicals derived from myrcene is lower (0.9 to 1.6×10-11 cm3 s-1) than predicted by SAR. Another explanation of the discrepancies would be that a significant fraction of products (yield: 0.3 to 0.6) from these reactions include OH and HO2 radicals instead of radical-terminating organic peroxides. Experiments also allowed us to determine the yields of organic oxidation products acetone (yield: 0.45±0.08) and formaldehyde (yield: 0.35±0.08). Acetone and formaldehyde are produced from different oxidation pathways, so that yields of these compounds reflect the branching ratios of the initial OH addition to myrcene. Yields determined in the experiments are consistent with branching ratios expected from SAR. The yield of organic nitrate was determined from the gas-phase budget analysis of reactive oxidized nitrogen in the chamber, giving a value of 0.13±0.03. In addition, the reaction rate constant for myrcene + OH was determined from the measured myrcene concentration, yielding a value of (2.3±0.3)×10-10 cm3 s-1.}, language = {en} }