Preprints
https://doi.org/10.5194/egusphere-2024-531
https://doi.org/10.5194/egusphere-2024-531
20 Mar 2024
 | 20 Mar 2024
Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Characterization of the new BATCH Teflon chamber and on-line analysis of isomeric multifunctional photooxidation products

Finja Löher, Esther Borrás, Amalia Muñoz, and Anke Christine Nölscher

Abstract. The photooxidation of volatile organic compounds (VOCs) in the troposphere has important implications for air quality, weather, and climate. A deeper understanding of the underlying mechanisms can be achieved by studying these reactions under controlled conditions and analyzing the emerging photooxidation products. This requires dedicated laboratory infrastructure as well as sensitive and selective analytical techniques. Here, we constructed a new 300 L indoor Teflon atmospheric simulation chamber as part of the Bayreuth ATmospheric simulation CHambers (BATCH) infrastructure. The chamber was irradiated by a bandpass-filtered solar simulator that enabled experiments with realistic photon fluxes and OH radical concentrations. It was coupled to a proton-transfer-reaction – time-of-flight – mass spectrometer (PTR-ToF-MS) and a solid phase microextraction – gas chromatography – mass spectrometry (SPME-GC-MS) system for the on-line analysis of the precursor VOC and its oxidation products in the gas phase. As part of the SPME-GC-MS method, multifunctional oxygenated compounds (carbonyls, alcohols, carboxylic acids) were derivatized with O-(2,3,4,5,6-Pentafluorobenzyl)-hydroxylamine (PFBHA) and N-trimethylsilyl-N-methyl trifluoroacetamide (MSTFA). We designed a permeation source for the on-line addition of internal standards to improve method reproducibility. The joint setup was tested and validated by studying the OH radical-induced photooxidation of toluene, one of the most abundant aromatic hydrocarbons in the atmosphere. For chamber characterization, we first derived the photolysis rates for several typical toluene products in the irradiated BATCH Teflon chamber (1.77×10−8 – 3.02×10−4 s−1). Additionally, wall loss rates were determined empirically (4.54×10−6 – 8.53×10−5 s−1), and then parameterized according to fundamental molecular properties. For the cresols and benzyl alcohol, we compiled a weighted calibration factor for the PTR-ToF-MS, taking into account isomer-specific sensitivities as well as the relative distribution as determined by the SPME-GC-MS. The weighted calibration improved the instrumental agreement to 15 %, whereas the PTR-ToF-MS overestimated the sum of the isomers by 25 % compared to the SPME-GC-MS concentrations when using the averaged calibration factor. Thus, the combined data set offered insight into both temporal trends and the isomeric composition. Finally, we conducted six toluene photooxidation experiments to evaluate the ring-retaining first generation products. Based on the loss-corrected concentrations, we derived formation yields for o-cresol (8.0±1.8 %), m-cresol (0.4±0.1 %), p-cresol (2.4±0.6 %), benzyl alcohol (0.5±0.1 %), and benzaldehyde (4.6±1.7 %) under NOx-free conditions at T = 298±1 K. These yields are consistent with previous studies and therefore serve as proof-of-concept for our applied methods.

Finja Löher, Esther Borrás, Amalia Muñoz, and Anke Christine Nölscher

Status: open (until 25 Apr 2024)

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Finja Löher, Esther Borrás, Amalia Muñoz, and Anke Christine Nölscher
Finja Löher, Esther Borrás, Amalia Muñoz, and Anke Christine Nölscher

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Short summary
We constructed and characterized a new indoor Teflon atmospheric simulation chamber. We evaluated wall losses, photolysis rates, and secondary reactions of multifunctional VOC photooxidation products in the chamber. Two on-line analytical methods (SPME-GC-MS, PTR-ToF-MS) were used for gaining isomeric information and high temporal resolution data. These data were combined and cross-compared. For method validation, we studied the formation yields of the main ring-retaining products of toluene.