Universität zu Köln

Färber, Michelle ORCID: 0009-0003-0302-6142 (2025). Investigation of current and future anthropogenic chemical regimes in simulation chamber experiments. PhD thesis, Universität zu Köln.

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Abstract

Air pollution is a societal challenge, affecting millions of people world-wide living in urban conglomerates. In cities, emissions are mostly from anthropogenic activities such as traffic, industry, cooking, and use of volatile care products. These emissions are not only hazardous for human health, they also undergo chemical degradation driven by oxidants, forming secondary pollutants such as ozone (O3) and particles. Main tropospheric oxidants are the hydroxyl radical (OH), dominating oxidation processes during the day, the nitrate radical (NO3), predominantly available during the night, and ozone. In the reaction chain of the atmospheric oxidation of volatile organic compounds (VOCs), peroxy (RO2) and hydroperoxy (HO2) radicals are formed, which oxidise nitric oxide (NO) to nitrogen dioxide (NO2), the latter being the main tropospheric source of ozone following its photolysis. Understanding atmospheric oxidation processes is crucial for mitigating air pollution and tackling current and future air quality challenges. In many different field studies, performed in or close to urban areas, measured HO2 and/or RO2 radical concentrations could not be reproduced by chemical model calculations, which represent the current understanding of the atmospheric chemistry. Even though chemical models carry uncertainties, the observed discrepancies in particular for RO2 radicals often exceeded a factor of three, making air quality prediction challenging. Data collected during field campaigns are very valuable in highlighting where our gap of knowledge for atmospheric chemical processes lies. Laboratory studies and experiments in atmospheric simulation chambers can then focus on investigating such processes in a confined and controlled environment. In this thesis, first the performance and comparability of several different atmospheric simulation chambers were studied. Oxidation experiments of alpha-pinene were performed in nine different simulation chambers, which are part of the EUROCHAMP-2020 consortium. Chamber effects, such as the release of small oxygenated compounds from the chamber wall or the loss of trace gases or particles on the chamber wall were characterised. Furthermore, yields of pinonaldehyde, formaldehyde, and acetone, which are products from the oxidation of alpha-pinene by OH, could be derived for experiments in five different chambers. A high variability of the yields of pinonaldehyde and formaldehyde was observed, which is also reflected in the available data from the literature. In contrast, obtained acetone yields agree within the combined uncertainties for the different chambers and within the uncertainties with reported literature values. Overall, well-characterised simulation chambers offer a great opportunity to investigate atmospheric chemistry in a controlled environment. The goal is to simplify the complexity of field studies while still keeping the conditions comparable to the real atmosphere. The main part of the thesis is on the investigation of the daytime and nighttime oxidation of anthropogenic VOCs in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. Measured trace gas and radical concentrations were compared to zero-dimensional box model calculations, based on the Master Chemical Mechanism (MCM) and complemented by an updated ozonolysis scheme for alkenes, and by state-of-the-art peroxy and alkoxy chemistry from structure-activity relationships (SAR). Photooxidation experiments were performed for a variety of anthropogenic VOCs at different levels of NO, mimicking current (high NO) and future (low NO) chemical regimes. The VOCs investigated were chosen according to their alkoxy chemistry, forming HO2 either in a single-step reaction (propane, propene, trans-2-hexene) or in a multi-step reaction involving the regeneration of RO2 (iso-pentane, n-hexane), which results in a different number of ozone molecules produced per oxidised VOC molecule. A comparison between measured trace gases and radicals with results from the MCM showed overall a good agreement (within 17 %) for most VOCs. An improved agreement of HO2 and RO2 radical concentrations, in experiments with n-hexane, was found for the MCM complemented by SAR, assuming a factor of ~ 1.4 higher organic nitrate yields for first-generation RO2 and RO2 isomerisation reactions. HO2/RO2 ratios were derived from measured and modelled radical concentrations, showing a 20 % smaller ratio for the VOCs forming HO2 in a multi-step reaction compared to VOCs forming HO2 in a single-step reaction. The production of odd oxygen (Ox = O3 + NO2) was calculated from modelled radical concentrations and from measured Ox for 3 < NO < 6 ppbv and for NO < 1 ppbv, where the Ox formation could additionally be determined from measured radical concentrations. Overall, a good agreement was found for the different approaches. In agreement with the observations of the HO2/RO2 ratio, a 20 % higher Ox production was observed for species, regenerating another RO2 radical before eventually forming HO2. Overall, the model-measurement discrepancies of the Ox production rates, as found in urban areas, were not observed in the performed chamber experiments. The nighttime oxidation of cis-2-butene and trans-2-hexene was tested in the presence of NO2 at different temperatures (from 3 °C to 32 °C). At low temperatures, time profiles of measured RO2 radical concentrations were significantly delayed and lower peak concentrations were reached than observed in the modelled RO2 radical time series. The model-measurement agreement could be significantly improved by including the formation of non-acyl peroxynitrates (RO2NO2) from the reaction of RO2 with NO2 in the chemical model for all formed non-acyl peroxy radicals. The formation of non-acyl RO2NO2, with the exception of methyl peroxynitrate, is not implemented in commonly used chemical mechanisms, such as the MCM, as it is thought to be negligible due to the short lifetime of alkyl (non-acyl) RO2NO2 of less than 1 s at 298 K. This study suggests that at 10 °C, 60 % of RO2 radicals are stored as corresponding peroxynitrates in the presence of only few ppbv of NO2, which may impact ambient RO2 and NOx (= NO + NO2) concentrations. In addition, a recent model study found an increase of NOx of up to 25 % on the ground, when including the formation of non-acyl RO2NO2. This suggests that these reactions should be included in chemical mechanisms for a better representation of the underlying chemistry.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Färber, Michelle faerbermichelle@web.de orcid.org/0009-0003-0302-6142 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-752931
Series Name:	Schriften des Forschungszentrum Jülichs, Reihe Energie & Umwelt / Energy & Environment
Volume:	657
Date:	2025
Publisher:	Forschungszentrum Jülich GmbH, Zentralbibliothek, Verlag
Place of Publication:	Jülich
ISSN:	1866-1793
ISBN:	978-3-95806-809-4
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Außeruniversitäre Forschungseinrichtungen > Forschungszentrum Jülich
Subjects:	Physics Chemistry and allied sciences
Uncontrolled Keywords:	Keywords Language anthropogenic VOCs English ozone formation English peroxynitrates English radical measurement English photooxidation English nighttime chemistry English multi-chamber comparison English SAPHIR English atmospheric simulation chambers English SAR English MCM English
Date of oral exam:	30 April 2024
Referee:	Name Academic Title Fuchs, Hendrik Prof. Dr. Wiesen, Peter Prof. Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/75293