BayCEER Workshop 2023

Unraveling Photooxidation Mechanisms: Introduction of the Bayreuth Atmospheric Simulation Chamber and an On-Line Double Derivatization SPME-GC-MS Method

P 4.3 in Thinking sharp: Insights from inbetween

The Earth’s ecosystems and human activities copiously release volatile organic compounds (VOC) into the air. In a sunlit atmosphere, these compounds readily undergo photooxidation, affecting air quality, weather, and radiative forcing. To improve photochemical models and support policies, the underlying gas-phase reaction mechanisms need to be elucidated in detail. This requires controlled experiments as well as the sensitive and selective analysis of the formed oxidation products.

Here, we present the re-commissioned Bayreuth Atmospheric Simulation Chamber and its capabilities to mimic photooxidation processes under various realistic tropospheric conditions. As an example system, we investigated the reaction of OH radicals with toluene, one of the most abundant aromatic hydrocarbons in the atmosphere. Chamber- and compound-specific losses (photolysis, follow-up chemistry, wall losses) were characterized and corrected to derive reliable product formation yields.

The emerging multifunctional oxidized products (e.g. aldehydes, ketones, acids, alcohols) were determined in low mixing ratios (pptv - ppbv) using two on-line methods: Firstly, a novel solid phase microextraction – gas chromatography – mass spectrometry (SPME-GC-MS) method with double derivatization resolved the prevailing isomers. A customized permeation source was set up for the on-line addition of internal standards to ensure reproducibility and compensate fiber effects. Secondly, a proton transfer reaction – time of flight – mass spectrometer (PTR-ToF-MS) enhanced the temporal resolution and the range of analyzed compounds. Here, we discuss the implementation, capabilities, and limitations of the developed methods.

Jointly, the chamber and the presented analytical methods provide a means of performing mechanistic and kinetic photooxidation studies of isolated VOC precursors, of investigating atmospheric particle aging, and of evaluating the fate of mixed VOC systems such as from specific emission sources.

Keywords: Atmospheric Simulation Chamber, Photooxidation Mechanisms, On-line Double Derivatization SPME-GC-MS, Method Development