RESEARCH
Staff
Reactions in aqueous phase: Kinetic and mechanistic studies
Laser-based studies of atmospheric radical reactions in aqueous solution are a major activity in the laboratory. These reactions are of importance for chemical processes in haze particles, fog, rain and clouds. In order to understand oxidation processes of organic compounds (alcohols, carbonyls, acids, aromatics) within the tropospheric multiphase system reactions of OH, NO3, SOX- and halogen radicals are characterized as a function of temperature, pH and ionic strength. Measurements were done with a laser flash photolysis long path absorption (LFP-LPA) set-up. Kinetic studies were done directly or using competition kinetic applying a pulsed (Excimer) and different continuous wave lasers (Argon ion laser, diode laser, He-Ne-laser, He-Cd-laser). The experimental set-up consists of a multi gas excimer laser (Kr/F - λ = 248 nm, Xe/Cl - λ = 308 nm or Xe/F - λ = 351 nm), which has a pulse length of 10 - 40 ns. Analytical light sources are both continuous wave lasers and lamps. In order to increase the optical path length of the analytical light through the reaction cell two mirrors in White configuration are used (Fig. 1).
Fig. 1: LFP-LPA set-up used for
kinetic studies of NO3 radical
reactions in aqueous solution.
Detectors applied are photodiodes, photomultiplier, diode arrays and CCD-cameras. Therefore, experimental set-ups can be used for both kinetic studies and spectroscopic studies of short-lived transient species (e.g., organic peroxy radicals, Fig. 2).
Fig. 2: LFP-LPA set-up used for kinetic
and spectroscopic studies of OH radical
reactions in aqueous solution.
Furthermore, the formation of stable oxidation products inside the reaction cell can be investigated applying modern analytical techniques (HPLC-MS, HPLC-DAD, IC-EC).
A current project is the development and characterization of a fog chamber (Fig. 3). This chamber will allow investigations on multiphase chemical processes in fog. For this purpose an impactor is used to probe fog droplets inside the chamber. Droplet size distributions in the chamber can be measured with a Argon ion laser.
(For further information see Poster 1, Poster 2, Poster 3)
Fig. 3: Fog chamber for the investigations
on chemical processes in fog.
Photochemistry
The Photochemistry of iron-organic complexes plays an important role in cloud droplets and deliquescent particles. Radicals can be generated through complex photolysis which influences the oxidation budget of the troposphere and thereby the conversion of many compounds as well as the red-ox cycling of iron. Laboratory investigations of photochemical quantum yields and photochemical reaction mechanisms are necessary to understand the impact of iron-complex photochemistry including the subsequent reactions. Atmospherically relevant iron-organic complexes are studied with the help of speciation-calculations based on known complex stability constants, laser-flash photolysis followed by UV-VIS spectroscopy and suitable radical scavenging techniques.
Phase transfer processes
For the investigation of phase transfer processes and the reactivity of trace gases and radicals on liquid surfaces a single droplet experiment is available. The uptake of organic substrates into aqueous drops can be measured using a flow tube reactor. The experimental set up comprises a single, stable pendant drop on a pipette's tip integrated in the flow reactor tube (Fig. 4). The analysis of the liquid phase is performed by UV-VIS spectroscopy (UV-VIS spectrometer / CCD - camera). Furthermore, a FT-IR spectrometer is available for the analysis of the gas phase. This experiment allows the measurement on fundamental phase transfer parameters, e.g. mass accommodation coefficient α and rate constants of possible surface reactions.
Fig. 4: The section for drop generation and analysis in the single drop experiment.
(For further information see Poster 4.)
The obtained data from the laboratory studies are implemented in a multiphase chemistry mechanism (CAPRAM). The extension of the CAPRAM mechanism will be done in close collaboration with the IfT department of modeling in order to implement new chemical mechanisms in current models. Based on laboratory experiments, different applications (clouds or aerosol chemistry) modules are already developed for the further improvement of higher scale models.
Current topics
Projects
TRACES (Ocean-Atmosphere-Land Impacts on Tropical Atlantic Ecosystems).
(WGL Network, "Pact for Research and Innovation")
