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Case Studies

Studying Atmospheric Chemistry with Absorption Spectroscopy

Studying Atmospheric Chemistry with Absorption Spectroscopy

Researcher Objectives

Dr. Daniel Stone of the University of Leeds in the United Kingdom investigates oxidation processes in atmospheric and combustion chemistry. He is particularly interested in the chemistry of reactive species such as OH, HO2, and Criegee intermediates (R2COO) that control atmospheric composition and fuel combustion. His research requires a combination of laboratory experiments, field measurements, and numerical modeling.

“The ability to set up FERGIE so quickly… no alignment was necessary… allowed me to focus on integrating it within the experiment straight away.”

— Dr. Daniel Stone

Past laboratory experiments conducted by Dr. Stone to investigate the kinetics of the CH2OO Criegee intermediate provided the first direct measurements of CH2OO reaction kinetics as a function of pressure, obtained by monitoring the HCHO reaction products by laser-induced fluorescence spectroscopy (Stone et al., 2014). This work also indicated significant yields of CH2OO following photolysis of CH2I2 in the presence of O2 under atmospheric conditions (Stone et al., 2013), which impacts the understanding of oxidation chemistry in iodine-rich coastal regions.

Dr. Stone’s future work will involve the development of a quantum cascade laser (QCL) infrared absorption experiment to monitor Criegee intermediates directly under atmospheric conditions, and to monitor the production of SO3 in reactions of Criegee intermediates with SO2. These experiments will enable assessment of the atmospheric impacts of Criegee chemistry on production of sulfuric acid and sulfate aerosol, and thus on air quality and climate change.

 

“Once FERGIE was integrated into the experiment setup, I was free to determine the triggering and able to produce relevant time-dependent data within a single measurement day.”

Click images below to enlarge and see captions.

FERGIE in Action

Recently, Dr. Stone designed an experiment with the FERGIE system to measure the transient absorption of a gas/gas mixture after flash photolysis using a high-power laser pulse. A fiber was readily connected to the FERGIE fiber port in the experiment setup (see Figure 1) and the trigger input of FERGIE was utilized to sync acquisition with an external delay generator.

For the experiment (see Figure 2), FERGIE’s spectra kinetics mode (with a window height of 50 rows) provided a time scale of ~290 µsec per spectrum. This enabled detection of fast changes in the absorbance, on the msec to sub-msec scale (see Figure 3). A spectrum taken a few msec before the flash photolysis pulse was employed as an absorbance reference.

Dr. Stone repeated the experiment 100 times to increase sensitivity. Afterward, a free-space coupling was attempted using a pinhole slit
(see Figure 4).

“The time scale can be reduced by a factor of 5x to 6x by reducing the spectra kinetics window height.”

Dr. Mark WaterlandDr. Daniel Stone
NERC Fellow / University Academic Fellow
School of Chemistry
University of Leeds, United Kingdom

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