Chemistry and Photochemistry of atmospherically important peroxy radicals
Dr Mark Blitz (SoC), Prof Paul Seakins (SoC)Contact email: firstname.lastname@example.org
The oxidizing capacity of the troposphere is controlled by radical reactions, where the OH radical plays the dominant role. For this reason it is important to know OH sources / sinks. Knowledge on this can be critically assessed from measurement of the OH concentration, [OH], in the atmosphere, where the majority of OH measurements have been made with the low pressure laser induced fluorescence (LIF) FAGE technique (Fluorescence Assay by Gaseous Expansion). Atmospheric chemical models are used to indicate how well these sources / sinks are understood. In general, these models do a reasonable job in predicting the [OH] in remote and urban environments, where the chemistry is simple and NO dominated, respectively. In pristine forested, tropical environments, where the chemistry is dominated by isoprene emitted from the trees, models do a poor job and under predicts the [OH], with differences in the measured and modelled [OH] up to a factor of 10 reported.[3, 4] This dramatic enhancement in the measured [OH] compared to modelled has also been observed in less pristine environments, where there is a wider range of volatile organic hydrocarbons, including isoprene, but low concentration of nitrogen oxides, [NOX].
There has been much scientific endeavour into identifying missing sources of OH, where explanations have revolved around isoprene. In the atmosphere the reaction between OH and isoprene, C5H8, is fast and its addition product is rapidly captured by oxygen:
where HO-C5H8-O2 is a peroxy radical, RO2. The literature has showed that while some RO2 will react with HO2 to form OH but this is not a significant channel for HO-C5H8-O2. A more promising source of OH from HO-C5H8-O2 has been identified by Peeters and co-workers, where using ab initio theoretical calculations an efficient isomerization route from HO-C5H8-O2 to hydroperoxyaldehyde (HPALD) was identified; HPALD is expected to efficiently photolyse to OH:
Experiments have determined the rate of this isomerization, and it was slower than the calculated rate implying that this mechanism cannot explain all the missing [OH], but it is still a significant minor source.
More recently we have studied the likelihood that HO-C5H8-O2 is a photochemical source of OH:
Figure 1: Left: Apparatus to determine the OH yield from RO2 photolysis, where RO2 is generated by the photolysis of H2O2 in the presence of C5H8/O2 using the excimer laser. RO2 is photolysed over a range of the wavelengths using the high energy dye laser and the resultant OH is detected using the probe laser, 308 nm; Right: the three laser experiment yields the blue trace the signal before t=0 is equal to H2O2 photolysis, which allows the RO2 signal, after t=0, to be quantified.
We have measured the efficiency of P1, see Figure 1, where two lasers are used to make RO2 and then photolyse it. The third laser is used to probe OH, 308 nm, blue. The results from our UV measurement on P1 have been put into a chemical model. Only small changes in [OH] was observed.
Therefore the high observed [OH] in specific field campaigns remains unexplained and attracts much speculation.
This project will determine the photochemical parameters (P1) of a range of (mainly aromatic) peroxy radical species that are known to have significant absorption in the 300 – 400 nm range, with particular attention to species that can generate OH. In addition, the chemical isomerization of many peroxy radicals to OH/HO2 (R3-type reactions) is still and unknown:
RO2 -> OH/HO2
The reason these isomerization rate constants are unknown is because they are slow, too slow to be studied in the laboratory at room temperature. However, our laboratory apparatus allow reactions to be studies at temperature up to 800 K. Hence at the appropriate temperature the isomerization rate constants can be determined, which can then readily be extrapolated to atmospheric conditions via reaction rate theory.
The impact of these laboratory generated parameters is assessed in atmospheric models. From the purely chemical model (MCM: http://mcm.leeds.ac.uk/MCM/), which can assess local air quality and composition, to the 3-D chemical transport models that can assess such things on a global scale.
The student will work under the supervision of Dr Mark Blitz and Prof Paul Seakins within the Atmospheric and Planetary Chemistry research group, School of Chemistry. Within the group there is a wealth of knowledge running many laser based experiments, which require regular maintenance, repair and development. You will learn the skills to carry out experiments to determine the relevant data, which is often discussed in the weekly group meeting. The repair and development of the experiments is carried out in conjunction with the senior members of the group, who can provide help / guidance. Potentially you will be conversing the University Workshops (Mechanical and Electronic) and outside companies with aspects of your experiment. By the end of your PhD you should be independently carrying out experiments, with good knowledge of future experiments.
Computers are used to control the experiment and process the data produced. LabVIEW is used to control the experiment and Origin is used to process the raw experimental data. Gaussian and MESMER is used to theoretically test the experimental data, before using it is put into atmospheric models. The University runs a number of computer courses in order to run the software used in the group. In addition the University also runs workshops on the generic skills for students that ultimately prepares you for your viva, (http://www.emeskillstraining.leeds.ac.uk/).
The majority of this project is laboratory based where lasers will be used to both create / photolyse / monitor species over conditions that will generate the parameters relevant to atmospheric chemistry. These experiments are controlled by hardware and software, so a strong experimental background with good knowledge of computers is desirable. The data collected from these experiments is then processed, parameterized and often modelled using theoretical descriptions. This data processing uses computer software such Gaussian / Origin / MATLAB and code developed within the group, MESMER, https://www.chem.leeds.ac.uk/mesmer.html. So some computer programming skills will be required.
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