PHOtolysis Reaction Mechanisms by Emerging and New Technologies - PhoRMENT

Photochemistry controls a vast array of the natural and man-made chemical processes: photosynthesis, plasma technology, solar energy, combustion and atmospheric chemistry. An important example is atmospheric photo-oxidation, which drives the atmospheric radical propagation cycles that breakdown primary pollutant emissions, but also controls the formation of secondary pollutants such as ozone and oxygenated volatile organic compounds (VOC), which can have significant impacts on climate, air quality and human health.

Despite the clear importance of the subject, experimental limitations have inhibited the study of even the simplest processes by which small gas-phase molecules interact with chemically active ultra violet (UV) light. In this project you will exploit new and emerging technologies such as low cost UV LEDs, chemosensors (developed in York) for sensitive and selective free-radical detection, online mass spectrometry, and modern theoretical computational methods to study atmospherically important photolysis reactions.

Among the largest classes of chemicals emitted or produced in the atmosphere are "carbonyls" - organic molecules containing one or more carbonyl functionality. Carbonyls are used in in a vast array of industrial applications, including directly as solvents, pesticides and biofuels, and as reagents for production of pharmaceuticals and aromachemicals. Carbonyls are also ubiquitous in both indoor and outdoor air. Direct emissions are supplemented by in-situ production as virtually all organic compounds break-down through atmospheric oxidation processes via multiple generations of carbonyl intermediates (Figure 1), where they can significantly impact on air quality and health. As an example, carbonyl photolysis has been shown to drive large wintertime ozone formation in the oil and gas "fracking" fields of the Unitah Basin in northeastern Utah.

The photochemistry of carbonyls is therefore both chemically interesting and important. Unusually amongst atmospheric organics, they are broken down by abundant UV-A radiation (e.g. Figure 2). Therefore this project is particularly timely, as the photochemical environment is rapidly changing indoors. LED lighting is replacing fluorescent and incandescent technology, whilst UV and plasma based air filtration systems are increasingly used in an effort to enhance indoor air quality and to suppress virus spread.

Objectives:

To determine absorption cross-sections and photolysis quantum yields for a variety of important gas-phase carbonyl species; to assess photolysis rates and product branching ratios, over a range of indoor and outdoor conditions, and hence air quality impacts; to identify how chemical structure and additional functionalities in carbonyls impact upon photolysis rates.

Experimental Approach:

(1) Characterisation and development of a newly commissioned fast-flow reactor, coupled to the use of chemosensor radical traps and on-line mass spectrometry for quantum yield determinations
(2) UV-vis spectroscopy techniques for absorption cross-section measurements
(3) GAUSSIAN quantum chemical toolkit for theoretical thermodynamic calculations and chemical structure determination
(4) Development of models incorporating the master chemical mechanism (MCM, mcm.york.ac.uk) for experimental design and environmental impact assessment