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Title: Investigating urban atmospheric chemistry using a time of flight chemical ionisation mass spectrometer
Author: Priestley, Michael
ISNI:       0000 0004 8510 2283
Awarding Body: University of Manchester
Current Institution: University of Manchester
Date of Award: 2018
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Gases in the atmosphere can alter the climate system and reduce air quality. This is felt strongly in cities where emissions of gases are high and where people are likely to be exposed to them. Many chemical processes are poorly understood due to the highly complex interactions and variable composition of the urban atmosphere. This drives the development of new atmospheric measurement instruments that can reliably measure gases of very low concentration in real time. The iodide time of flight chemical ionisation mass spectrometer (ToF-CIMS) is one such instrument that is able to probe reactive atmospheric systems including molecules that contain chlorine and bromine atoms as well as molecules that contain many oxygens and carbons, all of which are of relevance for the study of urban air. An iodide ToF-CIMS was used at the University of Manchester for a two week period in October and November 2014 to detect previously unidentified gasses relevant to climate and air quality. A biomass burning event (Guy Fawkes Night) was sampled, from which markers of combustion (HCN and HNCO) and other newly detected nitrogen containing species (amides) were quantified and their emission ratios (NEMR) to CO calculated. The HCN NEMR of 1.11 ± 0.62 ppt ppb-1, whilst low is comparable to other biomass burning studies. Newly detected molecules with chlorine, oxygen and carbon atoms were also identified. Their contribution to free chlorine atoms was calculated and compared with the contribution from other compounds that contain chlorine atoms, but not carbon atoms. The detection of day time chlorine gas suggests sunlight is involved in its production and is a significant source of chlorine atoms throughout the day (74%), more so than nitryl chloride (23%) when it is very sunny. The newly detected molecules with chlorine, oxygen and carbon atoms are a negligible source of chlorine atoms (3%) in either sunny or not sunny conditions. The iodide ToF-CIMS was also deployed at a chamber where many products of benzene reacting with hydroxyl radicals identified. The concentrations of oxides of nitrogen were altered between experiments to see if any different products were made. These measurements were contrasted with a nitrate ToF-CIMS which is able to detect different products to the iodide ToF-CIMS. Detection overlap between the instruments was observed, however different ratios of oxygen atoms per molecule to carbon atoms per molecule were found for molecules that contained the name number of carbon. Products identified by iodide ToF-CIMS in the chamber were then later identified in the Manchester urban air, if they contained 6 carbon atoms and 0.3 - 0.5 nitrogen atoms per 1 carbon atom and 1.5 - 2.0 oxygen atoms per 1 carbon atom. This suggests the products sampled in the chamber may not represent those in Manchester urban air.
Supervisor: Gallagher, Martin ; Coe, Hugh Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available