Laboratory studies of the products, mechanisms and kinetics of heterogeneous reactions on organic aerosol proxies

• Main Author: Last, Deborah J.
• Published: University of Manchester 2008
• Subjects: 551.5
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.491664

Aerosol particles have a significant impact on atmospheric processes through interacting directly with solar radiation, influencing cloud cover by acting as cloud condensation nuclei and by providing a surface on which heterogeneous reactions can take place. This thesis describes the development of an Aerosol Flow Tube (AFT) system with FT-IR detection for the study of heterogeneous processes occurring on aerosols. The major structural components, gas handling, particle generation, size distribution characterisation, and collection systems, and infrared optics are detailed, and high quality spectra of a range of inorganic, organic, and mixed inorganic-organic aerosols presented. Concurrent to the development ofthe AFT-FTIR system, a variety of analytical techniques were employed to investigate the ozonolysis of solution phase and thin film proxies. Infrared spectroscopy and mass spectrometry identified acids, aldehydes, esters, and ketones as products of the reaction, some of these being high molecular weight (HMW) compounds. These were concluded to be formed by combinations of Criegee intermediates (CI) with acid and aldehyde functionalities, leading to HMW products through a variety of linear and branching polymerisation pathways. Evidence was found of additional reaction pathways for CIs in the presence of aromatic rings, the nature of which bears further investigation. The distribution ofHMW products was observed to change with increased ozone exposure, leading to an increased concentration ofcyclic peroxides, suggesting some oxidation or decomposition of the initially formed aacyloxyalkyl hydroperoxides (AAHP) and peroxyhemiacetallinkages. The kinetics ofthe heterogeneous reaction of ozone with unsaturated organics was investigated in thin films in an ATR-IR cell and aerosols in the AFT. In the thin film studies a reactive uptake coefficient of (6.8±OA) xl 0-5 was obtained for oleic acid, an order of magnitude or so lower than that observed by most other researchers, whilst maleic acid and anthracene oxidation was found to be too slow to reliably study under the same conditions, but in the presence of oleic acid reactive uptake coefficients were found to be enhanced by factors of 6 and 10 respectively. In the AFT the decay of oleic acid aerosol was found to be well modelled by assuming a diffusion limited reaction, with reactive uptake coefficient (y) of 8.3xlO-4, in excellent agreement with other studies. The ozonolysis of anthracene coated on ammonium sulphate particles and as an internally mixed aerosol with oleic acid was investigated, and despite the considerable experimental challenges in studying such systems, the increased reactivity ofanthracene in the presence of oleic acid was again observed.