MATRIX-ISOLATION AND COMPUTATIONAL STUDIES OF TRANSIENT POLYHALOGENATED INTERMEDIATES AND WEAKLY-BOUND COMPLEXES.

As time goes by and new innovations are brought up to improve living conditions, the human impact on the environment becomes more significant. It has been shown that man-made halogenated compounds play a key role in many real-world chemical processes. For example, in combustion, these compounds are used as fire retardant agents, and in atmospheric chemistry, they initiate ozone depletion reactions. However little is known about the mechanisms governing these processes and many intermediates involved in these processes have been elusive to researchers for diverse reasons such as short lifetime and difficulty in distinguishing products from parents molecules. The studies compiled in this work are focused on exploring the photochemical behavior of various intermediates derived from polyhalogenated compounds. These intermediates are trapped in inert rigid matrix and characterized by using IR, UV/Vis spectroscopy supported by computational methods. The photochemistry is explored using selected wavelength appropriate to each species. In this work, the photolysis products of CF2I2, CF2Br2, CXBr3 (X=H, D, F), C2H4Br2, C2F4Br2 have been generated and trapped in Argon or Neon matrices and most of them were characterized for the first time. We have also studied weakly bound complexes (C2H4***Br2, C2H4***I2), formed in matrix by co-deposition of monomers or by trapping fragments resulting from high voltage discharge (H2CXBr***Br; X=H, Cl, Br). We hope that our results will contribute to better understand the photochemical behavior of polyhalogenated species, and to some extent help to understand mechanism in different phenomena involving these species

New threats to the ozone layer: exploring ozone-depleting substances in the troposphere and stratosphere.

The release into the atmosphere of trace gases that contain chlorine and bromine can significantly impact stratospheric ozone depletion. The production and consumption of many ozone-depleting substances has been phased out under an international agreement called the Montreal Protocol on Substances that Deplete the Ozone Layer. Further monitoring is required to ensure the overall abundance of ozone-depleting substances continues to decrease in the atmosphere. This thesis focuses on East Asia which is a region where there can be rapid transport of ozone-depleting substances into the stratosphere and where continuing emissions of many ozone-depleting substances have been reported in recent years. In this thesis, a variety of halogenated trace gases were measured in air samples collected at ground level measurement sites and during aircraft campaigns via a gas chromatograph mass spectrometer system (GC-MS) to investigate new threats to the ozone layer. As part of the StratoClim project, a number of halogenated trace gases were analysed in air samples collected on board the Geophysica high altitude research aircraft over the Mediterranean, Nepal and northern India during the summers of 2016 and 2017 to investigate the composition of the Asian summer monsoon in the upper troposphere and lower stratosphere. Chlorinated very short-lived ozone-depleting substances (VSLSs) were found to be enhanced above mixing ratios measured in the tropical tropopause layer in 2013-2014. This indicates that large emissions of chlorinated VSLSs in East and South Asia can be rapidly transported by the Asian monsoon into the lower stratosphere, before they have broken down to insignificant levels, and therefore they can contribute to ozone depletion. Chlorinated VSLSs contribute only a small fraction of the total amount of chlorine in the tropopause region and the lower stratosphere. However, their contribution could delay the long-term recovery of the ozone layer. This thesis also presents updated long-term trends and global annual emissions of CFC-113a (CCl3CF3). The mixing ratios of CFC-113a are still increasing substantially and global atmospheric emissions of CFC-113a remained at about 1.7 Gg yr-1 between 2012 and 2016 after an increase in emissions in 2010−2012. Complementary ground-based observations in Taiwan suggest the presence of persistent emissions of CFC-113a in East Asia. The emissions are relatively small and it is possible they are due to the few remaining allowed uses of CFC-113a. Furthermore, atmospheric observations of multiple halogenated trace gases in Taiwan used with backward trajectory modelling found that CFC-11 (CCl3F) emissions coming from eastern China had increased from 12 (10-14) Gg yr-1 in 2008−2011 to 19 (14−23) Gg yr-1 in 2014−2018. This is about one-quarter of global emissions and the increase contributed to the recently discovered global increase in CFC-11 emissions. These results independently support the findings of other recent studies. The increase in emissions is possibly due to illegal production of CFC-11