Tropospheric composition of organohalogens and alkyl nitrates: tropical and temperate case studies
The tropospheric composition of organohalogens and alkyl nitrates have been investigated from tropical and temperate environments. Ground based measurements and aircraft data are presented from the Oxidant and Particle Photochemical Processes (OP3) project, conducted in Borneo, 2008. Controlled experiments of temperate vegetation were also conducted to assess the emission of methyl halides from crop plants. Methyl halide results from OP3 contradict current assumptions of a strong source from tropical vegetation. High mixing ratios of methyl chloride and chloroform were observed in the boundary layer over oil palm plantations. OP3 aircraft data suggests that the oil palm plantations facilitate the formation of C2 to C4 alkyl nitrates. There was evidence that the southeast coast of Sabah is a source region for the bromocarbons measured. The short lived bromocarbons contribute to a bromine budget of 4- 6 ppt; this corroborates recent modelling estimates of their contribution to the stratospheric burden. In controlled experiments it was confirmed that the gene responsible for the emission of methyl halides is the HOL (HARMLESS TO THE OZONE LAYER) gene. The current WMO estimate for rapeseed contribution to the natural methyl bromide budget was shown to be an overestimate, based on the varieties studied in this thesis. Methyl iodide emissions from rice plants grown in soils were observed to be significantly lower than reported from rice paddies in the literature, suggesting that the growth conditions contribute to the production of methyl iodide.
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