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\chapter{Introduction}  \section{Trace Gases}  CO is produced globally from four main sources; biomass burning, fossil fuel burning, oxidation of non-methane hydrocarbons (NMHC), and oxidation of methane (CH$_{4}$) \cite{Seiler1987,Novelli1992}. Biomass burning in particular is a major source year round in the tropics and during summer and autumn in the high northern latitudes \cite{Galanter2000}.  In the southern hemisphere, the distribution of CO is largely affected by biomass burning \cite{Edwards2006,Gloudemans2006}. The main sink of CO in the troposphere is through reaction with the hydroxyl radical (OH). With a relatively long lifetime ranging from 2 weeks to a few months, depending on the levels of OH, CO is it an appropriate indicator of tropospheric pollution and transport \cite{Hough1991,Khalil1983,Holloway2000,Novelli1992}. In the mid to high northern latitudes the seasonal cycle of CO is driven by the variation of OH. CO is at a minimum in summer when OH concentrations are high and CO is at a maximum at the end of winter after a gradual build up due to the low OH concentrations. In the tropics, the maximum CO concentration occurs in September-October, correlating to the end of the biomass burning season \cite{Hough1991,Novelli1992}. CO has a relatively long lifetime, ranging from two weeks to a few months, depending on the levels of OH, making it an appropriate indi- cator of tropospheric pollution and transport \cite{Hough1991,Khalil1983,Holloway2000,Novelli1992}.