| Summary: | In this thesis the TOMCAT chemical transport model is used to investigate the processes which control the concentrations of CO and O3 in the Arctic troposphere. Particular focus is on understanding the main sources of CO, O3 and NOy species in the Arctic, distinguishing between natural and anthropogenic sources and the current drivers of interannual variability (IAV). First results from a new version of TOMCAT, with extended hydrocarbon chemistry and heterogeneous uptake of N2O5, shows better agreement with observed CO from MOPITT, surface stations and aircraft. Changes in simulated burdens demonstrate the importance of NMHC as a source of CO, O3 and PAN in the troposphere and show that the complexity of chemical schemes may have contributed to previously reported inter-model differences. The high PAN sensitivity to additional NMHC is particularly important in the Arctic as it is the dominant source of NOx in the Arctic lower troposphere, producing up to 30% of total O3 in the summer. This thesis contains the first source contribution analysis to consider impacts of fire emissions throughout the year in comparison to anthropogenic sources. Anthropogenic emissions are found to be the largest source of Arctic CO (48%), followed by methane (25%) and fires (13%). In summer, fire and anthropogenic sources contribute equally to the total CO burden. Boreal fires are the dominant source of O3 and NOx compared to anthropogenic emissions. North America contributes the largest amount (30%) to the total anthropogenic CO burden, followed by East Asia (26%), Europe (23%) and South Asia (9%). In contrast, North America makes the largest contribution (9%) to the Arctic O3 burden, followed by Europe (7%) and then Asia (6%). Overall, CO shows that the Arctic is most sensitive to emissions changes in Europe, then North America and then Asia. Fire emissions are the dominant driver of current Arctic CO IAV, causing 84-93% of observed variability. A statistically significant correlation is found between observed CO and the El Nino 3.4 index due to a link with fires. El Nino is strongly associated with increased fire emissions in regions of North, Central and South America, Africa, and Asia. In contrast, El Nino is associated with reduced fire emissions in eastern North America, Europe, southern Asia and Australia. The temperature dependence of fires in several regions indicates that fire activity will increase in a warmer climate.
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