The origin of carbon monoxide within the region covered by the Tropospheric Ozone Production about the Spring Equinox (TOPSE) measurement campaign during the winter and spring of 2000 is examined using a chemical transport model. The origin relates to the location (United States of America, Europe, Southeast Asia, and rest of the world) and time (June 1999 to May 2000) at which the CO was emitted from the surface. The model tracks CO from each source and from hydrocarbon oxidation, where each tagged CO reacts with OH. The analysis focuses on the relative distribution of each tagged CO. Changes in the transport and the photochemistry during the transition from the winter to spring months are particularly emphasized. In the high latitude lower troposphere, CO is strongly influenced by fresh emissions from Europe during the winter months. Otherwise, the analysis indicates the importance of the buildup of aged emissions during the winter season. The contribution from hydrocarbon oxidation is a significant fraction of the total CO, although the bulk of this oxidation occurs well away from the region of interest. During the spring months the transport of episodic emissions from Europe decreases in the lower troposphere and the age of CO generally decreases elsewhere.