We examined seasonal and interannual variability in the mixing ratios of ozone (O3) and carbon monoxide (CO) and their relationships at a rural low-elevation site (24 m, 43.11¿N, 70.95¿W) and higher-elevation site (406 m, 43.75¿N, 71.35¿W) in New Hampshire, United States. The data were obtained over 2 years (2001--2003) by the Atmospheric Investigation, Regional Modeling, Analysis, and Prediction (AIRMAP) program at the University of New Hampshire. An analysis of the frequency distributions of O3 and CO grouped by wind speed and direction and over four time periods of the day showed that the most polluted air masses arrived from the southeasterly and westerly wind sectors during the local afternoon hours (1800--2300 UT, -5 hours for U.S. eastern standard time (EST)). In summer within this time window a well-defined positive O3-CO correlation with a slope ~0.37 existed in these air masses. Applying this relationship together with the CO emission inventory and its temporal profile from the emission model SMOKE, we estimate that 370 Mmol d-1 of O3 are exported from the northeastern United States within this time window during summer. However, over 70% of the data were obtained under weak wind conditions (<2 m s-1) when CO and O3 exhibited a dispersed relationship. An analysis using the ratio NO/NOy and the mixing ratios of NOy revealed a dominance of photochemically aged air masses with significant NOy removal accompanying high levels of O3. This result implies that elevated O3 in southern New Hampshire can largely be attributed to long-range transport of air masses from outside the region. Furthermore, the O3-CO relationship in each season produced a smooth lower CO boundary, which appears to represent air masses with a composition reflecting the evolutionary stages of photochemical aging and progressive phases of transport.