O₃ and CO in New England: Temporal variations and relationships

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 μmol 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/NO_y and the mixing ratios of NO_y revealed a dominance of photochemically aged air masses with significant NO _y 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. Furtherniore, 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.