Ozone Formation Sensitivity in the Great Lakes Region: Insights from a Weekday-Weekend Analysis Coupled with Examination of Trends over Space and Time

Ozone concentrations at surface monitors in parts of the Great Lakes region have consistently violated National Ambient Air Quality Standards (NAAQS) for O₃ over the last 40 years. As a result, many areas have been designated nonattainment of these standards. Emissions of the O₃ precursors, nitrogen oxides (NOx) and volatile organic compounds (VOC), have decreased dramatically since the 1990s. While these reductions have helped to decrease monitored O₃ concentrations, the O₃ concentration reductions have lagged behind the reductions in O₃ precursor emissions. Ozone is formed through complex, nonlinear reactions of NOx with VOC in the presence of sunlight. In order to develop the most effective O₃ control strategies, it is crucial to understand whether ozone formation is more sensitive to changes in NOx or VOC emissions. This study applies several linked analytical tools to air quality data in the Great Lakes region to determine whether ozone formation in the region is most sensitive to NOx or VOC emissions changes. This study also examines how the ozone-NOx-VOC chemistry has changed over the past decades.

This study takes advantage of three natural experiments in the atmosphere to examine the response of ozone chemistry to changes in NOx and VOC emissions. These experiments include changes between weekdays and weekends, changes over time, and changes over distance from city centers. In each of these cases, NOx emissions change more than VOC emissions, allowing insight into ozone formation chemistry in these areas. We used Classification and Regression Tree (CART) analysis to select days with ozone-conducive meteorology, usually characterized by high temperatures, low relative humidity, and/or location-specific transport directions and distances. We then examined the differences in ozone concentrations on weekdays and weekends at monitors over a 20-year time period to determine whether ozone formation at these sites was NOx- or VOC-sensitive or transitional during different times. We used cluster analysis to group monitors based on their ozone concentrations and weekday-weekend ozone differences. Examination of how ozone concentrations in these clustered monitors changed over the last 30 years and over space provided additional constraints on ozone formation chemistry in this region. The distance of monitors from lakeshores, particularly from Lake Michigan, was another important factor.

This analysis of ozone in the Great Lakes region shows that ozone concentrations have decreased in almost all areas over the past 30 years. As ozone has decreased, almost all areas have shifted towards more NOx-sensitive ozone formation. Most of the urban centers have shifted from VOC-sensitive in the 1990s to primarily NOx-sensitive by the 2010s, although areas of transitional chemistry remain in many city centers. These shifts were especially pronounced in the southern cities of St. Louis, Louisville, and Cincinnati. In contrast, most of the Chicago area appears to have chemistry that is shifting from VOC-sensitive to transitional, resulting in ozone concentrations that are increasing over time. The reductions in ozone concentrations appeared to have been driven by reductions in both NOx and VOC emissions. Importantly, reductions in reactive VOCs helped decrease ozone concentrations on ozone-conducive days even when ozone formation was heavily NOx-sensitive. This indicates that the greatest reductions in ozone will be achieved by a combination of NOx and VOC emissions.