Meteorological controls on ozone at Mount Washington, the highest peak in the northeastern United States

This study examined the synoptic and regional scale meteorological controls on summertime ozone at Mount Washington (1910 m), the highest peak in the northeastern United States. This site is part of the Atmospheric, Investigation, Regional Modeling, Analysis and Prediction (AIRMAP) air quality monitoring network located in New Hampshire, USA. The Appalachian Mountain Club has also monitored ozone at the summit and base of Mount Washington since 1987. A six-summer data set (1998-2003) from Mount Washington was used in our analysis of extreme ozone at this site. The extreme events were identified and examined in detail for two categories: (1) enhanced ozone, defined as 2-hour averages above the 90th percentile (~65 ppbv), and (2) depleted ozone, defined as 2-hour averages below the 10th percentile (~30 ppbv). Due to the high elevation of this site and the associated diurnal dynamics, afternoon and nighttime ozone events were studied separately. Mount Washington experiences a reversed diurnal ozone cycle, with ozone mixing ratios typically peaking after midnight. The mean and median hourly average ozone mixing ratios for the summer season lie between 39 and 53 ppbv. During the study period (1998-2003), the number of days each year with 8-hour averages greater than 80 ppbv has ranged from 0 in 2000 and 2003 to 8 in 2002. Although several multi-day regional ozone episodes were noted, most high ozone periods at Mount Washington during the six summers were short lived, frequently less than a day in length. An analysis of air mass transport to the site was conducted using backward trajectories. This analysis revealed distinct patterns in air mass history that help explain the variations in ozone extremes. Enhanced ozone events at Mount Washington were generally associated with westerly transport, while depleted ozone events corresponded to northwesterly transport. Coincident meteorological observations, provided by the Mount Washington Observatory, and synoptic conditions were also used to understand and interpret both depleted and enhanced ozone events. Contrary to similar studies, we have shown that most nighttime periods with ozone mixing ratios greater than 80 ppbv do not coincide with surface anticyclones centered over New England. Instead, periods of enhanced ozone commonly occur during regional warm sector flow. Our analysis has also identified a stratospheric contribution to a small percentage of enhanced ozone events at this high elevation site.