Summertime ozone at Mount Washington: Meteorological controls at the highest peak in the northeast

This study examined the synoptic and regional-scale meteorological controls on summertime O-3 at Mount Washington, the highest peak (1910 m) in the northeastern United States. Analysis of air mass transport to Mount Washington was conducted for the summers of 1998-2003 using backward trajectories. Distinct patterns in air mass history were revealed using this approach that helped explain extreme variations in O-3 mixing ratios. Most enhanced (greater than or equal to90th percentile) and depleted (less than or equal to10th percentile) O-3 events were short-lived and spread out over the summer months. Enhanced O-3 events at Mount Washington were generally associated with westerly transport, while depleted events corresponded to northwesterly transport. Periods of O-3 greater than 80 ppbv during nighttime periods coincided with westerly (71%) and southwesterly (29%) transport. Periods of elevated O-3 commonly occurred during regional warm sector flow or on the western edge of a surface anticyclone. Our analysis also identified a stratospheric contribution to a small percentage (similar to5%) of extreme O-3 events at the site, but more evidence is required to establish the significance of the contribution to background O-3 levels in this region.