Surface-Level Ozone Variability in the Gulf of Maine during ICARTT 2004

During July and August 2004, the PSI UV Ozone Photometer was deployed on a commercial cruise ferry to measure temporal and spatial variations in ozone off the New Hampshire coast. The MV Thomas Laighton, operated by the Isles of Shoals Steamship Company in Portsmouth, NH, provided a mobile platform from which to conduct twice daily measurement transects between the coastline and the Isles of Shoals area some 8 km offshore. Ozone mixing ratios, ambient air temperature, and GPS latitude and longitude were sampled at a 1 Hz data rate via a mast and forward-facing air sampling inlet extending into the free stream above the wheelhouse of the vessel. The spatial resolution of the 1 Hz measurements was 5 to 10 m. Previous measurements of this type, during NEAQS 2002, showed extensive spatial variability in off-shore ozone mixing ratios during high-ozone events. The 2004 measurements sampled primarily very low ozone levels associated with clean air from the north and east, as well as a limited set of medium-ozone events associated with southwesterly winds from the polluted urban corridor. As in 2002, the data show substantial spatial variability in ozone mixing ratios for a given transect. These include frequent small-scale depletions in ozone on the scale of tens of meters, due to titration of ozone by localized NOx emissions including the ship's own exhaust, and large scale ozone depletions on the scale of km, associated with medium-high-ozone events. The results are compared to calculated air parcel trajectories, and to ozone measurements from fixed-site instruments on Appledore Island and on a buoy stationed 20 km east of Appledore Island. In general, the off-shore ozone concentrations appear to be greatly elevated only during periods of southerly or southwesterly winds, and are modulated by a complex sea breeze/land breeze effect near the coastline.