OBSERVATIONS OF DEPLETED OZONE WITHIN THE BOUNDARY-LAYER OF THE WESTERN NORTH-ATLANTIC

Ozone measurements taken between 0.90 and 2.5 km above the surface and extending over an 800-km radius from Halifax, Nova Scotia, are presen ted from aircraft flights between August 21 and September 14, 1992. Th e mean ozone mixing ratio was found generally to be greater above the top of the mixed layer than near the sea surface. Eleven of the 32 ver tical profiles displayed an abrupt transition at the top of the bounda ry layer, with surface ozone mixing ratios having values of approximat e to 15-20 ppb and values above the boundary layer increasing to appro ximate to 50-60 ppb. This transition between low and high mixing ratio s was observed to occur over a vertical scale of less than 0.5 km in s oundings taken within 4 hours of each other over horizontal distances of the order of several hundred kilometers. There was a well-mixed bou ndary layer in all cases where these sudden transitions in the ozone p rofiles were observed. These profiles are associated with subsidence o ver land, followed by dry deposition within a hydrocarbon-poor, well-m ixed continental boundary layer. Ozone loss through surface deposition exceeded ozone production by the time the air masses arrived at the m aritime coastal waters. Two other broad categories of profiles are des cribed, the most common having the ozone mixing ratio increasing linea rly with height to the top of the sampling domain at 2.5 km. A third c ategory had only a local maximum in ozone with much smaller values hig her and lower in the atmosphere.