4. SYNOPTIC TRANSPORT REGIME

Ozone episodes in the Northeast occur primarily under the influence of high-pressure systems or in front of advancing cold fronts (Vukovich et al., 1977; Vukovich, 1979; Comrie and Yarnell, 1992). The episodes examined in this report fit this pattern, with the most widespread ozone exceedances occurring on the "back side" of the high. Figures 4-1 and 4-2 show that the July 14-15 episode occurred when a high-pressure system was centered over the southeast United States, with generally southerly to southwesterly flow near the surface and westerly to northwesterly flow aloft. The high-pressure system for the July 31-August 1 episode (Figures 4-3 and 4-4) was centered over southwestern Pennsylvania on July 31 and moved southeastward on August 1. The surface winds were generally light and variable throughout much of the study region on July 31, turning to southerly to southwesterly on August 1. The boundary layer winds aloft were light and variable until the evening of July 31 and then turned southwesterly to westerly to northwesterly in the lower (500 m), middle (1000 m) and upper (2000 m) boundary layer, respectively.

The winds at 0700 EST at the surface and in the upper boundary layer (1500 m ±) are shown in Figures 4-1 through 4-4. The upper-boundary-layer winds in the study area were generally from the west to northwest during both episodes. This aloft wind direction is characteristic of regional ozone episodes. Gaza (1996) has shown that for the New York City area, the daily average 850 mb (about 1500 m msl) wind direction was between west and northwest on over 95 percent of the ozone exceedance days from 1992-1995. During the day, the winds shifted from west to more northwesterly on average, as the highs migrated southeast. Over 80 percent of the average winds were between 5 and 10 m/s. If the winds were steady across the region, this would imply an average transport distance of roughly 400-800 km in 24 hours. For the Washington DC area, the vector mean winds at 850 mb for exceedance days from 1987-1995 were from 295 degrees at 3.8 m/s (Ryan et al., 1996). On the highest ozone days, the winds were more northerly at similar speeds. The lower speeds in the southern OTR might imply a shorter average transport distance of about 300 km in 24 hours.

The above extrapolations of transport distance can be tested by the trajectory analyses. The transport distances suggested by Gaza's and Ryan's analysis turn out to be supported by trajectory analyses such as those shown in Figures 3-6 to 3-8. The straight-line lengths of the trajectories at the south of the study region ranged from about 300-500 km in 24 hours, while the trajectories in the northern portion were in the 600-800 km range for the three days examined.