8. OZONE TRANSPORT ALOFT

Ozone can be transported aloft overnight and contribute to the regional background concentrations the next day. Ozone concentrations in the boundary layer above the surface layer were above clean air concentrations in the morning during both of the episodes discussed here. The July 31-August 1 episode provides a good example of overnight transport of this carryover ozone.

The winds aloft on both sides of the Appalachians for these days are shown in  Figures 8-1 and 8-2. Both Holbrook in southwest Pennsylvania on the west side (Figure 8-1) and Gettysburg on the east side (Figure 8-2) had relatively stagnant conditions in the lower 1500 m msl of the atmosphere until evening on July 31. Note that these figures show wind above ground level (agl) and that the ground elevation at Holbrook is 454 m msl. During the night and early morning hours, the winds aloft accelerated at both locations, with the strongest winds forming a jet between about 550 and 1000 m msl in both places. The surface winds remained calm or very light during the night.

At Gettysburg, substantial differences between the surface and aloft winds persisted until about 1600 EST on August 1, when the atmosphere was well mixed and winds were similar throughout the vertical sounding. After about 1800 EST, the aloft winds again decoupled from the surface, and the jet started to form again. The strongest winds aloft in the lower 1500 m of the atmosphere at both locations were during the night when frictional effects were decoupled, and the lightest winds aloft were during the day. Ozone and precursors that remained aloft on the night of July 31-August 1 could have been transported up to 400 km as shown in the trajectories in Figure 3-8 (and later in this section). From the trajectories (Figure 3-8) and the aloft winds (Figures 3-5, 5-9, 8-1, and 8-2) it is clear that the speed and direction of the aloft transport was substantially different than the near-surface transport, with the air aloft coming from the southwest to west and the air near the surface coming from the southeast to southwest overnight.

The amount of ozone available for transport overnight is indicated by the maximum ozone concentrations on July 31 (Figure 8-3) and by the vertical ozone profiles on the afternoon of July 31. Figure 8-3 shows that on July 31, ozone concentrations exceeding the federal standard were confined to the immediate vicinity of the major urban areas. This is consistent with the location of the high pressure on that day directly over the study area (Figure 4-3) and the light and variable winds aloft. Transport at both the surface and aloft on July 31 would have been minimal. The highest concentrations were near the western and southern OTR urban areas. Even though the exceedances were near urban areas, the maximum concentrations in nonurban areas still reached the 80-110 ppb range in the southern and western OTR, including Pennsylvania, New Jersey, Delaware, Maryland, and Virginia. In the northern and northeastern OTR, the nonurban concentrations generally were in the 60-90 ppb range.

In the rural southern and western portions of the OTR, Figure 8-4 shows that afternoon ozone concentrations as high as 110 ppb were well mixed to above 1500 m msl over large regions. This aloft ozone was available for transport overnight.

Figure 8-5 shows the ozone and wind speed aloft over Gettysburg during the night of July 31-August 1. Concentrations above 100 ppb can be seen aloft all night and were transported at speeds of 8-10 m/s for most of the night. Figure 8-6 shows vertical ozone profiles for several sites the next morning. This figure shows that ozone was carried over aloft throughout the study area at concentrations of 80 to over 100 ppb in westerly or southwesterly flow.

Figure 8-7 shows trajectories ending at 0500 EST on August 1. From these rough trajectories, it is clear that ozone from the central and southern portions of the eastern OTR on July 31 was carried over aloft to the coastal central and northern portions on August 1. Straight-line transport distances aloft appear to be on the order of 200-400 km in 12 hours overnight, with the longer transport in the northern OTR. In particular, the early-morning ozone seen aloft over Gettysburg in Figure 8-5 would have traveled over 200 km overnight.

Figure 8-8 shows the maximum ozone concentrations on the afternoon of August 1. On this day, exceedances were seen throughout the urban corridor. The high concentrations had spread north and east along the corridor, with the highest concentrations in Connecticut and Maine. Rural concentrations decreased slightly from the day before in parts of Virginia, but increased 20-40 ppb over much of the eastern and northern OTR. While it was shown earlier that the high concentrations in coastal Maine were associated with near-surface transport, the concentrations at most inland locations on that day could have been affected by the ozone transported aloft overnight when the mixing layer increased in midday. Figure 8-9 shows a vertical ozone profile for New Haven on the coast of Connecticut. The concentrations aloft were over 120 ppb through 1500 m msl. (Although the surface concentrations at New Haven were also high, they were disconnected from the aloft concentrations at the time of this spiral due to undercutting by marine air near the surface. At a farther inland location, the concentrations seen aloft would have been mixed to the surface by heating and turbulence). From the back trajectories shown earlier in Figure 3-8, the air aloft in this spiral would have been near the location of the Kunkletown, PA spiral in Figure 8-6 in the early morning. The morning aloft concentrations at Kunkletown were about 100 ppb through 1500 m msl. Thus the aloft carryover might have accounted for 100 ppb of the 120 ppb seen in the boundary layer downwind later in the day.

Similar carryover effects were seen on other days. Figure 8-10 shows vertical ozone profiles for sites in Pennsylvania and Virginia for early morning on July 14. Similar profiles were seen over New York, Connecticut, and Massachusetts on the same day. Carryover of 80-100 ppb of ozone on regional episode days appears to be a common occurrence in the Northeast, and trajectory analyses suggest that the ozone aloft can be transported hundreds of kilometers overnight.

On Figures 8-6 and 8-10, the ozone and NO_y plots indicate that the carryover in the morning above a few hundred meters agl consists mainly of ozone and aged precursors without many fresh emissions. On all of the plots on these two figures, the ozone and NO_y were inversely proportional near the surface, indicating that the NO_y included fresh emissions of NO that depleted the ozone during the night. Aloft, however, in all but one of the plots, increases in NO_y were accompanied by increases in ozone, indicating an aged airmass without fresh emissions in which the reactions had gone to completion. Since photochemical reactions do not occur at night, the ozone, NO_y, and presumably VOCs seen in the morning in these plots were transported overnight. In the Gettysburg spiral in Figure 8-6, a spike in NO_y can be seen at about 1300 m corresponding to a decrease in ozone. This is an indication of depletion of ozone by a plume emitted overnight. It is surprising that the plume was seen at such a high altitude, but this plot shows one of the few fresh plumes seen in the transport layer aloft. In the same plot, another plume can be seen just above the surface layer at a more typical altitude for an early morning plume. In the aircraft data examined, aloft plumes emitted overnight made up a small fraction of the air carried over in the residual layer above the surface.