2. OVERVIEW OF TRANSPORT AND MIXING PHENOMENA AND THEIR IMPLICATIONS

2.1 TRANSPORT AND MIXING PHENOMENA

The findings below are drawn from the analyses described in the following sections. The section(s) which provide support information are noted in parentheses.

• Transport of ozone and precursors is affected by flow regimes that differ with altitude. These regimes can be divided into three general categories plus a subset of the lowest-altitude regime. (Sections 3-6)

1) Boundary Layer Synoptic Transport 800-2000 m msl ±

- Transport across the Appalachians - from western OTR and Midwest

- Flow from west to northwest during most episodes

- Transport distance of 300-800 km (200-500 miles) in 24 hours

- Does not have strong diurnal component

2) Channeled Flows Below the Ridge Heights 200-800 m msl ±

- Includes low-level jets (occurred on 6 of 9 1995 regional O₃ episodes)

- Includes flow modified by the Appalachian lee trough

- Often transports substances from southwest along urban corridor, but other directions occur, including flow through gaps in the Appalachians

- Transport distance of 200-400 km (125-250 miles) overnight

3) Near Surface Flows 0-200 m msl ±

- Light winds in night and morning allow accumulation

- Fresh emissions and urban plumes move downwind and react during daytime

- O₃ aloft and aged precursors are entrained as the mixing layer deepens

- Transport is typically to the north through east along the urban corridor for 50-250 km (30-150 miles) by evening.

3a) Offshore Flows (subset of 3 above, see Section 6 for details) 0-200 m msl ±

- Light winds in night and morning allow accumulation onshore

- Accumulated urban emissions can transport offshore and react during daytime

- Transport is typically to the northeast through east: Boston to NH, ME; Philadelphia and NJ to Long Island; Baltimore/Washington across bay to DE

- Transport can be 200 km (125 miles) or more during daytime and evening

- Layer stays stable and thin over water, with minimal mixing and no added emissions

- Can cause high concentrations at shoreline; dilutes with mixing as transported inland

These flow regimes are shown schematically in Figure 2-1.

• The first two flow regimes above became decoupled from the surface flow and were accelerated on the episode nights studied. During the day, winds throughout the boundary layer become coupled, and winds can become similar through all three regimes. During the day, frictional effects can slow the winds aloft below the nighttime speeds. On most episode nights studied, a jet occurred in a layer just above the surface, where the speeds were higher than those above or below. On some nights, the winds aloft did not form a jet (i.e., they were not faster than the winds both above and below). They were, however, faster than the winds below. This is a normal occurrence on most summer nights. When the frictional effects are decoupled, the winds aloft will be faster than the surface winds. When the aloft winds are decoupled from the surface, ozone and precursors aloft can be transported without deposition to or input from the surface layer. (Sections 3-5)
• Trajectory analyses and aloft ozone measurements show ozone can be transported 300-800 km (200-500 miles) across the Appalachians at higher altitudes (800-2000 m msl) and 200-400 km (125-250 km) along the corridor or through gaps in the mountains below the ridge heights (200-800 m msl). (Sections 3-5, 8)
• Transport over distances of 200-800 km (125-800 miles) or more of ozone and aged precursors can result in urban corridor background ozone concentrations of 80-100 ppb. The resulting background ozone is a mixture of ozone transported from different locations at different altitudes. (Sections 4-6, 8)

• From early morning ozone and NO_y measurements aloft, long-range transport seems to contribute mainly ozone and aged precursors, but fresh primary NO_x emissions also can be transported overnight in these flows. (Section 8)

• Ozone of 80-100 ppb or more and aged precursors can remain aloft overnight and be mixed with fresh emissions (or ozone) the next day. (Sections 8-9)

- Aloft wind measurements show the source of the air aloft can be very different from the surface air. (Sections 3-6)

- Resulting surface ozone concentrations depend on the same-day surface emissions as well as ozone mixed down from aloft. (Section 8, 9)

• Ozone aloft can influence surface concentrations through addition of aloft ozone and precursors to the surface concentrations as the mixing layer deepens and entrains them. (Section 8, 9)

• Simple two-layer box-model simulations for conditions similar to August 1, 1995 and with many simplifying assumptions were performed. Assumed VOC and NO_x emissions were varied by a factor of four to assess sensitivity to emissions, and two different chemical mechanisms were used. These simulations indicated that ozone carryover aloft might increase the maximum surface concentrations downwind of major urban areas by less than one third to over 100 percent of the aloft excess over 40 ppb (clean air) for the range of emissions used. Most of the simulations were in the bottom half of this range. On August 1, 1995, ozone carryover aloft was roughly 70-80 ppb. Given this range for the initial concentration in the upper box of the model, the maximum ozone values predicted for the lower box of the model increased by 13 to over 40 ppb from what they would have been with only 40 ppb of ozone aloft. This increase would be less if a higher ozone concentration were assumed for clean air. (Section 9)
• For the range of conditions simulated with the box model, roughly 60-75 percent of the maximum ozone seen in the lower layer was due to same day emissions in the lower layer plus natural background from aloft; while roughly 25-40 percent was due to carryover and formation in the upper box. However, reducing ozone concentrations in the upper box generally did not decrease the concentrations in the lower box in proportion. (Section 9)
• Maps of surface ozone concentrations, aircraft data downwind of cities, and analyses of offshore transport suggest that same-day urban plumes can add 75-100 ppb of ozone to observed nonurban background concentrations. (Section 6)
• Trajectory analyses and aircraft data indicate that same-day emissions transported offshore near the surface can cause ozone exceedances at downwind shoreline locations. These exceedances are not influenced significantly by ozone aloft. (Section 6)
• Mixing heights increased more slowly on widespread episode days. (Section 7)

- This can result in accumulation of higher precursor concentrations in the surface layer in the morning through lower mixing heights.

- This will result in higher precursor concentrations in layers transported offshore in the morning.

2.2 IMPLICATIONS REGARDING SPATIAL SCALE OF INFLUENCE

The findings summarized in Section 2.1 shed some light on the spatial scale of influence of ozone and precursors transported aloft overnight versus ozone formed from precursors emitted on the episode day. Our interpretation of the above findings leads to the following conclusions.

• The three-dimensional air quality and wind data show that aloft transport can result in the widespread occurrence of overnight ozone carryover aloft at average boundary layer concentrations of 70-90 ppb. When mixed to the surface, this transport can lead to surface ozone concentrations of 70-90 ppb. (See Sections 8, 9). This surface background contribution is roughly 30-50 ppb above clean air, assuming a clean-air value of about 40 ppb. This background carryover ozone can be caused by emissions or ambient ozone coming from 200-800 km (125-500 miles) away since the prior day.

The aircraft data and plots of surface maximum ozone show same-day plumes (or pulses) of ozone downwind of urban areas embedded in a regional background. The peak urban plume concentrations were up to 80-100 ppb higher than the nearby regional upwind or crosswind concentrations. Offshore transport of near-surface emissions was seen to cause downwind offshore and shoreline concentrations that were 80-100 ppb higher than the concentrations higher aloft and at downwind onshore areas not impacted by the transported plume. Under these conditions, even with clean air as a background, urban plumes resulting from same-day emissions could cause ozone concentrations to exceed federal standards 50-250 km (30-150 miles) downwind.