6. SURFACE FLOWS: URBAN PLUMES AND OFFSHORE TRANSPORT

Trajectory analyses shown in Figures 3-6, 3-7, and 3-8 and the vertical wind profiles in Figures 3-1, 5-3, 5-5, 5-7, and 5-9 showed that surface flows were disconnected from the aloft flows at night and early morning and were generally light, allowing emissions to accumulate overnight and be transported downwind during the day. The effects of the accumulated emissions can be detected downwind as ozone pulses when the ozone formed from the same-day urban emissions arrives at downwind receptors. In NARSTO-Northeast, urban plumes or pulses were documented by aircraft flights and surface measurements downwind of OTR cities and were traced offshore to downwind onshore receptors.

Figure 6-1, adapted from Zhang et al. (1996), shows the surface and aloft (500 m msl) ozone concentrations downwind of OTR urban areas in midafternoon on July 14. The surface concentrations at the top of the figure are hourly averages for the hour starting at 1500 EST (1600 EDT). The concentrations in the bottom part of the figure are instantaneous aircraft measurements during afternoon flights of two aircraft, one coming from Maine and the other from Virginia, and both landing in southeastern New York. The aircraft data shown are for those portions of the flights that were between 400 and 600 m msl. The surface mixing layer was deep enough by the time of the flights that surface emissions would have been mixed to the level shown. The aircraft measurements are described by Anderson et al. (1997).

Figure 6-1 shows ozone concentrations above the federal standards along the Connecticut shoreline and over Long Island Sound, over and offshore of central New Jersey, and in northeastern Maryland. Figure 6-2 shows the near-surface back trajectories from several receptor sites for the same afternoon. The trajectories generally come from the southwest and are consistent with transport since early morning from the New York City, Philadelphia, and Baltimore-Washington areas, respectively. The downwind ozone pulses of 150 to over 175 ppb are superimposed on regional background concentrations of 75-100 ppb throughout most of the OTR. This implies that the same-day urban emissions added 75-100 ppb of ozone over the regional background concentrations. Assuming a clean-air concentration of 40 ppb and assuming the regional background concentrations were due to transport aloft, the aloft transport contribution on this day would have been 35-60 ppb.

On August 1, high ozone concentrations were measured along the coast of Maine. The maximum concentrations, the time of occurrence of the peaks, and surface back trajectories from the times of the peaks are shown in Figure 6-3. The surface back trajectories for this figure were calculated manually by interpolating the hourly wind data from shoreline locations and offshore buoys. The trajectories were calculated manually because the CALMET model was not able to resolve the lowest-level flows over the ocean. Concentrations as high as 172 ppb at Phippsburg ME and over 150 ppb at several locations were observed along the Maine coast, while the highest concentration seen at an inland Maine site was 102 ppb. The progression of the time of the maxima from south to north along the coast suggested transport from the Boston area. The trajectory analyses confirmed this hypothesis. Surface winds for Boston, Cape Elizabeth, and two buoys are shown in Figure 6-4. The winds were offshore from the southwest during the night and morning hours, turning southerly offshore during the afternoon, and carrying the urban plume from eastern Massachusetts onshore along the Maine coast.

Offshore, the region of high ozone was confined below 400 m msl, as seen in Figure 6­5, which shows an aircraft spiral made offshore of the southern tip of Maine. The spiral location is shown in Figure 6-3. The spiral shows the highest ozone concentration immediately next to the surface. Above the surface layer were carried-over ozone concentrations of about 70 ppb between 400 and 900 m and 50 ppb above 1000 m msl.

An ozone concentration of 166 ppb (comparable to the maximum shoreline concentrations) was measured offshore of Phippsburg by the Scotia Prince ferry boat at the location shown in Figure 6-3. The concentrations measured each hour by the boat are shown in Figure 6-6. The ferry data show the ozone pulse near the shoreline, but also show that the pulse had passed the ferry port at Portland by 1900 EST.

Onshore, the highest concentrations in Maine were seen a few hours after the peak shoreline concentrations and were substantially lower. This is consistent with continued transport inland and with dilution of the surface-based ozone by the lower concentrations seen aloft. The dilution would occur as the surface air was mixed aloft by heating and turbulence when it crossed the shoreline. Farther inland, beyond the impact of the onshore flow, the maximum concentrations occurred earlier in the day and were consistent with the concentrations that would have occurred if the 50 and 70 ppb carryover values seen aloft in the aircraft spiral had been mixed to the surface.