: ad a fe metaham tliat saathanaesLee Lateriek? tt tial whee bee ied te ekBact ee ett Be a ae ak kci Ne a ceelethal tik ow tee 7 . . - . . . 154 centrations than the hour before, while lake front concentrations dropped even lower. The clean air had HEIGHT = TEMPERATURE - HEIGHT CURVE STABLE BOUNDARY LAYER NEAR NEUTRAL AIR UNSTABLE reached TAM-5 also, and SO, deereased sharply there from the 1000 reading. At 1400 and 1600 the whole city was quite clean, except for readings of about 0.3 ppm in the dewntown arca, undoubtedly from local PAPO sources. POROTA: COOL LAND WARM LAKE This is a case of a large morning SO, buildup. It was 14° F at Midway most of the night, so residential TEMPERATURE —————= sourees were undoubtedly strong. Winds were lght and skies clear during the night, so a nighttime radia- tion inversion was likely. This stable layer and the light surface winds probably caused the observed high SO. concentrations through the accumulation of SO. TEMPERATURE - HEIGHT CURVE i STABLE BOUNDARY LAYER bE = id in a shallow surface layer. Breakup of the radiation NEAR NEUTRAL AIR UNSTABLE FLOW =x COOL LAND WARM LAKE TEMPERATURE ————» Fig, 121—Schematie diagram of stability regimes for onshore flow from a warm lake. Top tall stack; bottom, short stack. noon, tightening the pressure gradient and thus increasing the wind speed. Figures 116-118 show the wind shifts at the TAM stations and the wind speed and SO. changes that occurred with them. Two different relationships were observed. Figure 116 shows that very high concentrations were observed at lake front stations 3 and 4 with southwest winds during the morning. At inland stations 1 and 7, the peak concentrations occurred in the east wind, as shown in Figure 117. The sameis true for inland stations 5, 6, and 8 as shown in Figure 118. The meso-scale charts (Figures 119 and 120) show another view of these events. At 0900 winds over the whole region were light from the southwest. Highest SO. concentrations occurred along the lake shore. At 1000 winds at the southern stations had switched to easterly. The northern stations still had southwesterly or westerly winds, and there was a calm zone hetween. The SO. pattern remained about the same. By 1100 winds were more generally from the cast, and the SOc had begun to move inland. It is clear from comparisons of 1000 and 1100 SOs readings at individual stations that concentrations were dropping at lake front stations and increasing inland. This trend continued, as shown on the 1200 map. Stations 6, 7, and 8, at the outer perimeter of Chicago, all had higher 30. con- inversion after sunrise could have brought additional SO. to the surface from plumesaloft. As winds swung to easterly and became stronger, this pollution pall was blown west and diluted somewhat, so stations on the west side of the city observed smaller peaks one or two hours after the wind shift. There is no evidence for any effects other than ad- vection for the high concentrations observed on the west side after the wind shift. This may be compared with the model for onshore flow from a warm lake shown in Figure 121. If a stable boundary layer were present, it would mhibit the downward transport of SO. from plumes aloft. As the figure shows, SO. from low-level sourees should remain concentrated near the ground. High concentrations were not observed after the morning pollution blew away, perhaps because of a reduced source strength in the afternoon and the stronger east winds. 9-10 May 1967: A Lake Breeze The surface weather map for 1200 on the 9th (Fig- ure 122) shows a high-pressure ridge line west of Chi- cago. The pressure gradient over Chicago was fairly tight in this case; winds were northwest at about 10 kt. The lake water temperature was 48° F, in contrast to a midmorning (1000) reading of 54°F at Midway. Increasing inland temperatures apparently caused convection and initiated the lake breeze soon atter 1000. Wind direction shifts during the day at six TAM stations are shown in Figures 123-125, together with simultaneous variations of wind speed and SQ. con- centration. These figures show that the pollution peaks occurred behind the lake breeze front, but not until several hours after the front passed. The 1000 CST map (Figure 126) shows moderate northwest. winds over the whole region with SO. concentrations less than 0.10 ppmat all stations. By 1000