8 g BB BR WIND SPEED, kt o 9 a o hw out the year. Beside the three pollution phenomena monthly totals. The highest frequencies in Table 61 come during summer. Fortunately, SO2 emissions are + oO 6 8 10 12 14 Is 18 a 9-10 MAY I967, CST 22 0 2 4 8 ol lowest then. There is little residential heating, and many Chicago industries find it cheaper to burn natural gas In summer. ~ _ nN 28 8 3 8 SUMMARY AND CONCLUSIONS WIND SPEED, kt oO on 0.6 04 Three high pollution cases involving different meteorological events were examined in detail using air quality and weather data taken near the surface. On a cold, nearly calm day in winter, when the city was several degrees warmer than the surrounding rural land area, an internal “heat-island” circulation apparently developed. Winds converged from east and west near the north-south axis of the city. Continuity indicated rising air over this axis and return flow to the east and west aloft. Measured SO. concentrations of 0.2 ppm in air arriving at shoreline ws 2 14 16 @ 2 22 o 2 4 9-10 MAY 1967, CST Fig. 124—Winds and SO, at TAM-2 and 7, 9-10 May1967 and subsequently blown about the city by shifting surface winds. Frequencies of Occurrence 9 10 Go 8 Ny 6 8 4 Ss ao ae: 0.2 WIND DIRECTION, degrees WIND DIRECTION, degrees Table 61 shows that several potential pollution- dealt with here, there are others that add to the Fic. 123.—Winds and SO. at TAM-6 and 8, 9-10 May1967 $0, CONCENTRATION, ppm The frequency of 1-2 per month for heat-island circulations is a preliminary figure based on a search of two years of data in the Argonne air pollution data file. The frequencies for offlake winds were determined from a search of Chicago local climatological data for 1965, 1966, and 1967. An offlake wind day is defined as a day when the prevailing direction was between 40 and 120 degrees. To qualify as a “lake warmer” case, the lake temperature had to be at least 3° F warmer than the maximum daily temperature at Midway; and vice versa for the “land warmer” cases. producing events normally occur each month through- 2 SO. CONCENTRATION, ppm o WIND DIRECTION, degrees 8 156 60 0 -_ a Each of the three cases examined involved high SQ. readings at several stations. Thus we know that the weather phenomena involved can produce high pollu- tion levels. Therefore, they deserve more attention. One important aspect to consider is how often they occur. Table 61 shows frequencies of occurrence for the three weather processes examined in this paper, namely heat island circulations, winds off the lake in response to the regional pressure gradient, and lake breezes, o it a wo 2 = & * o6 =z 2 & o4 ae 5Z a2 ao z 8 ™ o Lied 00 4 6 8 1 812 «(4 6 @ 20 22 O 2 4 9-10 MAY 1967, CST Fic. 125—Winds and SO. at TAM-3 and 4, 9-10 Mav 1967