waea tt tos, WR cat aden ot ane ne ncA aA ste ee ti FA algae 159 stations from over the lake strongly suggests that this circulation was recirculating air through the city. This may account for the very high concentrations of SOs TABLE61. FREQUENCIES OF OCCURRENCE OF SOME WEATHER PHENOMENA IMPORTANT TO CHiIcaGo AIR POLLUTION Two cases of winds off Lake Michigan were also examined. In one case, the lake was warmer than the land; in the other the land was warmer. Observed pollution patterns generally agree with those expected theoretically. However, more cases must be studied before we ean drawfirm conclusions. Two other possible meteorological pollution phenomena, namely nocturnal inversion trapping and morning inversion breakup fumigation may also have occurred in one of the cases, This shows that the me- Heat-island cir- observed that day. TEMPERATURE - HEIGHT x 2 CURVE UNSTABLE BOUNDARY LAYER FLOW PIPPLS STABLE COOL LAKE WARM LAND \ = @ Lake breeze 1-2 per month duringfall and This study and winter (preliminary) 1 per month, September through April 8 per month, March through September 2 per month, October through| February 3 per month reach Midway, Maythrough September , This study This study This study Murray and Trettel, Inc.“ teorological cause for any given pollution episode may be highly complex and involve several different physieal processes. More -tudy of these and other phenomena are would be desirable, of course, but these case studies offer hope that important information can come from data on hand now. We thank Mr. Ivan Brunk for information on Lake Michigan icc conditions on 19 January 1966, and Mr. Harry Moses for helpful discussions of the ease. NEAR NEUTRAL AIR Wl REFERENCES STABLE * WARM Land warmer* Reference needed. Iixtensive observations in four dimensions TEMPERATURE ———- UNSTABLE BOUNDARY LAYER eulation Offlake winds Lake warmer Frequency * Some of these mavbe lake breezes. NEAR NEUTRAL AIR = ‘ Event LAND COOL LAKE TEMPERATURE —= Fic, 128—Schematic diagram of stability regimes for onshore flow from a cool lake. Top. tall stack; bottom. short stack. (After Hewson and Olsson."”’) 1. Lowry, W. QO. The climate of cities. Set, Am. 217(2), 1523 (1967). 2. Hewson, I. 8S, and Olsson, L. E. Lake effects on air pollution dispersion. J. Air Poll. Control Assoc. 17(11), 757761 (1967). 3. Murray and Trettel, Inc. The climatology of air pollution in northeastern Illinois. Report prepared for The Northeastern Illinois Metropolitan Area Planning Commission, Murrav and Trettel, Inc., Northfield, Illinois, February 1966. MATHEMATICAL URBAN AIR POLLUTION MODELS* Harry Moses This paper provides a review of the use of mathematical models for urban air pollution problems. Discussed are the major contributions in this field beginning with the early work of Frenkiel to those of *Summary of a paper presented at the 67th Annual Meeting of the Air Pollution Control Association, New York. June 26-29, 1969. modern day operational models. To allow an assessment of work in this area, five tables are presented including the locations of the studies, types of tracers, sizes of the monitoring and meteorological networks, amounts of data collected, and the mathematical equations used. The urban air pollution model is considered as having four components. These are (1) the source in-