135 asked to convert to maximum use of high-sulfur coal or oil. During this week, industry was asked to recon- vert to minimum-sulfur fuels on a date selected on the basis of the weather forecast, with the Edison Company following one day later. Fortunately, the weather did cooperate this week. When the city converted to maximum use of high- sulfur fuels on July Ist, four of the eight TAI stations reported rapid and large increases in SQ. values (from 0.00 to 0.23 ppm in 15 min at one site, for ex- ample). Fluctuations during the balance of the day can be related to changes of wind direction. Very low ground-level SOs concentrations were often observed at night and early morning during the high SO2 output period with light winds and strong, ground-level inversions, showing that the pollutants were trapped above the stable layer and did not reach the surface. On July 3rd, when a lake breeze circulation covered the city, a SO. concentration of 0.47 ppm was observed at one station, This occurred several hours af- ter the lake breeze front passed the station, showing that the SO. was trapped in the relatively cool, stable air moving in from the lake. No high SOs levels were observed at any station in the zone of convergence along the lake breeze front. The data show that sources outside of the city are often responsible for high SOs levels inside the city. The third fuel-switch test in this serics was con- ducted between May 19th and May 30th, 1969. Dur- ing this experiment, the dual-fueled plants and the electrical generating plants were asked to convert to hich sulfur fuels on May 20th and May 26th. The conversions to low sulfur fuels were made one or two days later using a selective abatement strategcy; that is, only a limited numberof plants changed to low-sulfur fuels. The abatement strategy used depended on the actual and forecast weather conditions, observed and forecast SO» levels, location of dual-fuel plants with respeet to the SO. observing sites, diffusion calculations, and the availability of natural gas. Only those sourees contributing to areas of high SO. concentrations were asked to convert to gas. The strategy emploved in this test is quite similar to that which would be used during a real air pollution episode. The observed changes in SOs levels indicated that fuelswitches were effective in lowermg SOQ-s concentrations in the affeeted areas, and that the decreases were con- sistent with those computed bythe diffusion model. A complete analysis of this experiment in pollution abatement is in progress, CHICAGO AIRCRAFT SOUNDING PROGRAM J.E. Carson and D. MM. Nelson Atmospheric stability, as measured by the vertical tempera- ture distribution. is an important meteorological factor controlling diffusion rates. Until recently, no measurements of this parameter were being made in or near Chicago. The Chicago Department of Air Pollution Control has purchased a flight package to measure vertical profiles of air temperature and sulfur dioxide. Both helicopters and light airplanes have been used; Argonne and city scientists have made flights with the svstem. About fifteen flights were made during the period of this report; features of several of the more interesting flights are presented, Until recently, two types of meteorological measurements needed to accurately estimate diffusion conditions over Chicago were not being made. These are the vertical temperature gradient and winds in the zone of mixing and transport. The vertical temperature gradient (lapse rate) determines the atmospheric stability and (combined with wind speed and surface - roughness} the intensity and spectrum of turbulence. The depth of the mixing layer (thatis, the volume of air in which the pollutants can be mixed) is determined by the height above ground and intensity of stable layers (inversions) aloft; it is very difficult if not impossible to estimate this critical pollution parameter without direet measurements. The low-level lapse rate varies with time and height as well as horizontally in an urban area such as Chicago with its mixture of tall and low buildings, roads, parks, lake, ete. In dispersion calculations, the lapse rate in the first several hundred feet above ground is usually estimated from standard weather observations (wind speed, cloud cover, time of day, etc.) using ob- jective techniques, such as those proposed by Gifford™ and Pasquill.@ Unfortunately, these procedures cannot be used to provide accurate estimates of the mixing depth. Techniques to extrapolate the observations from the two nearest U. 8. Weather Bureau radiosonde stations (Peoria, Illinois, 140 miles southwest of the Loop and Green Bay, Wisconsin, 180 miles north) have been developed at Argonne and bya local private meteorological forecasting service (Sce Reference 3, p. 129). These procedures cannot be expected to be sufficiently accurate or detailed always to represent conditions over Chicago.