167 know which formula to apply. Solar radiation measurements will be made at four levels above ground up to about 1000 feet or greater. These will provide the distribution within the laver of measurement. Extrapolation techniques for determining the mixing depth may be developed by uw series of comparison measurements using helicopter and balloon data and the pyrheliometer measurements. These measurements will consist of vertical profiles of temperature and particulates in addition to those of other atmospherie constituents such as SO, or water vapor. A cheek on the systemis, of course, essential before operational reliability can be established. Aerosols may oceur in layers varying from 10’s to 100’s of feet in thickness above the primary mixing layer near the ground.“ Should this condition prevail, then the method as described here will be ineffective in determining the true mixing depth. However, work is currently in progress which will allow a determination of the position and thickness of such lavers. This involves a passive system using two pyranometers separated by a baseline of about 2000 feet. The covariance of the solar radiation flux observed by the two instruments involving 2 common volume arising from two intersecting cones of view is computed continuousiy, The height and extent of acrosol layers may be determined from the covariance measurements as the commonvolume is allowed to change its clevation above ground. The pyranometer system also requires validation by independent means such as by aireraft or balloons. Advantages of the Pyrheliometer System for Measuring Mixing Depth The technique described here is essentially a passive remote probe, but it would work best during cloudless daytime conditions. During cloudyconditions or during the night, it may be neeessury to develop an active svstem in which a source of electromagnetic energy is used. When conditions of severe pollution exist, un extensive antieyelone is generally present. With extensive high pressure areas, atmospheric subsidence is likely, with the result that little cloudiness is present. One may, therefore, expect to find that this system works well when needed during high pollution situations. As indicated in the schematic diagrams, Figures 131 and 133, the information obtained from the pyrheliome- ters may be passed through an electronic circuit in which the output represents the height of the mixing depth. This output may be transmitted to a central control point, such as an air pollution control office, where a recording meter provides continuous readings of the height of the mixing depth. The pyrheliometer technique represents distinct advantages over the systems currently available for measuring the mixing depth, such as those with helicopters, fixed winged aircraft, or bailoons. At best undercurrent systems, only 2 few measurements per day can be obtained. With a system such as described in this pauper, continuous measurements are provided. These, of course, would be invaluable for not only providing a better insight into meteorological processes, but-as a valuable tool for incident control. REFERENCES 1. MeCormick, R. A. and Kurfis, K. R. Vertical diffusion of aerosols over a city. Quart. J. Roy. Meteoral. Soc. 92(393), 392-396 (1966). 2. Gilroy, John. Private Communication, 1969. 3. Davidson, Ben. A summary of the New York urban air pollution dynamies research program. J. Atr Pollution Control Assoc. 1703), 154-158 (1967). THE TABULATION TECHNIQUE FOR FORECASTING CONCENTRATIONS OF URBAN AIR POLLUTANTS Harry Afoses, J. B. Anderson, and D. F. Gatz An urban air pollution model can be of considerable use to a municipality for imcident control, for the siting of new plants, and for assessing the effectiveness of abatement pro- cedures. Two types of urban air pollution models are avail- able: 1) the source-oriented model, and 2) the receptor-oriented model. In the tabulation prediction technique. combinations of meteorological variables are arranged in an ordered sequence in tabular form. For each combination of meteorological variables, a cumulative percentile distribution of the concentrations of a selected pollutant. eg. SO. is presented. Also included are relevant statistical parameters such as the interquartile range, the mean. or the number of cases. The development of a tabulation prediction technique requires a number of preliminary analyses. These include test- ing meteorological variables for their relative importance in influencing the pollutant concentrations, selecting optimum class intervals of the weather elements, and deciding the most effective arrangement of the independent variables in the tabulation. These ure examined in a discussion of the construction of this technique. The paper also discusses the continual upgrading of the tabulation prediction technique and its use in source surveillanee. An example is given of the tabulation prediction technique as applied to the city of Chicago. When material is injeeted into the atmosphere, whether it be from a distributed arca source or a point source, the meteorological conditions determine the