169 bution of SQ. concentrations for this combination of meteorological variables. plume rise and eddy mixing. There exists a critical direction were obtained from the appropriate Teleme- ating, the effect of wind speed on resulting levels of To develop the Tabulation Prediction Technique, values of SOz coneentrations, wind speed, and wind wind speed which results in a maximum concentration near the ground. Thus, there are a number of considerations one has to take into account in evalu- tered Automatic Monitoring (TAM) Station operated pollutant concentration. by the City of Chicago; values of other meteorological variables were obtained either from the Chicago Midway Airport or from the Argonne Meteorology Laboratory. THE INFLUENCE OF METEOROLOGICAL VARIABLES ON AIR POLLUTANT CONCENTRATIONS Since the Tabulation Prediction Scheme depends on the relations between observed SOc concentrations at a receptor and the ambient meteorological variables, it is necessary to understand the principles underlying the meteorological processes. Each of the meteorolog- ical variables, wind speed, wind direction, temperature, relative humidity, stability, net solar-terrestrial radiation flux, mixing depth, ceiling, or visibility, may play an importantrole. The relationships may be substantially nonlinear. The role played by meteorological variables in processes which affect SO. concentration levels is briefly discussed. Wind Speed The wind speed determines the urban ventilation rate. Further, the effluent from a point or distributed source is mixed with a large volume of air per unit time if the winds are strong and with a relatively small volume if the winds are light. Therefore, near the source, the low wind condition gives rise to high concentrations. In general, concentration levels vary inversely with the wind speed. The situation, however, is not as simple as it appears. During clear nights with strong inversions, there is a decoupling of the air motions in the lower layers, i.e, several hundred feet above the ground. Under these conditions, the surface winds are very light and Wind Direction The effeet of wind direction on a given receptor is obvious. If the wind blows directly from a strong source to the receptor, the concentrations will be high and vice versa. It should be pointed out, however, that even if the sources were uniformly distributed with respect to direction, there might still be a pronouneed direction effect due to variations in ground roughness. Temperature The primary importance of temperature is its effect on the amount of fuel used, especially for space heating. There are secondary effects which one would have to take into aceount for long trajectories, ie., exceeding three or four hours. Since SOs undergoes chemical reaction, especially oxidation, and since re- action rates double for every 10°C rise in tempera- ture, the removal rate is a function of temperature. With a 4-hr residence time for SO2 (probably at about 50° F) this effect has to be considered. Although at first sight it may appear that the SO. concentrations and temperature or degree-hours are linearly related, this appears not to be the case as indicated by Roberts,‘who points out that for temperatures above 32° F, the slope of the SOQ. vs. temperature curves differs from that below 32° F. Another point worthy of note is the wind chill fac- tor. With low temperatures and brisk winds cooling is enhaneed, resulting in greater use of fuel with higher SO» concentrations. Relative Humidity Ordinarily, one might expect that relative humidity is not an important variable in the diffusion process. the effluent from a stack 200 or 300 feet high will remain aloft. Therefore, such stacks contribute little to the concentrations observed near the ground even though the wind speed is low. Effluents from low stacks (a few feet to tens of feet above the building) are caught in the turbulent wake of the building; high ground-level pollution concentrations result. With clear skies during the daytime and light winds, looping may occur, bringing high concentrations to the ground. Therefore, with light winds, the observed SOs However, there are factors of which one must be aware; with high humidity, i.e., exceeding 90%, condensation of water droplets or ice crystals may occur which may scavenge SO2. Oxidation rates are influenced by the ambient relative humidity since the oxidation rates of SO. in aqueous solution differ from on the meteorological conditions. Not only is relative humidity of importance in determining levels of concentration, but high values concentrations would be very high or low, depending The concentration at the groundis affected both by those in the gaseous state. The oxidation of SO2 yields SOs, which reacts with water to form sulfuric acid. Further, chemical and physical action in the presence of metals or salts leads to the formation of sulfate particles. ak