ee ee RR ‘ . ” 4 } a Et eel . sami eet s : ti Bae oe wt ted . . . : hog . bag ed dee lla Aall cate lnelle acties corn a8 9 he cite ale rede De thar ah 170 of relative humidity for particular values of SO. concentration may have an important physiological ef- The Previous Hourly Reading of SO. Concentration must, therefore, always be aware of relative humidity levels in assessing the severity of an air pollution incident. lation Prediction Technique are those which are most influential in controlling the levels of SO. concentra- feet on people as well as plants and animals. One Aixing Depth From a physical standpoint, the height of the mix- ing depth is important sinee it determines the volume in which the contaminants will be mixed. tion. It is conceivable that at times there are conditions which are not accounted for by the selected variables resulting in concentration values different from those expected. Examples are pronounced shear in the lower layers or lack of information on the height of the mixing depth. If it is assumed that persistence of concentration levels is appreciable from one hour The Vertical Temperature Gradient The vertical temperature gradient represents a di- rect measure of the stability of the atmosphere. These measurements should be made in the urban area. Measurements several miles outside of the city in a rural area may be relevant, especially during extreme conditions when strong inversions are present. However, for optimum results, stability measurements should be made in the same area that predictions of pollution levels are made. to the next, the SO» concentration level would serve as a useful parameter. There are conditions, of course, when abrupt changes do oceur in the weather conditions, such as whena front is passing over the station and the usual pronounced wind direction change occurs. Further, during some parts of the day and under some weather regimes, both rapid meteorological and source strength changes are likely. Ceiling Height With relatively low ceilings, those formed essen- Net Radiation Flux This measurement represents the difference between the sum of incoming solar and sky radiation and the outgoing terrestrial radiation. It provides information on characteristic weather regimes relevant to the diffusive capacity of the atmosphere. For example, during clear sunny conditions, there is a surplus of incoming radiation. As a result, the ground surface warms, and convection inereases resulting in pronounced vertical mixing. The readings are strongly positive. During a clear night with strong outgoing radiation, the readings are invariably negative. This indicates a strong inversion, which results in a marked suppression of vertical motions. These data may also be used to indicate the degree of cloudiness. Visibility Visibility is directly related to the amount of particulate matter located in the lower layers of the atmosphere. Particle size is also an important factor in determining visibility. Not counting fog or other hydrometeors, low visibility denotes high concentrations of pollutants. At times, the pollutants The meteorological variables selected for the Tabu- are brought in from an external source such as the combustion products emanating from a forest fire even hundreds of miles away. More often, the particulates originate within the city and accumulate in the air because of poor diffusion conditions. Thus, visibility may be an indicator of pollutant concentration. tially by stratus clouds, the base of the clouds is related to the height of the mixing layer and thus provides a useful measure of pollution levels to be expected. Intermediate or high cloudiness, for example cloudiness over 7000 feet, also influences SO. concentration levels. These clouds affect the intensity of solar radiation and consequently the magnitude of’ convection in the lower layer which strongly influences the atmospheric diffusion rates. The ceiling height is, therefore, a related variable. Time of Day The time of day is important for several reasons. First, &@ maximum occurs in the early morning hours, primarily because the source strengths are increased during these hours. When people arise in the morning, they turn their thermostats up or put sev- eral shovels of coal into the furnace. A secondary maximum occurs shortly before sunset, due to increased use of sulfur-bearing fuel and to a decrease in the diffusive capacity of the atmosphere. Since there are substantial differences in concentration between the maximum and minimum values during a given day, the time of day must be seriously considered. SELECTION OF VARIABLES The relationships between SOs concentration and the meteorological variables are not linear, and some