United States Environmental Protection Agency Environmental Sciences Research Laboratory Research Triangle Park NC 27711 Research and Development EPA-600/S3-83-042 July 1983 &EPA Project Summary Meteorological Factors in the Formation of Regional Haze James G. Edinger and Timothy F. Press The purpose of this research project was to determine the role of mete- orological factors in the formation of widespread areas of haze in the eastern United States. Three case studies were made: A summer haze episode, an off-season haze episode and a non-haze episode. Results showed that over the course of 2 or 3 days emissions from widely separated sources such as St Louis, Chicago, Cincinnati and Pittsburgh are leafed together by vertical and horizontal shears and mixing by daytime convec- tion to form a dilution volume many hundreds to well overathousand km in extent and 2 or 3 km in depth. Almost all stations reporting haze during an episode were confined to this dilution volume and most of these in that part of the plume containing emissions that were 2 or 3 days old. The dilution volume associated with the off-season episode was of about the same magnitude as that of the summer case, but was shallower and horizontally more extensive. Both of these 3-day haze volumes were much smaller than the dilution volume as- sociated with the non-haze case which blanketed almost the entire eastern United States. This Project Summary was developed by EPA's Environmental Sciences Re- search Laboratory, Research Triangle Park, NC, to announce key findings of the research project that is fully doc- umented in a separate report of the same title (see Project Report ordering information at back). Introduction Extensive areas of hazy air covering major portions of the eastern United States have become a frequent summertime oc- currence during the last few decades. This regional haze, though well described by satellite observations and the conventional surface synoptic network, is not as well understood as the smaller, meso-scale visibility blight associated with plumes from individual point or area sources such as power plants and urban complexes. A variety of models have been developed for these meso-scale plumes. But as is reported in a recent study by the National Academy of Sciences, the modeling of regional haze is not nearly as far advanced The present study is directed toward improving our understanding of the mete- orological mechanisms involved in the formation of such large volumes of more or less uniformly polluted air. It is a diagnostic enterprise. Its purpose is to provide information useful for constructing models, of the regional haze formation process. Air quality simulation models describ- ing the long-range transport of air pollu- tion have been devised. Usually they are based on a requirement of mass balance. The processes and mechanisms included are: emission, chemical transformation, physical removal, turbulent diffusion and transport The air motions used in these models are observed atmospheric motions, not air movement calculated from fundamental physical principles. Typically they are the observed wind averaged over the depth of the polluted layer or the wind observed at some standard level interior to the polluted layer, often the 850 mb level. A certain amount of observed detail in the wind field is lost in the process of constructing vertically averaged wind fields or in ac- cepting the wind at any one level as being representative of the vertically averaged flow. These neglected (dispersive) motions ------- are parameterized in the models by eddy diffusion terms. Some air quality simulation models have taken vertical wind shear within the polluted layer into account for travel times out to 24 hours. In the work reported here the winds at all levels within the polluted layer are used to determine the long-range transport and dispersal of pollution out to 3 day's travel from the sources. It is done for a number of haze episodes chosen from 30 years of record (1948-1978). The purpose is to discover the meteorological mechanisms associated with the formation of these extensive volumes of hazy air. The analysis consists of constructing the previous 3-day history of the hazy air. It is assumed that the responsible pollutants came from the major air pollution sources in the eastern United States (large cities, power plants, etc.). A 3-day dilution volume is constructed for each major source. These individual dilution volumes overlap to form a conglomerate volume which encompasses all of the air which in 3 days passed over the major sources. Because of the overlap it may be assumed that this large conglomerate volume would also include air from any other source, of whatever size, located in the area inter- mediate in location to the major sources. In reconstructing the extensive volume of hazy air the major sources of emissions, i.e. the large urban centers, are taken as continuous point sources. Their nighttime emissions are treated as plumes moving with the surface winds as reported on the 3-hourly surface synoptic maps. Their daytime emissions are assumed to mix vertically by thermal convection through the lowest 2 km of the atmosphere and to move with the observed wind fields at each level (50 mb intervals) within that layer. The shearing motions and transla- tion combined with the vertical mixing produce a dilution volume associated with the particular source. The effects of horizontal motions on a scale smaller than those described by the streamline analyses are neglected. The results of the investigation indicate that the widespread volumes of hazy air examined correspond closely with the conglomerate dilution volume of the air which during the previous three days passed over the area that includes St Louis, Chicago, Cincinnati and Pittsburgh and points in between. They also show that most of the haze is reported in air that contains blended emissions that are 2 or 3 days old. Sources closer to the Atlantic coast probably contribute to the haze but because of the absence of data over the adjacent ocean their influence could not be determined. Procedure The meteorological analysis makes use of the U.S. National Weather Service's surface and upper air data. Horizontal wind fields were constructed and analyzed for the 950,900,850 and 800 mb levels. Figure 1 is an example. Surface flow was determined from the photostatic records of the U.S. National Weather Service's 3- hrly surface synoptic maps. The vertical profiles of temperature and humidity were generated by computer graphics. The dilution volume developed during the first 24 hrs for any one source is constructed as follows. The initial instant is taken as 00 GCT. During the subsequent twelve hours, nighttime, a plume is laid down at or near ground level. Its location and configuration is determined by con- structing a streakline for 12 GCT from the 3-hrly surface wind maps. It is assumed that vertical mixing motions are absent during this twelve hour interval. During the subsequent twelve hour in- terval, 12 to 00 GCT, daytime, mixing motions distribute the emissions vertically through the layer from the surface up to the 800 mb level. It is assumed that at a time not far removed from 12 GCT the surface plume develops vertically into a curtain which is subject to winds at all levels up to 800 mb. Shears in the wind subsequently carry the emissions at various levels off in different directions, deforming and tilting the curtain. Streaklines con- structed at each level for the time 24 hours after emission began depict the location of this deformed curtain. Vertical mixing motions from the surface up to 800 mb transform the curtain into a volume, its horizontal projection being the area over which the emissions have been spread during the 24 hour interval. The volume which contains the first day's emissions from the source in question continues to grow during the next day due to shearing motion in the vertical and another 12 hours of vertical mixing, and is transported (usually) away from the source. To determine the shape of the dilution volume and its location at the end of the second day, trajectories(24 to48hr) origi- nating at significant points around the perimeter of the 24 hr volume at each level are constructed. The end points of these Figure 1. Streamline map, 00 GCT 25 February 1973 at 850 mb level. ------- trajectories define the surface of a volume, the horizontal projection of which is the area covered by the dilution volume at the end of the second day. To construct the dilution volume that contains not only emissions during the first 24 hours but emissions during the second day as well, one needs to construct the dilution volume for air moving over the source during the second day. This volume when added to the other constitutes the two day polluted wake of the source (its 48 hr dilution volume). In the case studies carried out these procedures were repeated for a third day's emissions so that the dilution volume containing three day's emissions from a given source were obtained. The selection of the 800 mb level as the upper limit of the vertical stirring and mixing followed unsuccessful efforts to define the top of the mixing layer by inspecting the plotted radiosonde tem- perature and humidity profiles at all stations in the eastern United States at both the00 and 12 GCT times. Careful analysis of the profiles together with the associated sur- face weather maps suggested that during typical summer haze episodes in the eastern United States there is no extensive daytime stable layer in the lowest few kilometers placing an upper limit on corrective mixing. To the contrary reports of towering cumulus frequently appear here and there within the hazy volume suggesting that the mixing in some places reaches and exceeds the 500 mb level. The upper limit to the vertical stirring and mixing is at best ill- defined and irregular. In all of the case studies the top of the stirred layer was assumed to be at the 800 mb level. In one case, in an attempt to improve the fit between the constructed haze volume and the actual haze observations the level was raised to 700 mb with some improvement of the fit with the observations. To select the major sources of emissions in the northeastern United States the sta- tistics appearing in EPA reports (1978) were used. In preliminary case studies nine major urban industrial areas were chosen to represent the sources of emis- sions. Each was treated as a point source. Several considerations resulted in the re- vision downward in the number of these sources to just four: St Louis, Chicago, Cincinnati and Pittsburgh. They were: (1) preliminary results suggested that it was emissions two or three days old that were the major contributors to the haze and some of the emissions from the cities along the Atlantic coast moved into the data void over the ocean in less than three days preventing the completion of the construction of their dilution volumes and (2) the two and three day dilution volumes from individual point sources overlapped each other to such a large extent that including sources more closely spaced than the four chosen would have only a small effect on the size of their combined dilution volume. Results Four haze episodes were selected for analysis from the historical record, 1948 to 1978. In addition, one non-haze episode which was meteorologically similar to the haze cases was examined. Figures 2,3, and 4 show the results of the study of an off-season (not summer) haze episode, OOGT 1 March 1973. Figure 2 shows the 00 GCT 1 March 1973 location of one-day-old emissions (released during the last 24 hrs) from St Louis, Chicago, Cincinnati and Pittsburgh. The black dots indicate stations reporting haze at 00 GCT I March 1973. Figure 3 gives the location at 00 GCT I March 1973 of two-day-old emissions (released between 24 and 48 hrs ago), and Figure 4 gives the location of the corresponding three-day- old emissions (released between 48 and 72 hrs ago). Examination of these figures and others not reproduced have shown that: (1) only a small fraction of the haze reported occurs in one-day-old emissions, less than one half occurs in two-day-old emissions, but almost 90% occurs in the three-day-old emissions, and (2) the majority of the haze reports are at locations where a superposi- tion of emissions from different sources or different times occurs. These results suggest that much of the haze forms in blended, aged emissions and at a con- siderable distance from the sources. The corresponding diagrams for the non-haze case revealed one-day-old plumes that are noticeably larger than those generated during the haze episodes. And the two-day-old emissions cover an area so large that it compares favorably with the largest of the three-day-old emissions volume for the haze episodes. It is not possible to determine how much larger the three-day-old emissions volume is for the non-haze case since its boundaries extend beyond the map boundaries on the north and in the southeast. It is apparent that much larger dilution volumes are generated in this non-haze case than during haze episodes. Figure 2. One-day-old emissions from St. Louis. Chicago. Cincinnati, and Pittsburgh. 00 GCT 1 March 1973. ------- Recommendation The analysis of the haze episodes sug- gests that it might be instructive to devise haze formation models that move the emissions with the winds at all levels within the lowest 2 km layer and that include vertical eddy diffusion but neglect horizontal eddy diffusion. Figure 3. Two-day-old emissions from St. Louis, Chicago, Cincinnati, and Pittsburgh, 00 GCT 1 March 1972. Figure 4. Three-day-old emissions from St. Louis, Chicago, Cincinnati, and Pittsburgh, 00 GCT 1 March 1973. ------- James G. Edinger and Timothy F. Press are with the University of California, Los Angeles, CA 90024. George C. Holzworth is the EPA Project Officer (see below). The complete report, entitled "Meteorological Factors in the Formation of Regional Haze," (Order No. PB 83-209 742; Cost: $10.00, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Environmental Sciences Research Laboratory U.S. Environmental Protection Agency Research Triangle Park, NC 27711 ------- United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Fees^aid" Environmental ProtectTn Agency EPA 335 Official Business Penalty for Private Use $300 ------- |