sao lh, + . a pceda bla da edehd betees lb fone Sa . 2 1 . . . Te RR et ee Aa hve itll Bend dieeae ek sel a a eli kd i 138 Lu ae 0.4 oO E o z o = om 2° = ac y= 03 ~~ <q a z bs) z °o oO G2 ~ oO Ww) 0.1 0 CERMAK | 315° | q7th | 63rd | | 7gith | gsth Fie. 100.—-SO, concentrations observed during horizontal traverse along County Line Road, 250 meters above groundlevel, April 1, 1969. 1000 CST, ENE (80°) at 10 mph at 1100. The Mid- way sounding was made at 1050 CST,after the frontal passage. The cool air behind the front was consider- ably moderated by the time it reached Midway Airport; a dry adiabatic layer about 1300 ft thick had been generated, with a stable layer above. The front did not pass Argonne until 1320 CST. The weather before the air mass change was very good: clear skies and light winds mostly from the north at 3 to 5 mph, The maximum temperature was 19°C (66° F), reached just before the frontal passage. After the frontal passage, the winds became ENEat 9 to 11 mph and the skies remained clear. The Argonne sounding, made at 1100 CST, showed an adiabatic layer to the top of the sounding at 3300 turbulence in the new air mass was quite low; for example, the standard deviation of the vertical wind direction (as measured by a bivane and sigma meter) 37.5 ft above the ground changed from 13° before the lake air arrived to 6° after. The standard deviation of the azimuth angle changed from 40° to 10°. These data show the reduced turbulence levels in cold air mass. By far the best SOz and temperature profile data collected by the helicopter to date were obtained between 0940 and 1100 CST on April 11, 1969. The flight path and sounding locations were selected using exist- ing weather conditions and modified during the flight itself to obtain maximum useful information. On April 11, a large cold high pressure area was ft. (The lowest two points on this sounding are from centered over the upper peninsula of Michigan, moving adiabatic surface layer.) middle clouds. Winds were ENE at 7 to 9 mph. This flow concentrated the pollutants emitted by the com- the Argonne weather tower and showthe usual super- The pall of pollution brought to Argonne was easily visible; there was enough dust in the air to reduce solar radiation intensity about 15 to 20%. Figure 98 shows the solar radiation data at Argonne on this date; data for May 18th, also a clear day, are included to show the reduction of insolation due to pol- luted air. These data show that the pollutants were within the newair mass, notahead of it. Wind data at Argonne showthat the intensity of ESE. The sky was mostly clear with a few scattered plex of industries and power plants along the Stevenson Expressway into a single visual plume. The flight pattern was altered to take advantage of this concentration of pollutants. The original plan was to make vertical soundings at Meigs and Midway Airports and at Argonne, the standard flight schedule. A decision was made while in the air to make a detour to Hinsdale Airport in order to make an additional sounding