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Chloride Content of Fog Water in Relation to Air Trajectory Samples

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Chloride Content of Fog Water in Relation to Air Trajectory Samples Chloride Content of Fog Water in Relation to Air Trajectory ROBERT H. CUNNINGHAM Massachusetts Institute of Technology, Cambridge URING THE SUMMER of 1939 fog to fifteen miles per hour, showed 3.6 samples were collected by the parts per million of chloride, while D author at the Bowdoin Scientific the next sample, after the wind had Station on Kent Island (lat. 44° 35' increased to eighteen miles an hour, N, long. 66° 45' W) in the Bay of and the sea had become choppy, Fundy, and by Prof. H. G. Houghton showed 392 p.p.m. of chloride. and Mr. A. C. Bemis at Mt. Washing- ton and Brooklin, Me. Both projects were conducted partly for the purpose of studying the chemical properties of fog in relation to the air mass type. The apparatus used to collect the fog at Kent Island was very simple. It consisted of a strip of nickel screen- ing about four feet high by two feet wide mounted vertically. A nickel- plated trough collected the drip from this screen. From the trough the water passed into a litre bottle. The screen was exposed perpendicular to the wind in the fog and was placed in an open field approximately twenty feet above and five hundred away from high water mark. The samples collected were taken back to Cambridge and analyzed in Prof. Houghton's laboratory at M. I. T. The amount of chloride in the fog water was determined by the titration of silver nitrate into the sample, using potassium chromate as an indicator. FIGURE 1. TRAJECTORIES OF SAMPLES. Dots The amount of sulphate was deter- on trajectories represent positions of air parti- mined by a photometric method. cles every twelve hours. Omitted trajectories: Numbers 16 and 17 are similar to numbers 7, In the collecting of the samples 8; number 18 is similar to number 10. The care was exercized so that no foreign 64° isotherm is a mean sea-surface isotherm for July and August {Mo. Wea, Rev., February, matter contaminated them. The screen 1938). It is approximately the northern limit of the rapid temperature gradient be- was exposed only in foggy weather, tween the Gulf Stream and the colder waters and before the first sample of a run to the north. was collected, one or more litres were In figure 1 the air trajectories for collected and thrown away. When most of the samples are plotted. These contamination from local salt spray trajectories were obtained from the does occur, the excess amount of M. I. T. weather maps according to chloride in the sample is easily recog- the method given by J. J. George.* nized, and the sample is discarded. Eighty percent of the geostrophic For example, fog water collected on wind was used over the abnormally an occasion in the summer of 1938 when the wind increased from five •A.M.S., BULLETIN, April, 1940, pp. 135ff. Unauthenticated | Downloaded 09/24/21 02:11 PM UTC [Vol. 22 cold Gulf of Maine waters. No de- 6, taken the next morning, is from air flection from gradient was made for that has come down almost directly the part of the trajectory over the from northern Canada. The amount ocean, as the frictional effect is small of chloride in these samples clearly and the orientation of the isobars varies with the change of air mass. uncertain. No. 2 had a very large chloride con- A number of difficulties were en- tent (34.9 parts per million or milli- countered in plotting the trajectories grams per litre) ; no. 3 a moderately chiefly because of the weak pressure large content (8.7 p.p.m.) ; and nos. field in summer and strong local in- 4, 5, and 6 all had a small chloride fluences, such as sea-breeze effects. content (3.6, 4.4, and 3.8). Trajectories no. 9 and 10 illustrate the difficulty of following an air par- ticle near the center of a weak high. No. 9 seems to have gone around the northern side of the high, while 10 went around the southern side. A somewhat different map analysis could have, in this case, led one to draw of trajectory 10 similar to 9. In most of the cases the trajectories were not plotted for any great dis- tance over the land, for in the sum- mer the large amount of vertical con- vection introduces too great an error. While there has yet to be found a relationship between air mass or tra- jectory and the salt content of the fog samples from Mt. Washington and Brooklin, Me., the Kent Island data show quite a definite correlation be- tween the trajectory of the air and the amount of chloride in the fog. This relationship is shown in a single run when a front passes the FIGURE 2. CURVE A represents the number of Island. Because of the very cold wa- days air was over warm water versus p.p.m. chloride obtained only over warm water. ters of the Bay of Fundy in summer CURVE B represents the number of days air (45 to 55°F.) fog forms very rapidly was over cold water versus p.p.m. chloride obtained over cold water. Crosses are points in air passing over the Bay. There- for samples with trajectory over warm water, fore in many cases fog does not clear and dots for those with trajectory over cold water. off after a cold front passes the Island, so that fog samples can be col- Figure 2 is drawn in an attempt to lected continuously through a front. bring out the above relationship Samples collected on July 15 were graphically. The amount of chloride taken during a cold-front passage. is plotted against the number of days Sample no. 2 seems to have been taken the air was followed over water. For in pure TA air; sample no. 3 was the samples which appeared to have a taken in a mixture of TA and Pc; trajectory only over water colder than samples no. 4 and 5 were taken from 64° F. the actual amount of chloride progressively purer Pc. Sample no. found was used as the ordinate, and Unauthenticated | Downloaded 09/24/21 02:11 PM UTC curve B was drawn as an approxi- amount of chloride gathered over mate mean of the data. For samples warm water. with a prior trajectory over warm The two points far to the left of water, the number of days over warm curve A probably are inaccurately water is plotted against the amount placed as to the number of days the of chloride that it would have had at indicated samples were over warm the end of its warm-water trajectory. water, for there seems to have been To find this amount of chloride, the some mixing of air masses of different amount of chloride gathered during trajectories in these cases. the trajectory over cold water (found We de not pretend the curves drawn from curve B) was subtracted from to be "best fits," for all that was at- the actual amount of chloride found tempted was to show that there is a at Kent Is. As the gathering of difference of slope. Therefore, air that chloride over the colder water is has a trajectory over warm water slow, the above procedure affects collects chlorides faster than air com- curve A slightly, but allows us to ing over cold water. get an approximately correct idea of If the particles from the evaporated the relationship between the number droplets of spray act as condensation of days over warm water and the nuclei or are the source of chlorides FOG WATER SAMPLES 1939 (Full Description of headings given below) Total Percent No. Date Time Wind Sea CI, S04 Cl0 S04 (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) 1 July 8 1907-2050 S 2 SW 15 M 19.0 (a) — — >5 >5 T\ 1 2 15 1137-1400 SSE 10 s 6 M 34.8 13.3 72 28 >5 >5 TA 2 3 15 1400-1710 S 6 s 5 M 8.7 9.1 49 51 IV2 % NPC&TA* 3 4 15 1710-1945 s 5 WSW 3 LMS 3.6 7.1 34 66 IV,, V2 NPC 4 5 15 194^-2220 WSW 3 WNW 4 LMS 4.4 6 2 41 59 IY2 Y2 NPC 5 6 16 0713-1120 S 3 SW 2 SMS 3.8 6.7 36 64 % 0 Pc 6 7 26 1824-2008 sw 10 SSW 11 L 2.3 6.4 26 74 1 0 Pc 7 8 26 2008-2155 ssw 11 ssw 10 L 0.3 3.7 8 92 1 0 Pc 8 9 27 0628-1125 ssw 10 s 10 L 6.2 5.2 54 46 3 IY2 NPC 3 10 27 2000-2125 s 10 ssw 10 L 1.5 4.6 25 75 3 0 NPC 10 11 28 0706-0952 SSE 10 SSE in L 7.7 7.1 52 48 4 1% NPC 11 12 28 0952-1140 SSE 10 SSE li L 17.7 9.0 66 34 4 1% NPC&TAI 12 13 Aug. 4 0855-1122 SSE 12 SSE 12 M 4.9 8.1 36 62 2% 0 Pc 13 14 4 1953-2048 SSE 13 SSE 14 M 11.5 6.2 65 35 3% 2% TA&NPC| 1 14 15 5 2020-2147 w 4 w 9 LMS 1.5 4.7 24 76 % 0 NPC 15 16 8 1937-2120 s 10 SE 5 LMS 0.3 4.5 6 94 1 0 NPC 16 17 9 0726-1007 SSE 10 SSW 10 MMS 2.8 4.8 37 63 4 0 NPC 17 18 21 0659-0855 SE 10 SSE 14 L 0.8 6.7 11 89 2 0 NPA 18 19 Sept.
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