THE COMPUTED MOBILITY OF SMALL IONS IN THE ATMOSPHERE OVER THE OCEANS 



A considerable number of laboratory determinations 

 of the mobility of small Ions have been carried out over 

 a period of many years. The results indicate that at 

 normal temperature and pressure the mobility of the 

 negative lofi generally is appreciably greater than that 

 of the positive ion. In gaseous compounds the mobility 

 of both the negative and the positive ion is reduced from 

 that found in the elementary gases. The Introduction of 

 water vapor into dry air has a similar effect on the mo- 

 bilities. In both cases the reduction in mobility of the 

 negative ion is greater than that of the positive ion. The 

 ratio of negative to positive mobility consequently is re- 

 duced under these circumstances. Zeleny (1), using 

 x-rays as a source for ions, for example, found that the 

 mobility of the negative ion in dry air was 1.87 cm/sec/ 

 v/cm, while that of the positive Ion was 1.36 cm. In 

 moist air, on the other hand, the mobilities were 1.51 

 and 1.36, respectively, for the negative and positive ion, 

 a reduction in the ratio from 1.36 to 1.10, in going from 

 dry to moist air. In any attempt to estimate the mobility 

 of the small ion in the atmosphere from laboratory de- 

 terminations, it is necessary to allow for the effect of 

 various gaseous compounds that may appreciably alter 

 the mobility and also for moisture which always is pres- 

 ent in varying amounts. The picture is still further 

 complicated by the fact that there may be other factors 

 as well which appreciably influence the mobilities but 

 by different amounts. 



The answer to any question concerning the mobility 

 of the small ions and the relation between the negative - 

 and positive-ion mobility in the atmosphere probably can 

 best be answered through observation on ordinary air, 

 that is, air which has not been artificially ionized, dried, 

 or purified in any way. Simultaneous observations on 

 the ion content and conductivity during fair weather pro- 

 vide valuable material for such determinations. Obser- 

 vation as ordinarily carried out, however, must be used 

 with considerable caution because of various effects 

 which can appreciably alter the results. The presence 

 of a considerable number of large ions, for example, 

 may appreciably contribute to the current in the ion 

 counter and thus lead to an error in the determination of 

 the small-ion content of the atmosphere. This effect is 

 such as to increase the value attributed to the small-ion 

 content of the air and consequently to reduce the com- 

 puted value of the mobility. The negative and positive 

 large ions may not always be equally numerous and con- 

 sequently the ratio of negative- to positive-ion mobility, 

 as well as the absolute value of the mobility, then will be 

 affected. Another effect may arise from the earth's 

 electric field. The charge induced on the exposed part 

 of the ion counter by the normal potential-gradient re- 

 duces the number of negative ions measured with this 

 instrument. The reduced negative-ion count under these 

 circumstances will result in a corresponding increase in 

 the negative small -ion mobility. Since the positive -ion 

 content remains unaffected by the potential-gradient, 

 there will then be an increase in the ratio of negative- 

 to positive-ion mobility. 



Observations made over the ocean areas aboard the 

 Carnegie were such as largely to avoid these effects. 

 The large-ion content of the air over the ocean generally 



is small compared with that over land areas and there 

 is no evidence that the intermediate -ion content is of 

 any consequence. Any electric field at the observing 

 station was practically eliminated owing to the presence 

 of the sails overhead. The observations aboard the Car- 

 neeie were carried out in such a manner that the meas- 

 urementsof ion count andconductlvity were simultaneous, 

 and positive- and negative-ion and conductivity measure- 

 ments were alternated in order that negative -ion mobility 

 would center on the same average time as the value of 

 positive-ion mobility. Considering all these circum- 

 stances it appeared of interest to examine the values of 

 mobility computed from small-ion and conductivity 

 measurements of the last cruise of the Carnegie and, for 

 comparison purposes, values computed for previous 

 cruises as well. 



One might reasonably expect that the mobility of the 

 small ions in the atmosphere, even over the ocean, might 

 differ greatly from time to time owing to changes in the 

 various factors which are generally supposed to affect it. 

 On examining the values of the mobilities calculated from 

 ion- content and conductivity data it is found that they do 

 scatter over a considerable range in values. It would 

 seem desirable to determine to what extent this scatter 

 may be attributed to real changes in the mobility and to 

 what extent it may be due to errors in measuring the 

 ion content or the conductivity of the atmosphere. This 

 matter has been investigated and will be discussed in 

 what follows. 



Frequency curves of the computed mobility of the 

 positive and the negative ion have been constructed from 

 data taken during cruises TV, VI, and Vn, and are shown 

 in figure 11. In each case the resulting curve corre- 

 sponds closely to a probability curve, the latter being 

 drawn also in the figure. The mean values of the com- 

 puted mobilities differ considerably for the various 

 cruises. For cruises IV and VI the mobility of the posi- 

 tive ion is practically the same as that of the negative 

 ion, whereas for cruise VH it is appreciably less. The 

 mean approximate values are as follows: 1.4 cm for 

 cruise IV, 1.6 cm for cruise VI, and 1.3 and 1.4 cm for 

 the positive and negative ion, respectively, for cruise 

 Vn. The changes in the mean value from cruise to 

 cruise probably can be ascribed to changes in the instru- 

 ments not compensated for by appropriate changes in 

 constants. The difference in the mean computed mobili- 

 ties of the two signs of ions in the case of cruise Vn, 

 however, cannot be accounted for in this manner. The 

 difference in this case may be related in some way to a 

 change in meteorological and other conditions of the at- 

 mosphere, but as yet it must remain unexplained. 



The spread in computed mobilities, as illustrated by 

 the frequency curves, appears rather large, and further 

 consideration was given to this matter. As a step in 

 this direction the values of conductivity and ion content 

 were grouped and meaned for various values of mobility 

 over the range of 0.7 to 2.5 cm/sec/v/cm, the values of 

 mobility being taken in steps of 0.1 cm. The mean values 

 of conductivity and ion content for the various mobility 

 values have been plotted in figure 12A, for the combined 

 data of cruises IV and VI. To use all possible available 

 data, the results for both positive and negative ions have 



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