THE MOBILITY SPECTRUM AND THE ION CONCENTRATION OF LARGE IONS IN AIR MIXED WITH N~O GAS b y A. F. EL NADI (*) & N. FARAG (**)

Summary - - Using the divided electrode condenser it was possible to detect the large ion groups formed when small amounts of NsO gas were mixed with atmospheric air. Eight groups aPPeared with mobilities ranging from 12.50 • 10 -a to 0.60 X 10 -a cm/sec :volt/cm. When using the whole electrode condenser the results showed an increase in the total ion concentration of these large ions when small amounts of N20 gas were mixed with air. The results obtained in this work confirm that 1%O gas acts as a nucleus for condensation which is changed into a large ion by appropriating an electrical charge. Introduction.

I t was shown e x p e r i m e n t a l l y b y some investigators (1,2) (1916) t h a t , when large ions were produced in the air in various ways, the ions could be divided into a number of groups, each group having a definite mobility. According to LoEB (~) (1934) i f a gas is really pure it can be asserted t h a t ions of one m o b i l i t y only can be observed. I n a heterogeneous gas such as poorly dried air with organic vapours, and subjected to chemical change in a closed space as a result of prolonged exposure to ionizing agents, a continuous distribution of ionic mobilities, i.e. a m o b i l i t y spectrum, can be expected after the lapse of a considerable time. 1R. K. B o Y r x ~ (4) (1931) obtained, in a r o o m situated in the city of Dublin, not far from a n u m b e r of f a c t o r y chimneys where the air is polluted b y m a n y combustion products the following mobilities: (12.60, 6.35, 3.38, 2.16, i.28, 0.90 and 0.60) X 10 -4 cm/sec:volt/cm. J. A. Mc CLnrrA~D & P. J. NOrA~ (5) (1919) found in the air which had been passed over phosphorus groups of ions having the following mobilities: (12.00, 6.40, 3.10, 1.50, 0.85 and 0,53) X 10 -4 cm/sec : volt/cm. This p a p e r contains a s t u d y of the ion groups and their concentration in atmospheric air containing small amounts of I~20 gas (prepared in the laboratory).

(*) Assiout University, Faculty of Science, Assiout, Egypt, U.A.R. (**) Cairo University, Faculty of Science, Giza, Egypt, U.A.1R.

--

115

- -

Apparatus and Procedure. The a p p a r a t u s used in this s t u d y consisted of a cylindrical condenser composed of two concentric brass tubings (Fig. 1). The inner electrode of 4.76 cm external diameter was divided into three sections insulated from each other and fixed firmly together b y perspex cylinders. The lengths of the sections were 140, 20 and 20 cm in the direction of the air flow. The outer electrode (i60 cm in length a n d 7.30 c m internal diameter) was

Electl'omet er

Earth

[]

Ebonite

Fig. 1 - Large ion counter.

connected to a variable voltage supplied b y means of d r y batteries placed inside a shielded box. The cylindrical condenser was well shielded b y a t h i r d galvanized iron t u b e of 10.5 cm in diameter. The air was drawn from a rectangular wooden box of 240 litres volume, t h r o u g h a short t u b e of galvanized iron of 5 cm internal diameter, into the cylindrical condenser. The following two methods were a d o p t e d : 1) The divided electrode method, due to ZELE•u in which the first and t h i r d sections of the inner electrode were grounded, and t h e second section was connected to an electrometer of the Compton t y p e adjusted at a sensitivity of a b o u t 1800 m m / v o h . This m e t h o d was used to s t u d y the ion groups a n d their mobilities. 2) The non-divided or the whole electrode method, due to Mc CLELLA~D, in which the first and the second sections were connected to the electrometer while the t h i r d section was grounded. This method was used to s t u d y the ion concentration. I n the first case the nitrous oxide gas was passed into the suction box t h r o u g h a hole at the top of the box and the mixture of atmospheric air and the N20 gas was w i t h d r a w n with a s t e a d y velocity t h r o u g h the cylindrical condenser. The potential V applied to the outer electrode was v a r i e d and the ionisation current was measured, b y recording the time t t a k e n b y a light spot reflected from the electrometer rain-or to move 10 m m on the scale. Then a curve was drawn to show the relation between i/k and f , where k is the m o b i l i t y of the ion and f is the corrected value of ]/t I f - T,/T X 1/t, where T is the mean of the readings taken with a s t a n d a r d p o t e n t i a l (T,)]. Obtaining several characteristics of this t y pe, a s t u d y was made of the ion groups and their mobilities.

--

116

--

I n the second case a s t e a d y flow of the air containing the N20 gas was allowed to pass t h r o u g h the cylindrical condenser, and the ionisation current was measured while the potential applied was varied. A characteristic curve was t h e n drawn showing the relation between 1/k and N, where k is the m o b i l i t y and N is the n u m b e r of ions per cm 8 (i.e. the ion concentration). B y the aid of these characteristics a s t u d y of t h e ion concentration was made. As the ion concentration IV does n o t r e m a i n constant t h r o u g h o u t the series of observation, the current was measured with a s t a n d a r d p o t e n t i a l at short intervals, then every reading was corrected relative to the m e a n of these readings if t h e y are within a reasonable order, otherwise the whole series were repeated. The experiments have been carried out in a r o o m 9 metres from the ground level in the Physics D e p a r t m e n t , F a c u l t y of Science, Cairo University at Giza, Egypt.

Preparation of N20 Gas. A mixture of recrystallized a m m o n i u m n i t r a t e and pure sand in equal proportion b y weight was placed in a p y r e x flask provided with a glass stopper a n d an outlet tube connected t h r o u g h a hole at the top of the suction box. The mixture was t h e n gently heated to ensure a regular current of the gas. The gas was dried b y passing it over silica gel. NH3NO~ = N20 § 2H20. Nitrous oxide gas was found to have no chemical effect on the a p p a r a t u s and the connection wires.

Mode of Calculation. The m o b i l i t y of ions was calculated from the formula derived b y HOGG (6) (1939):

= <~/4~V(C1 § C~), where qb is the volume of air passing per second, V is the applied potential, C1 is the c a p a c i t y of the first section and C2 is t h a t of the second section. I n the case of the divided electrode m e t h o d the ion concentration N was calculated from the relation: N=

[

~

10

•

(C2§

em

2

]

X (C 1 § C2) X f ,

where :

S is the sensitivity of the electrometer in mm/volt, f is the corrected value of 1/t = T # / T x l / t , where T is the mean of the readings Ts t a k e n with the s t a n d a r d potential, and t is the time in seconds t a k e n b y the light spot to move 10 m m on the scale, Ca is the c a p a c i t y of the electrometer and its connections, and e is the electronic charge. I n the case of the whole electrode method the ion concentration N was calculated from the relation: (Q§ N =

C~) 300e(P

10 • ~- x f.

--

117 ---

Results. a) The divided electrode method - - Fig. 2 a n d Fig. 3 a r e t w o e x a m p l e s of t h e c h a r a c t e r i s t i c s o b t a i n e d w i t h t h e d i v i d e d e l e c t r o d e c o n d e n s e r for p o s i t i v e a n d n e g a t i v e ions.

Ifx16s

2 I[ "q

I_~_xI0s

i

i

i

i

i

L

i

i

~

~1

2

4

6

8

10

12

14

16

18

20

K

Fig. 2 - Characteristics obtained by the divided electrode condenser for positive large ion~ by a flow of a mixture of air and N20 gas.

fxlO

-3

36

28 24

2O

/ J/ l / / ,/ / \ I

I~

0

~

,

2

4

,

6

8

,

10

12

,

14

16

18

~

I__

20

K

xlO 3

Fig. 3 - Characteristics obtained by the divided electrode condenser for negative large ions of a mixture of air and •20 gas.

- - 118

--

I n the curve 2 seven sharp peaks appear indicating the existence of 7 ion groups. The mobilities and the ion concentration of these groups are given in Table 1. TASLE 1 - ( + ) ve. k X

10 .4

1/k

era/see : volt/era

N[emZ

1300 2600 4300 6000 10000 14600 16600

7.70 3.85 2.33 1.67 1.00 0.68 0.60

1434 1520 1070 667 496

248 527

F o r t y three regular curves were obtained, 24 curves for positive ions, and 19 for negative ions. This work was carried out during the period between F e b r u a r y 1959 to August 1959. The m a j o r i t y of the curves was done in Egypt's fair and steady weather. The relative humidity- varied from 26% to 54% and the temperature from 16 ~ C in winter to 32 ~ C i n summer. The percentage b y volume of N20 gas in the mixture u n d e r investigation ranged from 1.03~ to 2.16%. The average mobilities of the positive ion groups and also the negative ion groups are given in Table 2. TABLE 2. Group number

Mobility (+)

1 2 3 4 5 6 7 8

10.43 3.77 2.32 1.64 1.35 1.00 0,78 0.60

re.

~_ O.80 ~ 0.34 -- 0.20 ~ 0.12 ~= 0.11 ~ 0.07 i 0.08 •

• 10-4

(I)

10.99 3.89 2.36 1.66 1.31 1.01 0.79 0.60

ve.

:~ O.93 ~: 0.31. • 0.21 zk 0.15 :~ 0.12 ~ 0.07 • O.07 • 0.06

b) The whole electrode method ~ Fig. 4 and Fig. 5 are two examples obtained with the whole electrode condenser for positive and negative ions. I n both cases curve A was drawn b y a flow of air alone, and curve B b y a flow of air mixed with N20 gas. Four bends appear clearly in curve A, Fig. 5, representing four ion groups while in curve B six distinct bends can be seen corresponding to six ion groups. The mobilities and ion concentration of these groups are given in Table 3.

--

119 - Fig. 4 - Characteristics obtained by the whole electrode condenser of positive large ions. A : By a flow of air alone, B: By a flow of a mixture of alr and N~O gas.

NxlO 3

k

~

B

~

If"

A

, 2

4

6

8

10

I__x I0 z

12 14 16

18 20 K

Nx10 ~ 2.0 1,6

Fig. 5 - Characteristics obtained by the whole electrode condenser of negative large ions, A: By a flow of a mixture of air, B : By a flow of a'mixture of air and N20 gas.

1.2 .8 .4 3

1__xlO

TABLI~ 3 - ( - - ) r e . Curve A

Curve B

1/k

k • 10 -4

N/cm a

1/k

k • 10 -a

N/cm ~

1300 4400 9900 16800

7.69 2.27 1.01 0.60

1240 200 100 100

1000 2800 4400 7300 16800

10.00 3.57 2.27 1.37 0.60

1300 180 240 100 100

1640 2020

m

120

The total ion concentration in the case of atmospheric air is 1640, and in the case of air mixed with }q20 gas is 2020 with an increase of 380 i.e. 23.2% of the, ions present in air. TA~r~ 4 - ( + ) ve.

Total ion concentration Date 1959

5/10 15/10 19/10

26/10 27/10 2/11

3/11 9/11 10/11

Temp. oC

R.H.

% of ~qzO

%

28.0 30.0' 26.2 23.0 23.2 28.8 30.0 26.5 27.8

42 33 32 34 32 24 20 51 28

1.62 1.08 1.72 1.62 0.86 1.03 2.16 2.58 2.16

Air

Air &N~O gas

6100 5500 4750 2050 6350 4600 63OO 395O 4600

7550 6600 6600 2700 6800 5500 79O0 6200 6000

Percentage increase

23.8 20.0 38.9 31.7 7.1 19.6 25.4 57.0 30.4

A s u m m a r y of the results obtained b y this method is given in Tables 4 and 5. I t is seen from these tables that the increase in the total ion concentration varies with the concentration of 1NT20gas i n the mixture, the relative h u m i d i t y and the temperature.

T.srr

5 - ( - - ) re.

Total ion concentration Date 1959

5/10 12/10 13/10 26/10

2/11 7/11 9/11 9/11

10/11 11/11

Temp.

R.H.

oC

%

28.2 28.0 28.0 22.5 27.5 25.8 26.2 26.0 25.2 25.2

41 29 31 30 43 40

46 47 50 55

% of N20

1.62 1.29 0.86 1.62 2.16 1.62 1.08 1.29 1.03 2.16

Air

Air & N20 gas

3850 3500 7400 1640 7300 4100 6150 4850 6450 5500

4650 3950 8000 2020 10500 5100 6950 6100 7100 6900

Percentage increase

20.8 12.8 8.1 23.2 43.8 24.4 13.0 25.8 10.1 25.5

Discussion o f the results. The results obtained b y the divided electrode condenser show the existence of 8 groups of ions i n air mixed with small a m o u n t s of ~qzO gas. This is in good agreem e n t with the results perviordsy obtained b y some investigators under approximately similar conditions. This can be seen clearly from Table 6.

- -

121

- -

TABL~ 6.

Observer

Condition

Mobility of the ion groups obtained • 10 -4

R . K . BoYLA~ (1931)

J- A- Mc CLELLA~D & P . J . NOLAI~ (191~

City air near factory chimneys

Air passed over

12.60 6.35 3.38 2.16

12.00 6.40 3.10

phosphorus

1.50 1.28 0.90 0.60

0.85 0.53

Authors (1959) Air containing small amounts of N20 gas

( + ) re. 10.43 3.77 2.32 1.64 1.35 1.00 0.78 0.60

- - ) ve.

10.99 3.89 2.36 1.66 1.31 1.01 0.79 0.60

The results of the whole electrode m e t h o d show an increase in the t o t a l ion concentration of large ions with a m o b i l i t y ranging from 12.50 • I0 -4 to 0.60 X 10 -~ when small amounts of N20 gas are m i x e d with air. I n the case of positive ions this increase ranges from 7.1~o to 57.0~o giving an average value of 28.2~o. I n the case of negative ions the increase ranges from 8.1~ to 25.5~o giving an average value of 20.8~o. The above results confirm t h a t N20 gas acts as a nucleus for condensation which is changed into a large ion b y a p p r o p r i a t i n g an electrical charge. This supports the suggestion p u t b y H. NOm~DER & R. SIKS~A (7) (1952) t h a t N20 gas m a y be considered as an i m p o r t a n t source for the production of nuclei.

Conclusion. I t appears from the results obtained in this w o r k t h a t , when mixing small amounts of N20 gas w i t h air, new groups of large ions are formed. The mobilities of the ion groups detected in this case are: (10.43, 3.77, 2.32, 1.64, 1.00, 0.78 and 0.60) X 10 -4 for positive ions and (10.99, 3.89, 2.36, i.66, 1.31, 1.01, 0.79 and 0.60) X X 10 -4 for negative ions while in the case of atmospheric air alone four groups could be detected, n a m e l y : (6.93, 2.26, 1.00, 0.60) X 10 - 4 c m / s e c : v o l t / c m . There is also an increase in the t o t a l ion concentration of large ions w i t h a m o b i l i t y ranging from 12.50 X 10 -4 to 0.60 X 10 -4 w h e n small amounts of 1N-20 gas are m i x e d with air. This leads to the a s s u m p t i o n t h a t some of t h e large ions in the locality where t h e experiment has been carried out m a y be due to _N20 evolved b y the soil. This is s u p p o r t e d b y the infra-red measurements (s) which a p p e a r to favour the assumpt i o n t h a t the major source of the N20 gas in the atmosphere is the soil.

- -

122

- -

REFERENCES (I) NOLAN J. J. (1916): The Mobilities of Ions Produced by Spraying Distilled Water, Proc. Roy Irish. Acad., Vol. 33, A, pp. 9-23. - - (2) Mc CLELLAND J. A. & NOL~tN P. J. (1926): The Nature of Ions Produced by Bubbling Air through M'ercury. Proc. Roy. Irish. Acad. Vol. 33, A p. 24. - - (~) LOEB L. B, (1934): The Kinetic Theory of Gases, Mc Graw Hill, p. 543. - - (4) BOYLAN R. K. (1931): The Mobilities of Atmospheric Large Ions, Proc. Roy. Irish. Acad., Vol. 40, A, 4, p, 76. - - (5) Mc CL~LLAND J. A. & NOLA~ P. J. (1919): The Nature of the Ions produced by Phosphorus, Proc. Roy. Irish. Acad. Vol. 35, A, p. 1. - - (s) HocG A . R . (1939): The Intermediate Ions of the Atmosphere, Proc. Phys. Soc., 51, p. 1014. ~-- (7) NORINDE~ H. & SIKSNA R. (1952): Ions produced in the air at atmospheric pressure by ultraviolet light from a quartz-mercury arc, A r k i v for Fysik, B a n d 5, 23, p. 471. - - (s) ALTSHULLER A~BREY P. (1958): Natural sources of gaseous pollutants in the atmosphere, Tellus, vol. 12, p. 479.

(Received 29 December 1960)