Observation of ion clusters in ion-induced NH4Cl nucleation

J Aerosol Sol, Vol 17, No l, pp 95-105, 1986 Printed m Great Bnlmn

OBSERVATION

0021-8502/86 $3 00 +00D ~ 1986 Perpmon Preu Lid

OF ION CLUSTERS IN ION-INDUCED NH4C1 NUCLEATION C. M

BANIC* a n d J. V. IRIBARNE

McLennan Physical Laboratories, University of Toronto, Toronto, Ontario, Canada, M5S 1A7

(First received 25 June 1985; and In final form 26 Auoust 1985) Abstract--Aerosol par tlcles formed by the action of negattve ions in an atmosphere contmmng small amounts of NH3 and HCI were found to be charged The small ion clusters formed were observed with a mass spectrometer A mechanism for the negative-ion-reduced reaction forming NH(CI aerosol is proposed m terms of the mixed clusters CI- (HCI). (NH4CI)j.

INTRODUCTION Ion-Induced gas phase reactions leading to particle production are of interest from the point of view of nucleation theory and for the formation of particles and droplets in the atmosphere (Castleman, 1982). One of the primary roles of ions m nucleation is to act as nucleation centres It is known that the detailed structure of small ion clusters affects the influence of the ton on nucleation (Castlernan and Tang, 1972) and theories are being developed to account for this (Chan and Mohnen, 1980; Suck, 1981). However, ions can play more varied roles in nucleation phenomena than simply acting as nucleation centres. For example, in the ioninduced nucleation of sulphuric acid droplets from H 2 0 and SO2 the ions have been shown to be active in the oxidation of SO 2, but not in the actual droplet formation itself (Diamond et al., 1985). The influence of ions on the gas-to-particle conversion o f N H a and HCI has been studied (Banlc and Irlbarne, 1980) It was found that nucleation o f N H 3 and HCI proceeding in the absence of ions or neutral radicals occurred in accordance with the predictions of class~eal nucleation theory, the nucleation rate increased exponentially with increasing supersaturation above a threshold value Increasing relative humidity acted so as to reduce the threshold The curves obtained were consistent with the expression for the free energy of critical embryo formation based on the reaction NH3 + HCI = N H , C I , with water vapour acting as a catalyst only U p o n introduction of ions or radicals the threshold reactant concentrations for nucleation were reduced, the nucleation rates were increased, and the shape of the nucleation curves was altered, indicating a change in the reaction mechanism. With the apparatus used it was impossible to &stinguish between the effects of positive ions and neutral radicals The effect of negative ions, however, could be distinguished due to their stronger action. The influence of ions and radicals was most evident at low humidities and small reactant supersaturatlons The role of water vapour was still expected to be that of a catalyst. Part,cle sizes were found to be almost the same for particles formed under the influence of positive or negative ions, or in ran-free air. The mean particle radn after a 2.5 s aging ume were found to be 8 + 4 nm, 9 + 4 nm and 11 + 5 nm respectively, with a slight trend to larger sizes at higher huml&ties. In th~s paper the mechamsm of particle growth as influenced by negative and posit|ve ions is investigated Preliminary results have already been briefly presented (Banlc et al., 1983) EXPERIMENTAL The reactant gases were supplied as 1 03 + 0 02 % N H 3 and 0.62 _+0.01% HCI in N 2, and were further diluted by the addition of purified air to concentrations of 10-400 ppm(v). The * Present atfihatlon" Atmospheric Environment Service, 4905 Dufferm Street, Toronto, Ontano, Canada M3H 5T4 95

96

C M BANIC a n d J V IRIBARNE

gas handling system has already been described (Banlc and Iribarne, 1980). The experiments were carried out at atmospheric pressure and 296 K In order to elucidate the action of the ions in inducing nucleation an experiment was performed to determine the degree to which any particles produced were charged, and then in a separate experiment, the small ion clusters which were present In the nucleating system were observed with a mass spectrometer The chamber shown in Fig. 1 was used to determine whether or not the particles formed from NH 3 and HCI by the action of Ions were charged. This chamber has three main sections" the reactant and ion introduction system (A, B, C and D), the reaction region (R) and a large ion (charged particle) trap (E). The electrodes D and E have dimension 3 x 3 cm and 5 cm x 1 m respectively, with an electrode separation of 7_5 cm The electrode pairs D and E are 10 cm apart The gas flows admitted at A carried the NH 3 and HCI in separate channels The gas flows entering at B were of purified air which acted to shield the reactants from the walls and so prevented reactant loss due to heterogeneous nucleation. The gas flow in the tube was laminar, and the reactants could mix In the region R by molecular dtffusion The total flow through the chamber was 101mm-~ giving an average flow velocity of 4 cm s-1 and an average gas residence time in the chamber of ~ 25 s Ions of positive or negative sign produced by a 100/~Cl 241Am foil (~t emitter) at C were introduced to the reaction region R, a field of 4000 V m - 1 applied between electrodes D eliminated the ions of undesired polarity and helped to introduce the others into the flow_ Before any nucleation occurs the Ions in this region are N H ~ (NH3) ,- (H20)m or C1-- (HCI)~- (H20), ~ with 0 ~< (n, m) ~< 5, (i.e. small molecular cluster ions). The field between electrodes D is sufficient to confine the small molecular cluster ions to the reaction regaon The ion concentration IS estimated to be 1 0 6 c m - 3 Neutral radicals produced by the ionizing radiation could enter the reaction region by diffusion. The reactant gases spend ~ 1 s in this region Any reaction which could occur without the presence of a small ion could continue for a longer time as the gases flowed down the tube until quenched by dilution with the purified air flow along the walls Any particles formed in the reaction regJon were carried by the gas flow to the regton between the electrodes E. By application of a suitable field between these electrodes charged particles could be trapped A General Electric condensation nucleus counter was used to detect the particles present in the outflow at F. Diffusion batteries could be Inserted before the counter to allow estimation of particle sizes The chamber used m the mass spectrometric study of the small ion clusters is shown in Fig 2 This chamber was constructed of brass, and no potentials were applied The dilute flows of N H a and HCI entered as two streams The reactant concentrations in this chamber were not accurately known since the mixing was not well controlled, and there were losses due to the formation of NH4CI on the walls. Since it is expected that the ions sampled are those produced very near the orifice the concentration of NH a should be close to that set m the NH 3 stream, and the concentration of HCI is expected to be much lower than present in the Inflow The concentrations introduced to the chamber for the spectra shown in this paper are 40 ppm(v) NH 3 and 170 ppm(v) HCI

F-'N

--i

E

D

I

± #~B

E

C

R/ITU

F i g 1 C h a m b e r used f o r the s t u d y o f a e r o s o l p r o d u c e d b y N H 3 a n d H C I m the presence o f Ions A - i n l e t s for c a r r i e r a n d N H 3 a n d H C I . B inlets for purified air, C at source, D 1on e x t r a c t m n electrodes, R r e a c t i o n region, E partzcle t r a p p i n g electrodes, F exit to particle c o u n t e r T h e c h a m b e r as s h o w n Is n o t to scale T h e overall l e n g t h Js ~ 1 5 m a n d the d m m e t e r o f the r e a c t i o n a n d a g e i n g tube is 7 5 c m

Ion clusters in lon-mdueed NH,CI nucleation

97

HCI

NH 3 ,nlet

(2 sogrc,e

~

moss

spect rornete~

I cm

Fig 2 Chamber used in the investigation of clustering of NH 3 and HCI to ions A bipolar ion population of ~ 10 a cm -3 was produced in the area near the orifice by a 670/~Ci 241Am source. A differentially-pumped quadrupole mass spectrometer (Thomson and Iribarne, 1979) was used to mass analyse the ion clusters produced RESULTS The influence of the trapping field applied between electrodes E on NH4CI aerosol produced m the presence of neutral radicals, and positive or negative ions, is shown in Table 1. Under conditions in which negative ions were able to induce nucleation, but positwe ions and neutral radials were not (conditions A and D in Table 1), application of a large ion trapping field stopped all particles Thus, when all particles are produced by the influence of negative ions all o f the particles are charged. Under conditions in which positxve ions and/or neutral radicals were able to induce nucleation (B, C and E in Table 1) the application of the trapping field to either positive-ion-induced or negative-ion-induced aerosol yielded the same result, i e the number of particles which was detected when positive ions and neutral radicals were present with no trapping field applied. Since all the particles are expected to be approximately the same size, these untrapped particles are neutral. Again, under these conditions, all particles formed by the action of negative ions are charged Since the same number of neutral particles are produced, regardless of the sign of ion introduced into the reaction region, these neutral particles must be the result o f a nucleatmn mechanism which is independent of the ions. This implies that the positive ions are ineffective in inducing nucleation, and that the enhanced nucleation noted by Banic and Iribarne (1980) in the presence of positive ions and neutral radicals may be due to the influence of the neutral radicals only. The results m Table 1 also show that fields of 10 kV m - 1 or less could not trap the particles, whereas fields of 40 or 70 kV m - 1 were effective at trapping particles produced by the action of negative ions. F r o m this result some estimate can be made regarding the number of charges carried by each particle The evolution o f negatwe ion peaks as shown by mass spectrometry for the entire mass range scanned is shown in Fig 3. Each peak cluster of interest is shown in expanded detail in Figs 4-7. The dominant negative ions m the purified air were O~--(H20)~.2 ' 3- Upon addition of HCI the 1on spectrum became dominated by C I - - ( H C I ) I , 2 with a small contribution from C I - - (H20)2-6 and C I - H 2 0 . (N2)0.1" (HC1)0, 1- In Figs 4-7 the emergence of the clusters CI--NH4CI, CI--HCI-NH4CI, C I - . ( N H , C I ) 2 and C i - - (HCI) 2 - NH4C1 can be clearly seen. The intensities of the peaks in the muitiplets were used as a check on the isotopic abundances of CI expected for the identifications made. Within the statistical errors and the error due to mass discrimination the peak intensities verified the above identificatmns. In a test for ion specdicity NH3 was not observed to cluster to 0 2 • (H20)2. 3. 4, SO~- or SO~-.

98

C M BANIC and J V IRIBARNE Table 1 Influence of the large-ion-trap on the amount of NH4CI aerosol detected Reactant parUal pressures N H 3, HCI, H 2 0 (Pa) (A)

06, 06, < 1

Sign

Trapping field*

Particles produced

o f runs

(kV m - 1)

(cm - 3)

+

0 40 0 40

0 0 400 0

+

0 40

4100 4000

-

0 40

9000 4O00

+

0 40

40000 40000

-

0 40

70000 40000

+

0 70

0 0

-

0 5 10 70

1700 1700 1200 0

+

0 5 10 70

800 800 800 700

(B)

(C)

(D)

(E)

0 9 , 0 9 , 400

1, 1 , 7 0 0

15,15,1

1 5, 1 5, 200

* Field due to the potenttal applied between electrodes E

The positive ions observed m purified air were H3 O÷ - (H20)~. Upon addition of NH3 the dominant ions became N H 2 • (NH3)2, 3, 4. When HCI was added peaks appeared at 88, 90 and 105, 107 amu. A possible identity for these peaks is NH~'- (NH3h, 2" NH4CI. There were no peaks observed indicating attachment of two or more HC1 molecules. DISCUSSION The data given in Table 1 show that aerosol particles produced by the gas-to-particle conversion of NH3 and HCI in the presence of negative ions are charged, whereas those produced m the presence of positive ions are not, and further indicate that the posmve ions do not induce nucleation. Consideration must be given to the different mechanisms by which the particles produced in the presence of negative ions could become charged. Charged particles could be produced by (1) neutral particles becoming charged by diffusion charging, or unipolar charging in a field, or by (2) particle growth about an ion core The charging mechanisms gwen in (1) are unhkely since they would lead to charged particles for both positive and negative ions, and this is not the case. An estimate of the degree of charging produced by the mechanisms of (1) can be made. Since small molecular cluster ions are removed from the gas flow by the electrodes D any particles formed are in proximity to the ions for at most Is. Ira maximum particle diameter of 10 nm after 1 s aging time is assumed, then unipolar char~ng m the field would lead to 10 -4 of the particles carrying a charge and diffusion charging to 10- 2 of the particles being charged (Friedlander, 1977). However, the results m Table 1 show that ~ 100~o of the particles produced by the action of negative ions are charged. Thus the charge observed on the particles must be due to mechanism (2): particle growth about the ion. If the action of the negative ions is to serve as a centre for nucleation then each particle should be singly charged An estimate can be made of the number of elementary charges carrted by each particle Assuming top-hat flow, the gas carrying the aerosol particles takes

Ion clusters in ion-reduced NH,CI nucleation

02

99

Io 23.5 omu

68 PURIFIED

AIR

0 2" ( H 2 0 ) 1 , ~ . . 3 86 5O

J 104,106, 107,109, III

PURIFIED

AIR

+ HCI

125

CI- (HCI) n

134

162

PURIFIED AIR •, HCI +

I

NH 3

~L

Fig 3 Nucleatmn of ammomum chloride Change of ion spectrum when reactants are added The slgmficant changes occur m the regmns indicated by the brackets The peak multiplets 71, 73, 75, and 107, 109, 111 are Cl- (HCI)~ ,, 81, 83 and 117, 119, 121 are Cl- H20 N, (HCI)o' =, 89, 91 are C1- (H20)3 and CI- H20 HCI The multJplet family 98, 100; 134, 136, 138 has not been satisfactorily identified, but may be NO~- (HCI)1.2

20 s to pass between the electrodes E. T o be trapped, the particles must have a transverse velocity o f ~ 0 35 c m s - 1 due to the field applied at E. The diameter o f particles aged for 20 s was found to be ~ 20 nm. Fields o f 5, 40 and 70 k V m - ~ are sufficient to stop singly charged particles o f 20, 50 and 70 nm diameter respectively. Considering that the flow in the chamber is not really top-hat (the flow along the middle o f the tube ms > 4 c m s - l ) , and that the measured s~zes for the particles were estimates only, the results support the conclusmn that each charged particle carries one elementary charge. Having n o w established that the particles grow around an ion core there remains the question o f the detailed mechanism o f h o w this occurs It will be shown in the following

AS 17.1-(:]

100

C M BANICand J V IRIBARNE

8 0 - 1 0 4 ~nu

89(0F}

PURIFIED

AIR

PURIFIED ÷ HCl

AIR

98(0F} :OF)

PURIFED AIR + HCI ÷ NH5

c,'-~4cl 88

Fig 4 Detail from Fig 3 regmn 80--104 amu The multJplet88, 90, 92 corresponds to CI- NH4C1 OF = overflow m this scale

arguments that NH4CI aerosol growth is probably achieved by step-wise addition o f HCI and N H 3 to a C I - core The growth o f the ion clusters could be by addition o f N H 4 C l molecules formed m the gas phase or by addition o f N H 3 and HCI singly. The vapour pressure o f NH4CI is low, m the

Ion clusters m ran-reduced NH4CI nucleation

101

I10- 134 ainu PURIFIED AIR

r I I I I I I 117

PURIFIED AIR

+HCI

I L

119

'271 124

t PURIFIED AIR ÷ HCI ÷ NH 5

CI- HCI NH4CI

Fig 5 D e t a d from Fig 3 region llO-134amu The multlplet 124, 126, 128 corresponds to CI- HCI NH4CI

ppb range (Wagner and Newmann, 1961) and the extent of dissociation in the gas phase is high (de Kruif, 1982). Thus the concentration of NH4CI in the gas phase would be very much lower than the N H 3 and HCI concentrations, and so the nucleation occurring about negative ions Is very probably the result of addition of N H 3 and HCI singly to the ions. It is, however, difficult to explain the difference in the ability of positive and negative ions to induce nucleation. There may be energetic or kinetic differences which can account for the observatmns. One hint is the apparent speclficRy of the nucleation to the clusters CI- •(HCI), since N H 3 was not observed to cluster to several other non-chloride ions, and, as found by Keesee et al. (1984) clusters only weakly to CI- in the absence of HCI. The effect of other cluster ions, such as B r - - (HCI) n, should be investigated.

102

C M BANIC and J V IRIBARNE

135- 155

ainu

PURIFIED AIR

136 3F) PURIFIED AIR +HCI

CI- (HCI) 3

I

i I

145

145

PURIFIED AIR + HCI

I

+ NH 3

i

CI- ( N H 4 C l ) 2 Cl- (HCI) 3

141

' I

i

47 [/

II,il

I

j j

i

J4# Fig 6 Detad from Fig 3 region 135--155 ainu The quadruplet 141, 143, 145, 147 in the lowest spectrum corresponds to CI- (NH4CI)2, supenmposed to the triplet 143, 145, 147 corresponding to CI- (HCI)a O F = overflow m this scale

The influence of possible kinetic differences, such as different potential barriers to be overcome by the approaching hgand, are unknown for this system However, the available thermodynamic data can be examined for potential differences between the action of positive and negative ions For nucleation to occur by ad&tlon o f N H 3 and HCI to the ions the relationship between the enth_alpies of dissociation (AHd) must be A H d (CI-

- (HCI). ~ C1- -(HCI)._ ]) > AH d(NH4CI --* NH a + HC1)

AHd(NH~" - (NH3) . ~ NH~" (NHa),_ 1) > AHd(NH4C1 --, N H a + HCI) Estimates for the enthalpy of dissociation (AHd) of NH4CI molecules m the gas phase are 42+_ 13 kJmol - t (Goldfinger and Verhaagen, 1967), and 67 kJmo1-1 (Clement] and Gayles, 1967). The AH d for N H 2 (NH3) . and CI--(HCI). clusters losing one hgand molecule, as well as some predicted equilibrium ion intensity distributions, are given m

Ion clusters in ion-reduced NH4CI nucleation

103

155-175 .mu

PURIFIED AIR

I

[ ] I I

iGI

PURIFED ÷ HCI

AIR

1631

J

162

1 164

PURIFIED AIR

÷

HCI

* NH 3

Ci° (HCI}2 NH4DI

Ftg 7 Detad from Fig 3 region 155-175 ainu The multiplet 160, 162, 164, 166 corresponds to CI-. (HCI)2- NH4CI

Table 2. The relationship between the A/-/d's and the energy levels o f the various reactant states is given in Fig. 8. Since the AH d for N H , C i may be as large as 70 kJ mol - 1, inspection o f Table 2 and Fig. 8 leads to the conclusion that only the n = 1 ion clusters, both positive and negative, would be assured o f leading to stable clusters with NH4CI attached for reacUons o f the type: N H ~ • (NH3) . + HCI - , N H ~ - (NH,CI)- (NH3) . C I - - (HCI). + N H 3 --, C I - - NH4CI - (HCI),. Since the C I - . ( H C I ) . distribution is skewed to lower n than is the N H ~ . ( N H 3 ) . distribution, the probability o f N H 3 associating with C I - ' H C I is greater than HCI

104

C M BANIC and J V IRIBARNE

Table 2 Properties of NH2 (NH3). and CI AHd (kJ mol - ~) NH + (NH3).* CI- (HCI).~

(HCI).

Abundance at 296 K (~;,) NH~ (NH3).* CI- (HCI).I at 1, 2 Pa NH a at l, 2 Pat HCI

0

104

99

73

64

58

49

52

43

1 2 3

0,1 004

11, 58

73 0, 56 6

86, 88 0

266, 41 9

3, 63

02,05

--

4

* Calculated from results given m Payzant et al (1973) *Calculated from results given in Yamdagm and Kebarle (1974) a)

CI"

CI-+HCI÷NH3 9

-71CI-+NH qCI

HCI+NH~' X~ CI- NH4CI b)

CI-HCI+HCI+NH3

c'

\ C I - HCINH4CI/ Fig 8 Energy level dmgrams showing formation of a cluster 1on (a) stable against dJssoclahon and (b) unstable against &ssooatlon following reachon between NH 3 and HCI on an ion The dissociation enthalples m kJ mol-l are shown

associating with N H ~ - N H 3 . In the nucleation experiments only a few percent o f the ions are effective, and this is consistent with the ion distributions given T h e ion spectra a n d the a b o v e discussion indicate the following m ech an i sm for particle production" C I - HC1 NH3 C1- N H 4 C I HCI C I - N H 4 C I HC1 NHa CI-

(NH4CI h_

The addition o f the second N H a m a y be stabilized by interactions with the N H , C I already clustered. Since the critical radius for nucleation o f N H 4 C I was found to be 0.55 n m in the absence o f ions (Banlc and Irlbarne, 1980) it would not be unreasonable to assume that C I - (NH4CI) ., with n = 2 or 3, is the cluster o f critical size, and that larger clusters are not seen because o f extremely fast growth to particle size.

Ion clusters m ton-mduced NH4CI nucleauon

105

T h e new positive ion peaks o b s e r v e d in the present study m a y be due to the association o f N H 4 C I f o u n d in the gas phase to the positive cluster ion, b u t further g r o w t h m a y be impossible due to the low NH4C1 c o n c e n t r a t i o n s present. Coffey (1972) also investigated the positive ion clusters f o r m e d in the presence o f N H 3 a n d HCI with a mass spectrometer, a n d saw no indication o f particle g r o w t h a b o u t the tons. CONCLUSIONS A c o m p r e h e n s i v e study o f i o n - i n d u c e d gas-to-particle reactions o f N H 3 a n d HC1 has been made. It was f o u n d that negative ions, specifically C l - - HCI, c o u l d initiate the f o r m a t i o n o f a particle with the i o n r e m a i n i n g at the core. Positive ions a p p e a r to be ineffective at inducing nucleation T h e indication is that N H 4 C I particle g r o w t h is achieved b y step-wise a d d i t i o n o f HC1 a n d N H 3 to the core C l - . This is an example o f the g r o w t h o f m i x e d cluster ions to aerosol parUcles. Acknowledgements--We would hke to thank the Natural Soences and Engineering Research Council and the

Atmospheric Environment Serwce for financial support, and are grateful to Professor W J Megaw of York Umversity for the loan ofa Pollak counter C M B is grateful to Professors A W Castleman, Jr and R G Keesee of The Pennsylvama State University for helpful discussions REFERENCES Banic, C M and lnbarne, J V (1980) J oeophys. Res. 85, 7459_ Banic, C M, Diamond, G L and lnbarne, J. V (1983) Ion-induced gas-to-parUcle reacUons In Proc Atmos Electrloty (Edited by Ruhnke, L. H and Latham, J ) pp 36-39 A Deepak Castleman, A W, Jr. (1982) J Aerosol ScL 13, 73. Castleman, A_ W , Jr and Tang, I_ N (1972) J chem Phys_ 57, 3629 Chan, L Y and Mohnen, V_ A (1980) J_ Atmos SCL 37, 2323 ClemenU, E and Gayles, J N. (1967) J chern phys 47, 3837 Coffey, P E (1972) Exploswe Growth Reuctzons Induced by the NH,~ (H20)2 Clustgr Pubhcatton No 204. Atmospheric Sciences Research Center, State Umverslty of New York at Albany, Albanyl de Krulf, C G_ (1982) J. chem Phys_ 77, 6247 Dtamond, G L, Inbarne, J. V and Corr, D J (1985) 3 Aerosol Scl 16, 43 Fnedlander, S K. (1977) Smoke, Dust and Haze Chapter 2 John Wiley~Toronto Goldfinger, P. and Verhaegen, G (1969) J chern Phys 50, 1467 Keesee, R. G, Evans, D H, Passarella, R and Castleman, A W, Jr (1984) Presented at the American Chemical Society Meeting, Philadelphm_ Payzant, J D, Cunmngharn, A J and Kebarle, P (1973) Can 3 Chem. 51,324Z Suck, S H (1981) J chem Phys. 75, 5090. Thomson, B A and Inbarne, J V (1979)J chem Phys 71, 4451. Wagner, H and Newmann, K (1961) Z Phys Chern 28, 51 Yamdagm, R and Kebarle, P (1974) Can J Chem 52, 2449