MEASUREMENTS OF NATURAL DEPOSITION AND CONDENSATION-FREEZING ICE NUCLEI WITH A

CONTINUOUS FLOW CHAMBER

R. AL-NAIMI* and C. P. R. SAUNDERS Pure and Applied Physics Department, UMIST. P.O. Box SS, Manchester, M60 1QD. England

(First recriced 7 February 1985 and received for publicotion 20 May 1985)

Abstract-Ice nucleus concentrations in samples of city air have been determined by using a continuous flow diffusion chamber. The chamber has been set-up so that ice nuclei may be activated as they pass through regions of controlled supersaturation and temperature similar to those experienced in natural clouds. It has been found that two distinct modes of nucleation may be discerned by adjusting the flow rate through the chamber to permit a range of nucleus activation and crystal growth times. For short passage times, deposition nuclei may be activated both above and below water saturation. At longer growth times, and above water saturation, condensation-freezing nuclei have time to activate and grow. The total nucleus concentration detected is thus the sum of the two distinct types activated in these experiments. The resulting concentrations are considerably in excess of estimates obtained with a conventional filter technique and help to account for the discrepancies between observed ice nucleus and measured ice crystal concentrations in clouds.

Key word index: Ice nuclei, diffusion chambers.

ISTRODUCTION

The ice phase in clouds is initiated by ice nuclei which form a small portion of the total atmospheric par- ticulate concentration. Ice nuclei are important be- cause in supersaturated regions of clouds, ice crystals grow faster than the surrounding water droplets, leading to the rapid developement of precipitation particles. This process is active in many types of cloud. The detection and identification of ice nuclei continues to present problems manifested in the general observa- tion in clouds of a far smaller concentration of ice nuclei than of ice crystals. In the case of maritime cumulus clouds, where large differences were noted (Mossop er al., 1972). the problem has been solved in terms of an ice multiplication mechanism involving the freezing of supercooled droplets upon a soft-hail pellet. Under carefully controlled conditions, Hallett and Mossop (1974) found that at temperatures between - 3 and - 8C with droplets larger than 25 pm and less than 12pm, ice splinters were ejected from a riming surface. Griggs and Choularton (1983) have shown that this process is associated with the details of the drop freezing process. However, there are many other studies that have revealed large crystal/nucleus con- centration discrepancies that are not explicable by this process. For example, studies of Montana summer cumuli by Cooper and Rodi (1982) showed that conditions for the Hallett Mossop process were not commonly met. Other studies are listed in a review by Rogers and Vali (1978). The observation of shattering

* Current address: Physics Department, Al-Mustansiriyah University, Baghdad, Iraq.

of delicate ice crystals upon collision with soft-hail pellets was reported by Vardiman and Grant (1974) and crystal fragments in snowfall have also been reported by Jiusto and Weickmann (1973). It is therefore possible that in some clouds multiplication processes can lead to the observed high crystal concen- trations. However, studies by Cooper and Saunders (1980) in cold winter-time orographic clouds and by Cooper and Vali (1981) in mountain cap clouds revealed concentrations of ice crystals in excess of ice nucleus concentrations in conditions which precluded an ice multiplication mechanism. In the orographic clouds studied, the ice crystal concentration showed no dependence on the cloud temperature. In some cases it was possible to trace the crystal origins back to a particular temperature level and to relate crystal concentration to the ice initiation temperature. The data indicated a crystal concentration of 10 I- at - 13C with a ten-fold increase for every 7C of cooling. In the mountain cap clouds, most crystals were formed at the cloud condensation level. Both these studies indicate that a specific nucleation mechanism was taking place and two possibilities were suggested. One involves a condensation-freezing mechanism in which a nucleus, perhaps having both a soluble and insoluble component, would act firstly as a cloud condensation nucleus and secondly as a freezing nucleus. The second involves a contact nucleation mechanism whereby a supercooled droplet captures a sub-pm freezing nucleus by Brownian diffusion.

Many measurements of ice nucleus concentrations have been made by drawing sample air through a paper filter having submicrometer pore size so that ice nuclei are deposited on the surface. Subsequently, the nuclei may be activated and grown into ice crystals b)

1871

subjecting the filter to J known supersaturation below 0 C in a static diffusion chamber. A review oimeasure- ments using this technique has been made by Pruppacher and Klctt (1979). The method is con- venient in that filters may be sampled on location and processed later in the laboratory: however. it suffers from the disadvantage that a high concentration of particles ars trapped on the filter surface where they compete for the available water vapour during activ- ation. Thus, the method is liable to underestimate the concentration of ice nuclei. With the filter chamber set above ice saturation and either above or below water saturation, the nuclei activated on the filter are de- position (sublimation, or sorption) nuclei. This process involves the direct transfer of water molecules to the nucleus to form the ice phase. If supersaturation with respect to water can be achieved over the filter surface, and such a condition may be precluded by the competition for the available water vapour by all the aerosol on the filter, then condensation-freezing nuclei may be activated.

An alternative nucleus activation method, in which an air sample containing the nuclei is subjected to a controlled temperature and supersaturation, has been developed over the past few years. This continuous flow technique was first described by Schaller and Fukuta (1979) and has been developed by Fukuta and Tomlinson (1980), Rogers (1982b) and Hussain and Saunders (1984). In principle, the aerosol sample is drawn through a chamber between two ice coated walls having a controlled temperature difference. The airborne nuclei experience a supersaturation and are activated as deposition nuclei below water saturation and as deposition and;or condensation-freezing nuclei above water saturation. Rogers, and Hussain and Saunders studied natural aerosol and noted that the continuous flow technique detected a higher concen- tration of ice nuclei than did the corresponding filter method. In some cases, with the continuous flow method, the nucleus concentration was more sensitive to supersaturation increase above water saturation than below indicating that condensation-freezing nuclei may have been active in conditions above water

saturation. Hussain and Saunders (1984) developed a

continuous-flow chamber in which the 3O-cm wide walls were horizontal with a vertical separation of 7 mm; the 12O-cm length permitted crystals to grow to about lO-pm diameter depending on the supersatur- ation and flow rate involved. They compared the continuous flow with the filter method and found that the former detected a nucleus concentration typically between 1 and 10 /- for a range of supersaturations from 12 to IS, with respect to ice, while the filter method detected between 0.1 and 0.5 F-t. They also noted that the continuous flow technique detected an increasing number of ice nuclei as the Aitken nucleus concentration increased (detected by a Pollak counter). while the filter method detected a decreasing number of nuclei. This result was attributed to the increased

competition for vapour on the tilts: surface and it makes difficult a simple mathematical technique by which a filter determined concentration may be cor- rected by previous calibration with the continuous- flow method.

In the present work, the continuous-Row chamber has been re-built and improved. For the earlier model, calculations of crystal growth and trajectories in- dicated that crystals activated early in the chamber, or below the central plane. would tend to fall out during passage through the 7-mm high chamber. This prob- lem has been overcome by turning the chamber on its side, thus providing 30cm of fall depth. The new chamber has been used to measure natural ice nucleus concentrations in IManchester.

Thr continuous-jaw cambrr (CFC)

The principle ofcontinuous-flow chambers has been discussed by Hudson and Squires (1973, 1976) and Hussain and Saunders (1954) among others. Ice nuclei are activated and grow into ice crystals which can be counted. Figure 1 shows the CFC which consists of two parallel aluminium plates of thickness 2 cm through which cooled methanol solution may be pumped to control the plate surface temperatures, the whole being insulated with iO-cm thick polystyrene sheets. The plates measure 120 cm by 30 cm and are separated from each other by thermally insulating tufnol strips 1.3-cm thick which form the side walls of the chamber. Hinges along the chamber length permit the inside plate surfaces to lie open horizontally so 15 mm thick ice layers may be formed upon them which can easily be polished smooth. When the chamber is shut, the plates seal against silicone rubber gaskets along the length of the tufnol strips, the whole being tightly clamped to avoid leaks. The chamber may be run horizontally with the plates horizontal, or verti- cally with the long axis horizontal. These will be called the horizontal and vertical modes. respectively. The sample air is introduced at one end through a row of jets mid-way between the plates. On either side of the sample air, layers of dried filtered sheath air maintain the sample air in position. Smoke tests confirmed that the How is not turbulent so that the aerosol sample does not mtx with the sheath air during passage through the chamber.

Figure 2 is a schematic diagram of the equipment. The input air is drawn through a 15-m long, Z-cm diameter tube and then through diffusion dryers. A portion of the stream goes to an impactor designed to remove large particles. The concentration of natural aerosol particles of 5-10 pm size in an industrial city such as Manchester is about 20/- I, which could lead to serious errors if they were to pass through the CFC and be counted as activated ice nuclei. However, the problem is reduced by sedimentation in the tube and dryers and is avoided completely by using an inertial impactor to remove particles larger than about 3 urn. The impactor was designed to give a 50,, reduction of 3.jlm particles at a flow rate of 3 i min- . The d:sign

Measurements of natural deposition and conden~tion-frying ice nuclei 1873

TOP VIEW IV-H MODE1

to :ump and R:):s

sample air Input

EN0 VIEW outlet orlftce

hinge. mp.

-3&m-

Fig. 1. Schematic diagram of the continuous flow chamber.

I natural air.

Fig. 2. Block diagram of the ice nucleus measuring equipment.

followed Marple and Willeke (Liu, 1976) in that the airstream is directed at a flat plate covered with silicone grease which retains the impacting large particles. Smaller aerosol are carried in the airstream and pass into the CFC. Tests with the optic& particle counter confirmed that no particles larger than 5 pm enter the CFC. The impactor outlet separates the air into two parts. One part is used directly as sample air in a normal run, or as filtered air in a control run to check the background count. The other portion forms part of the sheath air after rejoining the main, dried air stream. The sheath air is freed of particles by two serially connected 0.2-pm filters. The purpose of passing part of the sheath air through the impactor is to allow for

variation in the sample flow rate without varying the impactor flow rate, which is always kept at 3 i min- I. As required, the sample flow rate may be adjusted between 0.5 and 3cC mint while the corresponding total flow rate through the chamber may be adjusted between 3 and 2 1 I min - . At all flow rates, the sample air is sandwiched by the sheath air in about a I:6 ratio.

The air is drawn out of the chamber by two pumps, one of which draws theair through a Royce model 225 optical particle counter which required a flow rate of 3f min- * and was set to detect particles larger than 5 pm. The other pump was used to adjust the desired totaI flow rate. The manifold connecting the chamber to the particle counter and second pump was designed

111 such a way that both output Rows carrted represen- tative sampIts of the airflow. Thus tt was a simple matter to correct the Royce count in order to obtain the concentration of ice crystals grown u-t the chamber upon ice nuclei. Temperature control of the plates was maintained by two Neslab re-circulatory coolers with a stabiltty of = 0.03C. Five copper-constantan ther- mocouples were mounted on each plate to monitor the temperatures to within = 0.0X at distances of 10, 30 60. 90 and I IOcm from the chamber inlet. The supersaturation vvith respect to ice 1 cm from the edge of the chamber was within IO, of that at the centre; along the chamber the supersaturation remained steady to within 4, while at any particular point in the chamber tt was constant to within O.h,.

Tlrror) oj oprrarion

Supersaturation in the chamber was calculated on the assumption of a linear temperature and water vapour pressure gradient across the chamber from the warmer to the colder ice coated plates. The saturation vapour pressure at any point may be determined from the Clausius-Clapeyron equation and this is found to be lower than the actual vapour pressure. Hence the equilibrium supersaturation in the chamber may be calculated at all points between the plates as a function of the plate temperatures; this is shown in Fig. 3. The incoming air is cooled and comes to temperature and vapour equilibrium at a certain distance from the chamber entrance; it is important that the distance of non-equilibrium be known. Thus, solutions of the non- steady state heat conduction and water vapour trans- port equations are required in order to determine the properties of the transien: region. Carsiaw and Jaeger (1951) give the required solutions which provide values of time constant of 0.63 and 0.55s respectively, for thermal and vapour diffusion for this chamber. The exponential terms in the solution become insignificant when the time in the chamber exceeds four time constants. For a flow rate of 12Cmin-, vapour

(.

equtlibrtum occurs at 1Scm and thermal equilibrium at 16cm from the chamber entrance. Such a situation would produce an undesirable transient supersatur- ation before thermal equilibrium is achieved. This problem has been considered by Fitzgerald (1970), Saxena and Fowler (1973) and Hudson and Squires ( 1973) among others. The conditions that can lead to transient supersaturations are; turbulent air flow, humid incoming air, and sample temperature colder than the warm plate temperature. The flow was shown not to be turbulent having Reynolds numbers less than 190. The air sample and sheath air were dried by passage through diffusion dryers and as an extra precaution, the first 20cm of the warm plate were left free of ice. The incoming sample air was always warmer than the warm plate, hence there were no transient supersaturations present.

During passage through the CFC under conditions of water supersaturation, water droplets will grow on cloud condensation nuclei while ice crystals grow on ice nuclei. It is important that the droplets do not reach j-pm diameter or they will lead to an erroneous ice nucleus count. Fukuta and Walter (1970) produced a modified particle growth equation which took into account the processes of molecuar exchange at the particle-vapour interface. This equation has been used to calculate the rate of growth of a water droplet and an ice crystal using 0.1 s time steps assuming an initial radius of 0.1 pm for particular values of supersatur- ation and temperature. Figure 4 shows clearly the higher crystal growth rate and also shows that droplets will not grow to 5pm in the time available in the chamber. In fact, the particles may evaporate slightly when they leave the chamber to pass through the particle counter and this doubly ensures that droplets remain too small to be detected. A further experiment was conducted with the chamber near OC when it activated cloud condensation nuclei which grew into

4 :RM PLATE

c5-

z v T c O-6- P 2 __.-. ._._.-. : 2 O.L- 1 m z

-cz!. -20 -:6 -12 -8 -4 0 L 5 12 16 20 2L COLD PLATE

Fig. 3. Calculated temperature and supersaturation with respect to water and ice as a iunction of distance from the cold plate.

Measurements of natural dcpusiGon and condensarion-freezing ice nuclei 1875

water droplets. Even though the growth rate of the water droplets is higher at warmer temperatures, no droplets as large as 5 pm were detected.

During passage through the chamber, the growing ice crystals fall under gravity, their velocity being described by an equation derived by Pith (1972). The corresponding fall distances are shown in Fig. 5 as a function of growth time for the conditions of Fig. 4. It can be seen that the horizontal chamber should strictly be iimited to growth times of 8s or less for crystals initiated near the start of the ice zone. In fact, the data obtained over a range of flow rates show that crystals are initiated at various positions in the chamber.

~~ 0 I, 0 12 16 20

growth time lsec)

Fig. 4. Calculated droplet and crystal sizes as a function of growth time at - 16C + 17; S, and + IS, Si.

6-

The optical particle counter

A beam of light is scattered from each particle in an air sample, the amount scattered being detected by a photodetector. The amplitudes of the resulting voltage pulses are sorted in a multichannel anaiyser and the particle size dist~bution may be determined. Two counters were available for this work, a ROYCO 225 and a Climet 208. They were both calibrated with monodk- persed glycerin particles produced by a Berglund-Liu aerosol generator. The Royce was found to agree with its calibration curve above 5pm and to slightly over- estimate the size below 3pm, which agrees with Oeseburg ef al. (1979). The Climet seriously undersized particles above 3 pm which may be due to liquid drop break-up in the fine input nozzle. Ho and Bell (1981) have found that the Climet has significantly diflerent calibration curves for dry and liquid particles. They concluded that liquid droplets could be undersized by about 30:, by using the dry particle calibration curve. Similar results were obtained by Makyren et al. (1982). Comparisons between the acutal droplet concentra- tion and the value determined from the manufacturers calibration curves showed that the Royce was 80% efficient at 5pm, while the Chmet was only 15 / efficient at 5 pm, but reached 50% for 1.5-m particles. These calibration results were confirmed in side by side comparisons of natural aerosol concentrations and in use in the detection of activated ice nuclei in the CFC. Hence, the Royce was preferred for these studies where particles greater than Sprn in size were counted.

Conuection in the chamber

The problem of convection in continuous-flow chambers has been discussed by Sinnarwalla and Alofs (1973), Hudsonand Squires (1976)and Rogers (1982b). The flow in the horizontal mode is stable; however, in the vertical mode convection may cause several prob-

18 20 growth time lsec)

Fig. 5. Ice crystal fall distances as a function of growth time.

[ems: sample displacement from the centre line, tran- sient high supersaturation near the entrance and exit where the convection driven flow crosses from one wall to the other, ice crystal growth on the top of the cold plate and particle losses in the chamber. With a verttcal chamber, Hudson and Squires tested the importance of the sample being in the central position and found that some leeway was permissible. Sinnerwalla and Alofs and Rogers used chambers in which the sample air flow was vertical, which is the worst possible orientation as far as convection is concerned because the convective flow crosses the sample flow near the entrance and exit. They found that the sample was displaced 1 mm towards the cold plate. In our vertical chamber with horizontal air flow, the problem is less severe, nevertheless, the sample input jets were angled slightly towards the warm plate to compensate for the thermal force effect. Transient high supersaturations are only a problem in the sample input region. Again our vertical mode causes little problem here because the input jets start at 4cm from the walls so that any convection will only occur in the sheath air. However, with vertical air Bow, the problem is significant as shown by some subsidiary experiments with another chamber in this orientation, in which crystals grew on the cold plate from where they were easily dislodged and gave a high spurious crystal count seven times the count obtained with the usual vertical mode. Further tests were made in which the air was withdrawn from the end of the chamber through an aperture ljcm from the top edge of the chamber and then through an aperture 15 cm from the bottom; the concentrations detected agreed to within 1Oo.0 confirming that the sample withdrawal position and convection are not critical areas of concern.

After several hours of operation, ice crystals grow on the cold plate due to vapour diffusion from the warm to the cold plate and some of these crystals may be blown off to be counted as activated ice nuclei. To overcome this problem, the chamber was opened for re-polishing of the ice layers daily and after every 2--3 h of operation a thin electrically heated rod was moved between the plates to mcit the crystals. Between sample runs, dried, filtered air was drawn through the chamber and a value of the small background count was obtained. This value was used to correct the sample count and when it became unacceptably high, the crystals were removed or the ice plates were re- polished.

RESULTS

Early results showed that the ice nucleus concen- tration detected was a function of the flow rate through the chamber. Evidently, particles grow in the chamber and at low flow rates, they fall out before leaving the chamber. Conversely, particles may take some time to be activated and hence would not be observed at high flow rates. Therefore, experiments have been per-

formed to mvestigate the effect of residence time in the chamber which IS the time available for particles to grow as they pass from the start of the ice layer on the warmer plate to the end of the chamber. The product of residence time in s and the flow rate in i min- is 180.

In order to compare the behaviour of the new CFC with the previous UMIST model, thechamber was run in the horizontal mode; the results and conditions used are shown in Fig. 6. Every data set was obtained in a period of about 2 h and every experimental point is from a run of j-min duration. The initial increase of concentration with growth time is well defined and could indicate the activation of one particular kind of nucleus. The decay of concentration detected with longer times is more gradual, possibly due to a net balance between ice crystal losses and the activation of other types of ice nuclei. The peak values of concen- tration varied from day to day and were affected by the weather which was generally characterized by a pre- warm front with SW winds on 31 January, followed by a post-depression with high NW winds on 1 February and on 2 February, an anticyclone with W winds centred S of the U.K. This weather-dependent result is similar to reports by Schnell et al. (1980) who observed a decrease in nucleus concentration following a cold frontal passage and persistent low concentration measured during a period of large scale subsidence. Hussain and Saunders (1984) also found a low ice nucleus concentration at or behind the passage of a cold front, and higher concentrations at the passage of a warm front.

The new chamber was used horizontally and then vertically under similar chamber conditions on the same day in order to avoid air mass changes. Sets of experiments were carried out at mid-plane tempera- tures of about - 12, - 16 and - 2OC and with a range of values of supersaturation. The pairs of data sets displayed in Figs 7,s and 9 were typical results showing the different behaviour for the two chamber orien- tations. The data are plotted as a function of the maximum possible growth time. The 10 sets of graphs shown in these figures were obtained on 10 days, thus intercomparisons between them are complicated by air mass changes.

These figures show clearly that in the vertical mode more ice nuclei were detected than in the horizontal mode particularly at longer residence times, pre- sumably due to the reduced fall out losses. In most cases, both modes show a bi-modal response of the ice nucleus concentration to time, with a more marked behaviour in the vertical mode, particularly at the coldest temperature used.

The bi-modal nature of the results suggests that two distinct activation processes are taking place at two different growth times. The most rapid activation process is that ofdeposition nucleation and this is most likely to be associated with the first peak. At longer times, the activated deposition nuclei fall out during passage through the chamber and the longer time

Measurements of natural deposition and condensation-freezing ice nuclei 1877

SW=-0 lL%lj 1, -. x_

. . *i-_--L _____ ___--. -_

--.__ I I I I -v

10 20 30 Lo 50 60 t, (secsl

< 31 P-2-831 1 (Cl swPlo)

t. ~secsl --L 7

-1 t

Q ,

I z :I 12-2-831

3 I I

Fig. 6. The influence of growth time on nuclei activated in the continuous flow chamber in the horizontal mode on 3 consecutive days with changing weather conditions.

available permits condensation-freezing nuclei and possibly very small contact nuclei to become active. Of these two processes, that of condensation freezing seems the more likely because the contact process requires a long time for the growing droplets to capture a nucleus by Brownian diffusion unless the nucleus is exceptionally small. This point will be discussed later; for now, the assumption of condensa- tion freezing rather than contact nucleation will be made.

Further investigations have been made of the effects of temperature, supersaturation and exposure time on nucleus activation in the chamber. Hussain and Saunders (1984) noted that atmospheric ice nuclei experience minute to minute variability of about 20 % of the mean activated nucleus concentration. These variations have been smoothed by averaging data from a number of runs under identical conditions. The data were collected in the summer under stable conditions in order to minimize air mass variability. The chamber was run in the vertical mode at three values of mid- plane temperature in turn. The flow rates were ad- justed to cover values appropriate to deposition nucle- ation and then condensation-freezing nucleation. Table 1 contains the results of several experiments, of IO-min duration. The same results are plotted in various ways in Figs 10, 11 and 12.

Figure 10 shows that both nucleation modes ex- perience an increased activation rate at lower tempera- tures and higher supersaturations with a more pro- nounced effect for thecondensation-freezing nuclei. At - 20C the deposition nuclei increase by a factor of 3 for a 5 % change in water saturation, while the corresponding condensation-freezing nuclei increase

by a factor of 6. Figures 10 and 11 show that as water saturation is reached, the rate of activation of de- position nuclei decreases indicating that most de- position nuclei are activated below water saturation. Figure 11 leads to the approximate result that the rate of increase of condensation-freezing nucleus activity is about a factor of seven per 1OC cooling which is about three times the corresponding deposition nucleus increase. Figure 12 shows how the condensation- freezing nuclei became more active as nominal water saturation is approached. The large activity difference between the two types of nucleus at - 20C indicates the relative importance of condensation-freezing nuclei at colder temperatures in the atmosphere.

DISCUSSION

Ice crystals in clouds may be initiated by one of three distinct processes; namely, the deposition of water vapour directly on to an insoluble nucleus, contact nucleation whereby a supercooled droplet captures a freezing nucleus on its surface, and immersion freezing whereby a nucleus in supercooled water becomes active. A particular example of the immersion freezing process is that of condensation-freezing when a newly activated cloud condensation nucleus is frozen by an insoluble ice nucleating portion of the nucleus. Ice nucleation occurs on active sites on an ice nucleus surface (Roberts and Hallett, 1968; FIetcher, 1969). Such sites could exist at growth steps, dislocations, cracks and at the edges of the nucleating particle. For a deposition ice nucleus having a hydrophobic, low energy surface, water molecules may join a disordered

t- !secs)

Fig. 7. Comparison of detected nucleus concen- trations for the horizontal (dashed lines) and vertical modes of the continuous flow chamber

(T- - 12C).

water cluster on an active site. Such a cluster can more readily achieve an ice-like structure than an oriented film of molecules which appear on hydrophilic sur- faces. The role of active sites in the case of condensation-freezing and contact nucleation is stiI1 unresolved.

The possibility must be considered that extremely smah contact nuciei may be captured by and then freeze supercooled droplets in the WC. However, calculations of the required nucleus size indicate that they would have to be smaller than 6 x 10 -4 $m which is smatler than any reported active ice nuclei. The calculation of the nucleus size is based on the therory of the capture by droplets of small aerosol particles by Brownian diffusion. This process is most efficient for the smallest particles. The classical theory was used by Sax and Goldsmith (1972) to determine the contact nucleating ability of silver iodide particles through which supercooled droplets passed. For the small particles used, other diffusive effects could be ignored and the rate of depletion by Brownian diffusion of a

0 10 20 30 LO 50 60 t- lsecsl

Fig. 8. Comparison of detected nucleus con- centrations for the horizontal (dashed lines) and vertical modes of the continuous flow

chamber (T - - 16C).

concentration of nuclei, nP was given by

-dnP = 4nr,n,n,Dpdt (1)

where rc and ne are the droplet radii and concentration while DP is the diffusion coefficient for the contact nuclei. If we assume that during passage through the CFC, all the contact nuclei are captured then dn, = nr,. Measurements of droplet concentration in the CFC indicated that IOcm- were larger than 0.5 pm while the droplet growth equation shows the maximum radius that droplets attain is 2pm under the highest supersaturations used, so t, was taken as 2~m. For a passage time of 30s. the required diffusion coefficient from Equation (I) is about 1 cm* s-. D, is given by (Mason, 1957)

RT(l19 x 106/rr,) N,6n~rp

Measurements of natural deposition and condensation-freezing ice nuclei 1879

b-

t

\ \ -zo3c 1 \ -027%

\ \

O-- 60 tm iSKs.1 tn ISKS)

Fig. 9. Comparison of detected nuclei concentrations for the horizontal (dashed lines) and vertical modes of the continuous flow chamber (T - - ZOC).

where R is the universal gas constant, T the absolute temperature, N,, Avagadros number, fc the air vis- cosity and rp the contact nucleus radius. Hence, for D, = 1, rp= 10b4pm. Cooper and Vali (1981) pointed out that the above theory applies to steady state conditions when the region around a droplet has already been depleted of contact nuclei and a particle concentration gradient has been set-up. This situation is not relevant to the present work where it is the first capture of a contact nucleus that is assumed to initiate freezing. Cooper and Vali provided a theory based on the treatment of contact nuclei as a molecular species and derived an expression for a characteristic time for nucleus collection. From this, a value of the nucleus radius is calculable:

r; = 1.5k7(2r;n,t)

where k is Boltzmanns constant. For the same values as above, r,, = 3 x 10e4m.

The determination of the size or the size spectrum of ice nuclei is still not fully realised. Allee et al. (1968) using a Goetz spectrometer technique, found that most of the ice nuclei in suburban air were larger than l.Opm, a result supported by Prodi et al. (1982). According to Byers (1965), 15 y0 of ice nuclei are larger than l.Opm, 18 %are in the range0.7-l.Opm, 38%are in the range 0.4-0.7~m and 29 y0 are in the range 0.1-0.4gm. Georgii and Kleinjung (1963) found a lower limit of about O.O2pm, while Vali (1966, 1968) has reported that active nuclei remained in water samples after passage through O.Ol-pm pore-sized filters. However, Fletcher (1958) showed that the nucleation ability of ice nuclei of radii larger than 0.1 pm is independent of size, while for radii less than about 0.01 pm their ability is highly reduced. Thus the theoretical requirement in the present experiments

that active contact nuclei must be smaller than about 10e3 pm probably precludes them from our consider- ations. Therefore it is assumed that the CFC is responding to deposition and condensation-freezing nuclei.

Crystals and water droplets growing in the CFC will deplete the available water vapour supply and will release latent heat. However, for 20 ice crystals i- of radius 10pm at a mid-plane temperature of - 16C the reduction in the saturation ratio over ice is negligible. For water droplets of l-pm radius in a concentration of lOOr~rn_~, the water vapour is de- pleted by only 0.02 yO, hence there are no problems due to vapour depletion in the CFC.

CONCLUSION

The use of the continuous flow chamber in these experiments has shown clearly that more nuclei may be detected by running the chamber vertically rather than horizontally. In both orientations, crystals grow and fall out during passage through the chamber, but this problem is reduced considerably in the vertical mode. In fact, by varying the flow rate, advantage has been made of the fall-out effect in that two distinct modes of nucleation may be discerned. The time taken by an incoming sample to reach supersaturation with respect to water is of the order of 1 s longer than the time to reach supersaturation with respect to ice, therefore there is a delay in the activation of water droplets relative to ice. Also, because of the equilibrium vapour pressure difference between water and ice, the higher supersaturation with respect to ice leads to a faster growth of ice particles. Thus, there is a strong indi- cation that the bi-modal nature of the detected ice

R. AL-NAIW and C. P. R. SAI_&DERS

:: !_: -4 -3 -2 -1 0 *l

SW i%)

_- 3 es I b. DEPOSITION NUCLEI. f

Im -ll -3 -2 -1 0 '1

SW (%I

Fig. 10. The condensation-freezing and deposition nuc- lei concentrations as functions of temperature and

supersaturation with respect to water.

nucleus concentration, as a function of growth time in the chamber, is due to the activation of two types of ice nuclei. Short growth times permit deposition nuclei to be activated early in the chamber, grow relatively quickly, and reach the end of the chamber before falling out. At lower flow speeds and in conditions above water saturation, there is time for the condensation-freezing nuclei to first grow on the cloud condensation nucleus portion of a mixed nucleus and then to be nucleated by the insoluble ice nucleus part.

Figure 10 indicates the increase of ice nucieus concentration for both types of nucIei. The marked increase shown in part a of the figure is due to the increase in activity of conden~tion-frying nuciei as water saturation is approached. Evidently, the mixed composition nuclei have a reduced equilibrium vapour pressure due to the soluble part of the nucleus. In Fig. 11, the large increase in condensation-freezing nuclei activity as the temperature is reduced indicates their importance in clouds and in particular bears out the hypothesis of Cooper and Vali that these nuclei may be active in cold, wintertime clouds. The technique of activating the two types of nucleus separately under the same supersaturation conditions with one piece of apparatus is a considerable step forwad. Previously, a specific device was needed for a specific activation

Measurements of natural deposition and condensation-freezing ice nuclei 1881

L sww a CONOENSATiON iREZING NUGLEI

I

-20 -16 -12 T :?I

b. OEWSITlON NUCLEI

-- 4 -- 8- z

6-

SW(%i

:- --a -I-

-20 -16 -12

Fig. 11. The condensation-freezing and deposition nuc- lei concentration as functions of temperature and super-

saturation with respect to water.

mode, or by using the same device under different supersaturation conditions, above and below water saturation, the two modes could be separately identi- fied. In clouds, both activation processes will take place under appropriate conditions but the total ice nuclei activity can be obtained from experiments such as these by the addition of the two separate deposition and condensation-freezing nuclei concentrations. Thus Fig. 11 reveals total concentrations of five ice nuclei f- i at -lZC,seven!-at -16Cand12I- at - 2OC for conditions of water saturation. These concentrations are of the same order as observed crystal concentrations in some types of clouds.

Acknowledgement-This work was supported by the Natural Environment Research Council.

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_- =

swwol

4 L- ., 3 I a. -I z

2- -2 -

& o- -____ _____ ________ -----

-2v I -12 T IYI

I ------. / 0 _ _ _ _ _ _ _ _ _ _ _ ______ --_--_-

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-L -3 -2 -1 0 1 SW 1%)

-20C

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