Ice-forming nuclei of maritime origin

J Aero,Tol S¢i, Vol 17, No l, pp 23-46, 1986 Printed m Great Bntam

ICE-FORMING

0021-8502/86 $3 O0 +0 O0 © 1986 Pergamon P r m L~I

NUCLEI OF MARITIME

ORIGIN

J. ROSINSKI a n d P . L. HAAGENSON P.O. Box 3000, National Center for Atmospheric Research*, Boulder, CO 80307, U S.A.

and C . T . NAGAMOTO a n d F . PARUNGO National Oceamc and Atmospheric AdmmlstraUon, Boulder, CO 80303, U S,A.

(Fwst received 24 April 1985, and in final form 19 July 1985) AMtract--Aerosol particles collected over the Puctfic Ocean between 14 February and 7 May 1984 were examined for their ability to nucleate ice by freezing, sorption, and condensation-followed-byfreezing Aerosol particles nucleating ice by freezing m the temperature range from - 7 3° to - 11 1°C along the western coast o f North America were m the size range from 6 to 8/~m m diameter. A very low concentratson of particles smaller than 0.5 lan nucleating ice at - 12°C was present over the South Pacific No ice-forrmng nuclei, active by sorptmn in the temperature range between - 5 and - 17°C, were found. Concentrations o f IFN active by condensation-followed-by-freezing were 100 m - 3 at -3.3°C and 3 x 104 m -3 (30 I . - l ) at and below - 4 0 ° C . These concentrations were found to be independent of temperature for the temperature range from - 4 ° to -14°C. The fraction of the aerosol population nucleating ice below --4°C was ~ 10-3 Aerosol paxtlcles supplying IFN were below 0.5/~m in diameter and seemed to consist mostly of orgamc matter. They were present only over the South Equatorial Current and were associated with biological actavity in that current. The formation of frozen droplets by condensatlon-followed-by-free~ngm clouds is time-dependent and consequently depends on the evoluuon of clouds.

1. I N T R O D U C T I O N

Aerosol particles supply cloud condensation nuclei (CCN) (Courier, 1875; Aitken, 1880) and ice-forming nuclei (IFN) (Wegener, 1911) in the atmosphere. The first class of particles, the CCN, initiate a phase transition from water vapor to liquid water under natural atmospheric conditions. The CCN are transferred into the cloud droplets they form; once formed, the droplets continue to grow by condensation during the initial stage of a developing cloud. In a maritime air mass, the concentration of CCN (and of the cloud droplets they produce) is about 50 cm- 3; the total concentration of aerosol particles in the size range which can supply CCN is around five to ten times higher. The population of atmospheric aerosol particles which can act as IFN, on the other hand, cannot be as simply determined and compared with the background aerosol concentration. The ratio of IFN to the total number of aerosol particles larger than 0.1/~m diameter is 1 : 106 or smaller, based on measurements performed in different parts of the world. (A minimum diameter of 0.1 ~m for IFN was determined from experiments in which naturally occurring IFN were separated from the ice crystals they formed [Rosinski et al., 1980].) But the mode of ice nucleation by aerosol particles must be known in order to evaluate their contribution to the formaUon of ice in natural clouds. Aerosol particles can collide with cloud drops and initiate a liquid (supercooled water)-to-solid (ice) phase transition. Ice can form at the time of collision or after the captured particles have resided on the surface of a drop for a while. This mode is called nucleation of ice by contact. It is size- and temperature-dependent; the former determines the dynamics of capture and the latter determines conditions of ice nucleation itself. If an aerosol particle is captured by a drop and transferred into its interior, becoming a hydrosol particle, then it can nucleate ice by freezing. A particle active as a freezing nucleus can also be transferred rote a drop if it is a mixed aerosol particle, that is, if it consists of a water-soluble part (CCN) and an insoluble particle (IFN). This process can be called

* The National Center for Atmospheric Research ~s funded by the National Science Foundation 23

24

J ROSINSKI et al

condensation-followed-by-freezing. A third mode of ice nucleation, which is basically independent of particle size, Is called nucleation by sorptlon. There are aerosol particles whose surfaces are composed of chemical compounds which can adsorb water vapor molecules at some specific sites and produce directly a gas (water vapor)-to-solid (ice) phase transition. The background concentration of natural IFN can be defined as the concentration of IFN determined as a function of temperature alone (freezing mode), or as a function of temperature and supersaturation (sorptlon mode) (Rosinski and Lecmski, 1983) at some location where there are no foreign sources of aerosol particles. The size distribution of the background aerosol particles is automatically defined by the dispersing mechanisms operating in that area. Every measurement of an IFN concentration should be accompamed by the construction of an air parcel trajectory for that location. Only then will it be possible to draw meaningful conclusions about changes in concentrations and sources of IFN Such trajectories should be investigated----even over the vast open spaces of an ocean--to be sure that the aerosol part,cles which have nucleated ice were of local origin. We used lsentropic trajectory analysis (constant potential temperature, 0) to determine the transport history of the sampled air The isentroplc transport model used m this study is described by Haagenson and Shap,ro (1979). It has been used successfully m boundary-layer transport application (Clark et al., 1983, Ferek et al., 1983; Lazrus et al., 1983). Application of the model involving transport m the southern Hemisphere and in equatorial regions is discussed by Crutzen et al. (1985) Concentrations of IFN present in different maritime air masses have been found by many observers to be very low (see references in Pruppacher and Klett, 1978, p. 253; Fall and Schnell, 1985). In spite of this, an experimental program was set up to determme concentrations of different-sized aerosol particles capable of acting as IFN over the surface of the Pacific Ocean. Ice nucleation by sorptlon, freezing, and condensation-followed-byfreezing were investigated in order to estabhsh meaningful ratios of IFN concentrations nucleating ice by different modes to different-sized aerosol populations Only some of the phys~co-chemical properties of marine aerosols (Podzlmek, 1982) were measured m this program. Aerosol particles were collected during the "Marine Sinks of Atmospheric CO2 and Marine Sources of Acid Rain Precursors" cruise aboard the National Oceanic and Atmospheric Admimstratlon's research ship Discoverer The principal goal of this cruise was to conduct hydrographic and chemical observations in the central and western South Pacific. The period covered by the crmse was from 14 February to 7 May 1984. 2 EXPERIMENTAL SECTION Aerosol particles were collected simultaneously by two sampling systems (Fig 1). The first (Fig. la) consisted of three filter holders connected to a pump. The second (Fig. lb) was an Andersen sampler (Andersen, 1958) with two backup filter holders. The filters used in both samplers were 47 #m-diameter membrane filters with a nominal pore diameter of 0.22/~m (Milhpore Corp., Bedford, MA 01730). The samphng area of the filters used in the filter holders (A, B and C in Fig la, 7 and 8 in Fig lb) was 9 6 cm 2 The sampling area of the filters mounted m the Andersen sampler (1-6 in Fig lb) was 17 3 cm 2 The number of holes (jets impinging on a substrate) per each stage (7 6 cm diameter) was 400 and per filter was 149 There was a "dead" area m the center of each Andersen sampling plate equal to 0.8 cm 2. The area outside a Millipore filter mounted on the Andersen plate was coated with silicon grease to minimize travel to the next stage of any particles which might be dislodged after impinging on the metal plate. Filters were attached to plates at four spots on the outer edge with an ordinary lacquer. The flow was adjusted to 5 1 mln- 1 per filter Aerosol particles deposited in the Andersen sampler were therefore deposited on each plate from a flow of 101ram-1_ Seventy-two samples were taken during the ship's cruise, 12 were designed specifically for detection o f l F N and only the results obtained from these samples will be discussed in detail in the text. Some other measurements will be used when necessary to support the conclusions

Ice-fornnng nuclei of maritime ongm

~,

(a)

~, /Fdter CI / / L o c k l n q Washer ~ / M e t a , Screen I ~

i

(b) I I

25

Pump.~

d,F,m

)

j

2

>106 6.2-106

>S 6-8

3

40-62

5-7

4

2 3-40

4-6

5

09-23

I -S

6

0,3-09 05-15

Pump

©

Fig. 1. Schemauc dmgram o f the sampling system. (a)Milhpore filter holders, and (b) Andersen sampler, separation of different sized aerosol particles at two an'flows: i = 28 and j = 101rain - I

The ~ce nucleation temperature spectra of aerosol particles active as freezing nuclei were produced by means of a modified membrane filter/drop freezing technique (Schnell, 19?9). Membrane filters were impregnated from below with 0.014 cm 3 of Vaseline per cm" of filter area ( ~ 125 #m thick layer). Water drops 1.0 mm in diameter were placed on the pattern of spots present on the filters from the Andersen impactor. The temperature of the water drops was + 1°C and that o f the filter surface was - 4°C. Drops were melted and refrozen in some of the experiments. In cases where refreezing was performed more than twice, additional water was added to each drop to replemsh losses due to evaporation. The water used in all experiments was "Water, Baker Analyzed H P C L reagent." A dynamic developing chamber invented by Gerhard Langer (Langer and Rodgers, 1975) was used to detect, and to determine concentrations of, IFN active by sorption and by condensation-followed-by-freezing. In experiments to detect nucleation by sorption, filters were exposed for 15 rain to water vapor at supersaturation with respect to saturation over ice but below saturation over liqmd water. In investigations of IFN active by the condensationfollowed-by-freezing mode, filters were exposed at 1 ~o and 2 ~ supersaturation with respect to saturation over supercooled liquid water (Sw) for up to 30 s and then kept at 0.5 or 1 supersaturation for different periods of time (up to 120 rnin). Many experiments were performed at S w ~ 6 ~o; it was found that the temperature of ice nucleation was independent of the rate of growth of droplets, and, at this supersaturation, the first ice crystals appeared in minutes (usually between 3 and 15) and not m tens or hundreds of minutes, thus considerably shortening the time of the experiments. During the collecUon of aerosol particles on membrane filters for the nucleation studies, Nuclepore filters were also set up to collect them for determination of their size distributions and elemental chemical compositions. As a further venficauon of the sizes of the collected aerosol particles, approximately 1 cm 2 of Nuclepore filter was mounted outside each membrane filter used in the Andersen sampler. The area of each Nuclepore filter was then scanned using an energy-dispersive spectrometer interfaced with a scanning electron microscope. 3. R E S U L T S AND S P E C I F I C D I S C U S S I O N S The transfer of an aerosol particle into the liquid or solid phase of a cloud depends on the stze of the aerosol parucle and of the cloud parUcles; this does not apply to particles nucleating ice or water by sorption where aerosol particles are directly transferred Into cloud particles through nucleation. The Andersen ]mpactor was used to separate aerosol particles

26

J

ROSINSKI

et al

by size so that one could have a better understanding of the formation of ice particles in clouds. a. Ice nucleation through freezin 9

The temperature spectra from the freezing of 20 water drops (labeled a through to t) placed on the pattern of jets impinging on the Mlllipore filter are given in Tables 1-12, corresponding to samples l-12 Water drops on filters 7, 8, A, B and C were placed at random. Aerosol particles collected in the vicinity of the American west coast (Tables 1 and 2, line 2, columns a-t) contained particles in the 6-8/~m diameter size range which nucleated Ice in the - 7 3 °- - 11.1 °C temperature range. Melting of the drops destroyed the Ice-nucleating ability of the IFN. Perhaps some of the gaseous components evaporated during the first freezing, leaving drops deficient in the ice-nucleating complex. In any case, the freezing drop technique applied to liqmd precipitation would be unable to detect these I F N if they produced ice during the evolution of rain. Temperatures of freezing of the melted drops were lowered by about 10°C All of these particles appeared to be aggregates of smaller particles. Melting and refreezmg was used to find out if drops were freezing at random, or If there were ice-forming nuclei that nucleated ice at specific temperatures. Examples of drops freezing at the same temperatures are k-t, hne 5 and a-t, line 6, both in Table 1. Another example of the presence of freezing nuclei (aerosol particles in the 5-7 tam diameter size range) is the case a-t, line 3, in Table 2, where all drops froze between - 15.2 ° and - 15.3°C. Experiments with melting and refreezmg of drops were terminated because drops in general were freezing at much lower temperatures than expected and the temperatures were approaching the background temperature range of - 18.2 °- - 21.4°C (average of 200 drops). It was still possible to see the presence of I F N from a single freezing experiment in which all the drops froze at one temperature or in the narrow temperature range (Table 3, line 6, and Table 5, line 6, for aerosol particles with 0.5-1.5/~m diameter size range). Water drops were placed on spots (filters 1-6) where aerosol particles were collected. Two such spots are shown in Fig. 2. Most of the particles were found to be, or to contain, sodium chloride The mass of particles In Table 6, line 6 is about 5 x 10-1o g; the molal solution of sodium chloride for a water drop of 1 m m diameter is of the order of 10 - s tool 1-~. This concentration of dissolved salt is not sufficient to alter the freezing point of drops. But the drops usually supercooled below the lowest freezing temperature of the background. In addition, the data show that lowering of the freezing temperature took place to a much higher degree in the Southern Hemisphere (Tables 4-12, columns r, s and t). There must be, therefore, some chemical compounds present in the aerosol particle population which suppress freezing of water. At the same time, a few, very effective I F N must be present to cause freezing of some drops at relatively warm temperatures. In the South Pacific, examples are: Table 4, line 7, a, b and c ( - 1 1 9, - 1 2 . 0 and - 13.8°C), Table 5, line 7, a - t ( - 1 1 . 9 , - 14.2°C), Table 9, line 7, a ( - 12.6°C) and Table 12, hne 7, a ( - 13.9°C). There Is a similarity in these data; all I F N nucleating ice at warmer temperatures in spite of the presence of chemical compounds suppressing freezing are in the particle diameter stze range below 0.5/zm. These particles are m every water drop placed on the filter; therefore, why do they not nucleate ice; Maybe, in the presence of the "infinite" dilution, there is simply a destruction of ice-nucleating sites or of the entire nuclei. This could be visuahzed as desorptlon of some ions from the ice-nucleating surface. If this is the case, then the chemical composition of condensation nuclei (mixed particles) plays a very important role m the freezing of drops in clouds over the Pacific Ocean b Ice nucleatmn by sorption

All samples were exposed to water vapor supersaturation with respect to water vapor saturation over ice, but below water vapor saturation over liquid supercooled water. The temperature range was between - 5 ° and - 17°C No ice crystals were detected on filters and consequently I F N active by sorption were absent

Fig. 2 Aerosol particles collected on stages 5 and 6 of the Andersen sampler (airflow = 101 min- t).

g::t

,-i

O

_o

28

J ROSINSKIet al

Fig. 4 Examples o f water droplets and ice parU¢les formed on filters: (a) appearance of the first ice crystal; (b-l)condensation pattern on filter from Andersen sampler; (b-2)condensation pattern and tee crystals, (c-l)droplets formed on condemmtxon nuclei, and (c-2)all droplets are frozen at T -- - 6.1 °C; (d) ice crystals growing by using water vapor from evaporating droplets (T = - 17.7°C), and (e) large ice crystals showing positions of water-insoluble partmles transferred from another filter.

17.4

178

15.3 18.8

18 1 20.5

16.5 187

20.5 206

18.1

17.9

7.8

7.6

182

3

4

5

6

7

8

A

B

b8d

18.3

18.1

18.3

20.6 207

16.6 17.8

19.9 20.0

15 5 201

7.5 18.3 17.4

7.3 165 17.1

2

7.7 156 17.2

142 16.7 19.0 17.1

d

f 143 149 19.3 18.7

g

79 7.9 80 1 4 . 9 172 176 18.1 1 6 . 9 17.5

14.2 14.2 16.1 155 155 14.8 15.2 14 7

e

20.0 190

14.4 18.4 16.5 18.3

j 14.4 176 16.7 16.6

k 14.4 17.8 17.7 18.9

1 14.5 20.7 189 19.4

m

20.0 20.2 20.2 20.3 1 8 . 2 1 8 . 4 2 2 . 1 21.4

20.3 20.5

17 7 19.1 20.4 204 1 9 . 4 20.5

1 7 . 9 18.0 1 8 . 5 206

209 20.7

188

189

21.5 21.6

216 218

217 21.7

1 9 . 6 200

20.1

1 9 . 2 20.0 20.2

20.1

19.6 21.2

20.5 21.3

20.6

1 9 . 6 197

1 9 . 1 1 9 . 4 1 9 . 5 1 9 . 6 1 9 . 8 1 9 . 9 20.0

19.5

191

21.5 217

1 9 . 6 1 9 . 6 1 9 . 7 1 9 . 7 197

21.3 21.3

1 8 . 8 1 9 . 4 1 9 . 4 19.6

1 8 . 8 19.0

18 7

1 8 . 9 19.1

175

18.4

194

21.0 21.2 2 1 . 3 21.3

1 8 . 9 1 9 . 3 194

20.7 207

14.6 161 16.3 16.8

o 14.7 172 16.9 16.4

p 14.7 16.1 17.9 16.5

q 15 1 21.1 17.6 19.5

r 15.4 18.2 170 17.2

s

15.7 17.4 19.5 19.1

t

88 89 9.1 9.3 9.9 9.9 10.8 156 1 8 . 5 1 5 . 5 1 6 . 4 1 7 . 3 173 17.6 1 7 . 6 17.5 17.1 1 8 . 2 1 6 . 3 1 6 . 9 176

14.5 17.6 16.7 16.6

n

198

217 21.9

21.9 22.0

21.4 2 0 . 1 20.5

21.3

2 1 . 1 21.4

20.8

21.6

21.5

20.9

21.6

216

21.9

21.9 2 1 . 1 21.3

21.8

21.6

21.3

22.0

21.9

21.3

22.1

22.0

21.4

221

22.1

20.0

20.0

22.0 22.0 2 2 . 1 2 2 . 1 22.1 2 2 . 1 2 2 . 1 2 2 . 1 22.2 22.2 1 9 . 8 1 9 . 8 1 9 . 8 1 9 . 8 1 9 . 8 20.0

21.9 21.9

18.2 19.3 21 1 20.7 1 8 . 2 1 9 . 3 2 1 . 1 20.7

1 9 . 7 1 9 . 8 1 9 . 8 1 9 . 8 1 9 . 8 1 9 . 8 1 9 . 9 20.0

198

21.7 21.9

21 2 21.2

20.7 20.7 20.8 20.8 20.9 20.9 2 2 . 1 1 8 . 9 2 0 . 1 1 9 . 2 2 1 . 1 21.1

192 20.8 1 9 . 2 20.8

20.5 20.6 20.6 1 9 . 4 1 9 . 9 21.7

1 8 . 2 1 8 . 3 1 8 . 5 1 8 . 8 1 8 . 9 1 9 . 1 1 9 . 2 1 9 . 9 20.4 187 1 8 . 7 210 1 8 . 3 1 8 . 5 1 7 . 4 1 8 . 6 185 18.8

1 6 . 8 1 6 . 8 1 7 . 2 1 7 . 2 1 7 . 3 1 7 . 6 1 7 . 7 1 7 . 8 1 8 . 3 1 7 . 9 20.2 2 0 . 1 1 9 . 6 173 1 7 . 8 1 6 . 9 20.2 1 9 . 2 1 8 . 2 183 179 202

20.0 180

14.3 20.1 18.6 19.2

i

8.1 8.1 8.2 8.2 8.2 8.4 1 6 . 5 1 7 . 3 1 7 . 2 1 8 . 4 1 7 . 6 177 17.1 16.9 17 2 1 7 . 9 1 6 . 7 19.0

14.3 154 19.4 14.3

h

1 5 . 7 1 5 . 9 1 7 . 3 1 7 . 3 1 7 . 4 17.6 1 7 . 3 1 8 . 8 184 174 1 8 . 5 18.6

7.6 172 16 3

14.1 15.8 163 15.6

14.0 18.3 171 196

13 8 14.3 145 13.8

1

c

b

a

Fdter

FreeTang temperature of drops, - T(°C)

Table 1 Results from experiments with freezing of drops with subsequent meltm8 and refreevang. PosltlOn of the slup. 37°48'N, 124°59'W. Local time: 0855-1130 h, 17 February 1984 (bgd = background)

,-i

o

o

Fdter

b

202

7.4 20.2

176 15.3

205 232

21,1 230

15.6 156 156

214 215

161

a

20.2

74 13 5

17,6 152

20,5 22.9

184 203

153 20,1 217

214 246

155

167

217 223

173 186 201

212 24.5

205 19.1

176 153

74 14 6

20.3

c

227 182

206 22.6

176 15,2

8.8 17.9

206

f

22.4 239 210

228 239

178 181 1 8 . 3 241 241 237

215 215

20.6 22.9

17.6 152

8.6 23.4

20.5

e

1 8 , 9 200

221 241

174 153 199

212 21.5

206 227

176 15,2

8.3 23.8

205

d

210

234 24,4

18,7 188 22.1

22.9 195

206 238

17.6 15,3

8.9 14 6

207

g

21.1

239 247

189 21 l 196

230 215

206 22.4

17.6 152

95 20 1

21.9

h

21.1

240 251

191 220 207

231 20,7

207 238

176 15,2

96 20.8

220

i

214

24.1 248

192 19.3 192

23.2 230

20.7 229

17.6 152

96 21 0

22.0

j

215

241 239

195 247 251

232 241

208 229

176 15.3

9.9 16.5

220

k 223

n

221

242 226

22.4

o

235 23.6

20.8 189

17.6 152

236 197

227

24.2 24A

197 17,6 231

238 221

239

2 4 , 3 244 244 248 231

109 22 7

224

p

239 251

20.9 232

177 152

11.0 22.0

22.7

q

242

244 218

243

24.4 25.2

1 9 . 9 20.4 198 221 199 23.1

23.9 183

209 242

177 177 15~3 152 2 0 . 8 209 22.7 2 3 . 1

17,7 152

1 0 . 2 1 0 . 2 104 22 1 23 5 22.8

22.2

m

1 9 , 6 1 9 , 6 196 21 2 21 2 22.0 2 2 . 3 2Z7 218

23.5 196

208 232

176 152

102 23 5

22.2

I

Freezing temperature of drops, - T(°C)

24.3

244 238

208 173 221

24,0 221

209 22.7

177 15,2

11 0 24.1

22,7

r

25.1

24.4 248

216 22.1 207

242 20.1

21,0 229

18.2 15.2

11.1 16 7

227

s

252

24.4 23.2

21,9 204 20.1

24.3 19.3

210 219

18.2 152

II 1 19 5

234

t

Table 2 Results from expenments with freez3ng of drops with subsequent melting and refreezmg. Posmon of the ship' 36°52'N, 125°26'W. Local time 1430-1630h, 17 February 1984

7

192

203

16 3

188

137 22 5

5

6

7

A

178 230

22.4

17 1

205

193

211

190 189

212

c

222

192 211

217

e

179 24 1

225

17 1

211

212 249

238

17 1

211

1 9 , 5 214

212

191 207

214

d

212 219

239

17 1

211

215

225

193 19,1

237

f

21,4 23 2

240

17 1

211

217

227

222 218

239

g

21,5 216

240

17 1

211

217

23,0

222 228

241

h

j

217 24 8

241

17 1

211

219

238

22,3 217

219 219

241

17 1

211

220

240

227 221

24_1 243

1

219 234

242

17 1

211

233

240

236 208

243

k

220 23 7

242

17 1

211

239

240

240 220

243

I

221 23 2

243

17 1

211

240

240

242 237

24,3

m

221 24 2

245

17 1

211

245

240

24,5 242

243

n

225 19 7

245

17 1

211

245

240

24,5 219

243

o

225 226

24,6

17 1

212

245

240

245 251

24,3

p

226 25 1

24,8

17 1

213

245

240

245 248

243

q

222 21,7

248

17 1

213

245

240

245 241

243

r

228 23,9

249

17 1

213

245

240

245 20,8

243

s

238 23 1

249

17 1

226

245

240

245 241

243

t

199

21 2

186

3

4

193

141

13,2

A

C

120

141

119

13,2

7

193

141

185

13,7

5

6

21 6

211

187

211

l

b

a

2

Filter

141

141

138

141

195

211

22,0

218

204

c

172

181

197

143

196

21,8

22 1

22.2

205

d

178

199

207

143

199

219

22 1

223

21.4

e

184

201

219

211

208

220

22,3

223

217

f

190

203

22,1

212

210

222

224

223

22.1

g

197

20,8

224

212

211

230

224

224

224

h

211

210

226

223

222

23,1

226

226

226

1

211

211

232

223

222

240

229

226

226

j

21,2

21.5

233

22,4

227

241

23 1

226

22.7

k

217

217

233

22,5

23,3

248

242

228

229

1

220

21.7

233

227

23,3

249

24 3

229

22,9

m

Freezing temperature of drops, - T(°C)

220

228

234

227

234

25,1

24 3

231

229

n

24,6

236

23,3

p

228

230

235

228

236

242

24,0

23,7

229

23,7

2 5 , 3 256

24,4

234

232

o

243

247

242

229

23,7

25,8

24 7

23.8

23.9

q

245

247

247

229

241

26,0

25 1

240

241

r

258

25,1

247

260

257

25,2

244 244

242

263

25 2

25,3

252

t

242

261

25 1

245

247

s

Table 4 Results from experiments with freezing of drops Position of the ship. 3°30'N, 150000` W. Local time 1100-1300, 1340-1500h, 3 March 1984

147 14 7

188

17 1

203

210

191

179 20,4

199

210

199

149 215

3

b

a

4

Filter

Freezing temperature of drops, - T(~C)

Table 3 Results from experiments with freezing of drops with subsequent melting and refreezang Position of the ship 34c52'N, 121026 ' W Local time 1300-1500, 18 February 1984

B

t~

O~

221

191

169

119 225

186

4

5

6

7

8

190

119 119

171

197

222

227

191

119 119

171

201

222

229

229

232

c

192

121 171

172

201

230

229

232

233

d

196

125 211

172

202

231

230

232

233

e

203

136 154

172

202

235

232

233

233

f

204

136 164

172

215

236

232

233

233

g

205

136 156

172

215

236

233

233

233

h

209

137 161

172

216

238

234

234

234

1

210

139 168

172

216

238

235

234

234

j

212

139 172

173

216

239

235

235

235

k

212

139 197

175

216

240

237

236

235

1

213

139 201

175

227

240

239

237

236

m

F r e e i n g temperature of drops, - T (°C)

213

139 189

175

228

241

240

239

236

n

214

139 195

176

230

242

240

239

238

o

214

140 204

176

231

243

241

240

238

p

214

141 228

176

232

243

242

241

239

q

223

141 216

176

239

251

243

241

239

r

233

141 202

177

242

256

251

252

244

s

234

142 193

177

244

261

259

261

252

t

199

215

20,4

227

197

214

184

188

3

4

5

6

7

A

198

194

196

228

205

206

197

204

1

b

a

2

Filter

194

197

216

205

228

206

208

198

c

194

198

226

20,6

228

208

208

199

d

194

207

228

208

230

214

213

200

e

195

209

229

208

230

216

214.

200

f

197

216

231

209

231

216

216

203

g

199

217

232

211

232

216

217

204

h

242

220

221

213

j

20,0

217

23,2

211

217

235

21_7 218

24,1

22.0

220

211

1

211

218

236

224

242

223

223

217

k

212

219

23,7

225

242

225

224

217

1

213

221

237

225

242

237

238

21 8

m

Freezing temperature of drops, - T CC)

215

221

240

225

242

238

239

219

n

216

227

240

226

24,2

239

239

234

o

219

229

24.1

230

242

239

239

237

p

221

230

242

231

242

24,0

239

237

q

231

231

243

232

242

240

242

238

r

239

235

243

241

243

242

242

246

s

244

246

243

242

243

243

243

249

t

Table 6 Results from experiments with freezing of drops. Posltton of the shtp. 02 ° 10' S, 150°O0'W Local time 1055 1300 h, 1330-1500, 5 March 1984

227

3

228

229

229

228

1

b

a

2

Fdter

Table 5 Results from experiments with freezing of drops with subsequent melting and refreezing Position of the ship 0°00', 150°O0'W Local time l100--1300 h, 1345-1500, 4 March 1984

.7

206

20.8

23.6

23.2

16.9

208

236

231

167

4

5

6

7

A

215

229

16.9

182 167

18.7

16.7

17.5 167

6

7

2 3

198

196 212

195 21.1

5

196

195

l

4

b

a

Filter

A

173

23.2

237

209

20.9

209

201

210

c

17.6

233

23.7

212

20.9

20.9

201

211

d

207

236

238

21,2

213

21.0

202

213

e

208

23.6

238

213

21,5

212

20.9

21.5

f

20.8

237

23.8

216

218

21.3

210

215

g

209

23.7

239

21.7

219

213

21.0

219

h

210

237

239

230

22,5

217

211

22.2

1

21.2

23.8

24.0

23.1

226

21.9

21.2

227

j

217

238

242

23.2

23.0

235

219

229

k

21.9

241

24.2

232

230

23.6

219

229

1

219

24.3

242

23,4

23.2

237

22.1

23.0

m

Freezing temperature of drops, - T (°C)

22.4

247

24.2

234

233

23.7

222

230

n

226

24.8

243

23.5

233

238

22.5

231

o

227

249

245

235

23,3

239

225

23.1

p

22.9

25.0

24.6

23.6

234

24.0

23.5

235

q

23.5

251

24.9

237

23.9

240

238

236

r

239

25.1

253

23.7

23.9

24.2

24.0

23.7

s

241

21.8

208 223

20.2

e

22.1 242 215

22.1

22.9 230

216

204 22.3

197

d

192 231 20,2 214

221

229

216

199 213

19.7

c

212

21.3 225

21 5

g

220

21.8 229

218

l

22.0

21.8 23.0

219

j

22.8

220 236

22.5

k

219

22.5 227

222

229

228

23.2 23.2 23.2 232

219

217 226

21.7

h

22.2 22.2 222

21,8 218

222

23,0 231

21.8

211 22.5

20.9

f

230

224 237

227

m

22.9 229

22.9 229

23.9 239

229

22.0 23.7

227

1

Freezing temperature of drops. - T (°C)

242

22.5 24.2

23.1

o

24.2

22.5 243

23.1

p

242

243

225 243

23 1

q

236

24.2

251

22.9 24.3

23 1

s

253

22.9 243

23 1

t

241

25.3

253

243

242

24.3 24.3

25.0 25.0 251

24.5 24.5 24.5

24.4

229 24.3

23.1

r

23.9 241

24.3 24.3 243 23.2 235

243

24.0 24.0 241

23.9

224 24.2

228

n

t

242

Table 8. Results from experiments with freezing of drops P o s m o n of the sMp. 40°00, S, 149054 ' w , Local time 1300-1450h, 21 March 1984

20.7

20.2

2

20.5

18.9

3

20.9

177

17.9

b

a

1

Fdter

Table 7 Results from experiments with freezing of drops P o s m o n of the s h i p 10°00' S, 150000 , W Local time' 1100-1300 h, 1345-1530, 7 March 1984

o_.

b

206

229

2 30

23,8

270

211

149

158

a

205

228

229

232

190

139

126

136

2 10

187

223

223

238

231

229

207

c

21 1

187

227

224

239

231

229

207

d

21 1

188

228

224

23,9

232

235

209

e

212

189

228

226

239

232

238

209

f

212

189

228

228

239

232

238

213

g

220

189

228

22.8

239

233

238

213

h

220

190

228

22.8

239

234

238

217

l

22,1

190

229

228

240

234

23.8

218

.I

22,1

196

231

230

240

237

238

219

k

222

198

231

23.0

241

238

238

219

1

222

199

23,1

23.1

241

238

23,8

232

m

Freezing temperature of drops, - T (°C)

222

218

231

231

241

238

238

232

n

222

223

231

232

242

238

238

234

o

223

223

231

233

24 2

23,9

238

23,5

p

223

229

232

236

245

24,3

238

235

q

245

245

23.8

237

s

231

229

236

233

232

23,6

2 3 _ 7 242

245

245

238

236

r

Fdter

189

228

206

214

192

207

188

159

159

161

191

157

188

226

215

181

b

a

208

198

223

211

229

195

236

233

c

e

208

198

225

216

230

218

240

220

21 0

229

217

230

22 6

240

2 3 ~8 238

d

220

211

229

217

230

229

240

238

f

221

217

230

226

230

229

240

238

g

221

221

230

226

230

229

241

238

h

221

22,2

23,0

238

23,1

230

241

238

1

227

22.4

230

238

231

230

242

240

j

229

226

23!

238

23,2

230

243

242

k

22,9

229

23,2

239

23,2

234

244

243

1

230

229

233

239

23,4

23,4

24,4

243

m

Freezing temperature of drops, - T (°C)

232

229

233

239

234

234

244

243

n

23,4

232

233

23,9

234

235

244

243

o

234

238

234

239

235

23,8

244

243

p

234

238

234

239

235

23,8

244

24.3

q

234

239

237

239

235

238

24,4

244

r

238

239

237

240

235

239

246

244

s

t

240

240

238

24,0

235

23,9

246

244

t

234

234

236

242

245

245

238

237

Table 10 Results from experiments with freezing of drops Posmon of the ship 57028 , S, 158°07'W Local time, 1610-1810 h, 28 March 1984

Fdter

Table 9 Results from experiments with freezing of drops Position of the ship 42°00 ' S, 150°54'W Local time 0800-1000 h, 22 March 1984

z

21.5

21.5

21.6

22 1

22.1

22.1

22 0

23 1

20.4

18.2

20.9

21 9

21.2

21 8

21 9

18 6

23 1

22.0

22.1

22.2

22 1

22.0

22.4

22.1

c

23 1

22.1

22.2

22.3

22.1

22.0

22.4

22.2

d

23.1

22.2

22.2

22 4

22.4

22.0

22.5

23 2

e

23.2

22.3

22 3

22 4

22 4

22 1

22.5

23.2

f

23 2

22 3

22 4

22.5

22 5

22 1

22 5

23.2

g

23 2

22.5

22.6

22.5

22 5

22 1

22 6

23.3

h

23.2

22.6

22.6

22 5

22 5

22.1

22.6

23.3

I

23 3

22 6

22.6

22.6

22.6

22 4

23.4

23 3

.I

23 5

22.6

22 9

22.9

22.8

22.4

23.5

23 4

k

23 5

22.8

23.0

23.0

22 9

22.4

23 5

23.4

I

23.5

23 0

23 0

23 0

22.9

22.5

23 5

23.4

m

Freezing temperature of drops, - T (°C)

23 5

23 0

23.0

23.0

23.0

22 7

23 7

23 5

n

23.5

23.1

23 1

23.1

23.0

22.8

23.7

23 5

o

23.6

23.2

23.1

23.2

23.0

22.9

23.7

23 5

p

23.6

23.4

23 4

23 4

23.4

22 9

23 7

23.5

q

23.6

23.5

23.4

23 5

23 5

23.0

23.8

23.6

r

236

23.1

23.7

13.9

194

4

5

6

7

A

22.2

3

22.5

14.9

24.0

233

236

23.5

19.8

23.6

19.8

19.8

1

b

a

22.5

22.2

24.1

233

237

23.7

21.6

237

c

22.6

22.4

24.2

234

23.8

238

21.7

237

d

22.6

22.7

24.2

23.4

23.9

239

23.8

23.7

e

22.6

22.7

24.3

23.4

24.2

239

238

23.7

f

23.0

22.8

244

23.4

24.2

23.9

239

238

g

231

22.8

24.4

23.4

24.3

24.0

239

238

h

23.1

22.8

24.4

23.5

24.5

24.5

23.9

238

l

23.1

23.4

24.5

23.7

24.5

24.5

24.0

23.8

j

236

23.4

24.5

24.0

24.5

24.5

243

24.0

k

237

23.5

24.7

24.1

24.5

246

243

24.1

I

23.7

23.5

25.0

24.4

24.5

25.6

244

24.1

m

23.7

23.6

25.1

244

245

25.6

24.4

24.6

n

Freezing temperature of drops, - T (°C)

23.8

23.7

25.3

244

24.5

25.6

24.5

24.7

o

240

25.1

25.3

24.6

24.6

<257

245

247

p

24.1

25.2

25.4

24.9

24.6

<25.7

24.5

24.7

q

24.1

25.3

25.4

25.2

<257

<25.7

24.5

251

r

s

23.8

23.5

23.6

23.7

23 5

23 7

23 8

23 7

24.1

25.3

<25.7

<25.7

<25.7

<25.7

25.1

251

s

Table 12. Results from expenments with freezing of drops. Position of the ship: 40000- S, 170000` W. Local time 1125-1355 h, 6 Apnl 1984

b

a

2

Fdter

Filter

Table 11. Results from experiments wRh freezing of drops. Position of the slap' 55°07 ' S, 158007 ' W. Local time: 1220-1245 h, 29 March 1984

24.1

25.3

<25.7

<25.7

<25.7

<257

252

253

t

23.9

23 5

23 7

23 8

23 8

23 8

23.8

23.7

t

O

o

0

0 0 0

II1 - 1 - 7 - A

IV

1 2 3

VII-

-4 -5 - 6 - 7 - A

1-4 5 - 6 - 7 A

2 3-5 6 7 A

VI-

-

V-I

0 0 0 0 0 100 10

0 100 0 0 0 100 104 103

0 3x104 2 x 104

0 5 2

0 0 0 0 0 0

0 0 0

0

0

0

-

0

4°

0

0 0 0 0 0 0

0

1~ - 7 - A

0

I-7-A

I -

3°-

II - 1-7 A

number

Sample and

0 0 0

0

0

0

6c

104 104 104

8 x 103 3x104 2 x 104

0

0 0 0 0 50 50

5° -

10 3

3 x 104

0 0

0 0 0 0 100 100

0 0 0

0

0

0

8°

0 103 800 200 130

3x104

0 0

0 0 0 0 130

0 0 0

0

0

0

7c -

103 800 200 150 104 104 104

0

0 0 0 20

0 30 24

0

0

0

0

0

0

0

0

0 0 0 35 200

9° -

10 °

0

4 4 0

0

0

0

0

-

11 ° 0

0

0

0

0

0

12 °

0

0 42 500 500

60

0

0

0

0

0

0

0

0

0

0

104 104 104

13 ° -

T e m p e r a t u r e , - T (°C), A T = I ° C

14 °

0

0

60

0

0

0

0

0

0

0

0

22

0

4 4 0

80

15 ° -

16 °

0

0

0 140 100

0

0

0

-

T a b l e 13 C o n c e n t r a u o n o f lee-forming nuclei a c t w e t h r o u g h condensation-foUowed-by-freezang, n u m b e r / m 3

17 ° 0

0

0

0

0

18 °

103

-

19 19 103

19 ° -

20

z

Ice-forming nucle~ of maritime ongm

O w ~

~Q

O~

0

O O 0 ~ O

O

Q O ~

O

~ O 0 ~

~O

I

I

I

I

l

l

l

l

l i l l

0

Q O ~

~

I

Q O ~

I

0

I

~

I

I

0

I

~

I

O Q O ~

I

I I I I

37

38

J ROSINSKIet al

c. Ice nucleation by condensatlon-followed-by-freezm# C o n d e n s a t m n o f water vapor generally takes place at water vapor supersaturation w~th respect to water vapor saturation over liquid water. There are some natural aerosol particles, though, which can start absorbing water just below (or even far below) the saturation vapor pressure and grow into droplets in the updraft Any mixed aerosol particles active as CCN produce droplets which consist of solutions o f water-soluble salts (Junge, 1952). At first these solutions are so concentrated that the dissolved salts suppress the freezing temperature and prevent them from freezing even in the presence o f freezing nucleL During further growth o f droplets, the solution becomes sufficiently diluted that the hydrosol particles may act as freezing nuclei. Results o f the measurements are presented in Table 13. This table was constructed as follows: R o m a n numerals I t h r o u g h XII refer to sampling at the locations given in Tables 1-12; Arabic numerals 1-8 and capital letters A, B and C refer to the filters shown in Fig. 1. N o t e that filters 7 and 8 are identical, as are filters A, B and C. The results from filters 7 and A are given m the table only. Some additional information, like photographs, from filter 8, or from B and C, may be used in the text but were omitted in the table. Samples o f aerosol particles collected between latitudes of 37 ° and 34°N did not nucleate ice by this process (Table 13, hnes I, II and III) at any temperature, in spite o f the presence o f I F N active through freczmg at quite warm temperatures ( - 7.1 ° to - 11.1 °C, Table 1, line 2, and Table 2, hne 2). Just north o f the equator the first I F N appeared in the - 8 ° to - 9 ° C temperature range in a low concentration; aerosol particles were below 0.5 # m diameter. At the equator the concentration o f I F N increased, and some started to nucleate ice between - 5 ° and - 6°C. At 2 ° 10' S latitude (Table 13, line VI-7) a few ice crystals appeared at - 3.3°C but a concentration o f 30 1- ~ was present for the - 4 ° to - 5°C temperature range. Th~s concentration did not change when ~ce was sublimed and particles were exposed agam to water vapor at supersaturations with respect to saturation over supercooled liquid water at temperatures down to - 8°C. The concentration may be actually underestimated because o f overcrowding o f ice needles and frozen droplets and difficulties with counting them. I F N also showed up between - 5 ° and - 6 ° C for particles in the 0.5-1.5 # m diameter size range. The concentratmn o f I F N active in the - 3 ° to - 4 ° C temperature range Increased to 100 m - 3 at latitude 10 ° S. At the same t~me, I F N active in the - 4 ° to - 5 ° C temperature range reached a concentration of 10 1- ~. Aerosol particles active as I F N at these warm temperatures were smaller than 0.5 g m diameter. Aerosol particles between 6 and 8/am diameter reached a concentration o f I 1- ~ at - 7°C. Between - 7 ° and - 8°C, ice-forming nuclei were present In all sizes of aerosol particles. It is not possible to know from these experiments whether the same chemical species that were associated with the submlcron-stzed aerosol particles were distributed over all sizes o f aerosol particles or whether the larger aerosol particles were o f a different chemical composition, which nucleated ice at lower temperatures. When sampling o f aerosol particles moved farther south (40000 , S, 149054 ' W; 42000 ' S, 150000 ' W; 57028 ' S, 158o07 , W; and 55007 , S, 158°07 ' W), the concentration o f I F N especially active at warmer temperatures decreased to zero. Subsequently the ship sailed northward and, at 40 ° S latitude and 170 ° W longitude, I F N nuclei were present again; they started to nucleate ice at - 8 ° C and the aerosol particles were below 1.5 # m diameter. At 5 ° S latitude and 170 ° W longitude, the concentration o f I F N active at - 4 . 5 ° C was again ~ 301 -~ The results o f the measurements are presented in Fig. 3 for the southward sailing along 150 ° W longitude. In the case o f condensation-followed-by-freezing, supersaturation governs the time o f growth o f droplets. It was observed in all the experiments that freezing never took place until a critical size o f droplets was reached. It was not possible to determine this size, but it seems that the first frozen droplets were somewhere between 25 and 35 # m diameter; they started to freeze at - 5.7°C (V-B). Filter (V-C) was first exposed to high supersaturation ( S , = 15 ~ ) at temperatures just below zero (some o f the droplets had grown up to 500 g m diameter) and then it was cooled slowly at S W~ 1 ~ The temperatures o f freezing o f different dmmeter droplets were: 25-35/am (?), - 5.7°C; 50 gm, - 6 ° to - 9°C; 100-250 gin, - 6 ° to - 12°C; and 300-500 gm, - 1 8 ° to - 1 9 ° C . This is based on observation o f selected droplets. It seems, therefore, on the basis o f this rather crude experiment, that larger droplets, like the large

Ice-forming nuclei of maritime origin 105

,,,,I,,,'

iO'O0'S

3,30'N E

39

104

Z --I.-- io3 nr F-~(.3 iO2 Z0 ¢,D I01

0

/ '

,=liH-i-tll

0

-5

-I0

L, -5

-I0 0

-5

-I0 0

LI=l ill

-5

-I0

. . . . 75 .....

'-;o

TEMPERATURE, T,*C F~g. 3 Concentrauon of ice-forming nuclei actwe by condensation-foUowed-by-free~ngat different latitudes along 150°W longitude over the Pactfic Ocean.

drops used m the freezing drop technique, could not respond, in this particular case, to freezing nuclei active at warm temperatures. This could be due either to the infinite dilution mentioned in the previous sectmn or to some unique chemical composition of the C N which originated these large drops. The only difference between conditions within drops slowly grown by condensation and large drops placed on a filter was that m the first case the hydrosol particles were exposed to continuously changing concentrations of water-soluble chemical compounds and in the second case they were exposed to a large volume of water from the time they were in contact with it. It was observed that, at very high supersaturations, it took a very short time to produce the first ice crystal, but at low supersaturatlons it required, sometimes, more than an hour for ice to appear. In both cases the temperature of ice nucleation was exactly the same. Supersaturation was therefore required to supply the absolute quantity of water vapor only for the growth of droplets through condensauon. Some of the data are given in Table 14. Particles below 0.5 #m diameter (Table 13; samples VI-7 and 8, and VII-7 and 8) nucleated ice at S w = 2 % and at warm temperatures in 15-30 rain. This means that formation of ice m clouds by this process is time-dependent. It took 120 and 15 rain at S w = 1% for the first ice crystal formation on aerosol particles collected at 3o30' N (IV) and 40o00 ' S (VIII), respectively; also, sample VIII was the only one in which freezing droplets were small and ice was formed at S w = 0.1%. Some examples of how ice particles look on filters are given in Fig. 4. Appearance of the first ice crystal ts seen in "a" (IV-C, T = - 8.9°C, S w = 1%, t = 20 rain); the condensation pattern (filter from Andersen sampler) is visible in " b - l " (V-6, T = - 12 5°C, S w = 1%, t = 5 min); and the condensation pattern and ice crystals are shown in "b-2" (IX-6, T = - 18 3°C, S w = 1%, t = 3 min); the surface of the filter is covered with hardly visible droplets, except for four larger drops in "c-l" (V-A, T = -6.1°C, S w = 20%, t = 3 man) and m "c-2" the enttre surface is iced up, with bundles of ice needles visible (S w = 1%, t = 30 rain); ice crystals are growing by using water vapor from evaporating droplets in "d" (VIII-6, T = -17.7°C, Sw < 0.5 %, t = 40 mm); "e" will be discussed later (VII-B, T = - 17.2°C, S , ~ 1%, t = 60 rain, 0.22 #m Durapore hydrophoblc Millipore filter).

Table 14 Approxamate ume of appearance of the first ice crystals on filters (minutes) -T(°C) 5_0 5_5 72 89 d,/am Sample No

Sw=25%=1%

1 1

45 45 1-5

- T ( ° C ) Sw=22 % - T ( ° C ) S . = 1 5 % = 1 % 42 6.5 73 87

30 15 3 3 6--8 VII

15 5

10

0 1-10 VII

120

-T(*C) Sw=l~/o 11 4 13 2 17 2

IV

15 15 20 1-5 VIII

=01~o 15 15 0 5-1 5

40

J ROSINSK1 et al

It is possible to have a system of dissolved salts m water from which, upon cooling, the first solid phase separating from the solution wdl be ice. A case hke this probably does not exist m nature. To check whether hydrosol particles really were nucleating ice, the following experiment was performed. About one cubic centimeter of water was poured on the surface of a filter containing particles in the 0.5-1.5/am diameter sine range. Water was moved around and then poured off the filter onto another filter (hydrophobic) producing three large ovalshaped pools. They were evaporated in an aerosol-free hood and then the filter was placed in the dynamic chamber at a temperature of - 17.2°C and S w ~ 1 ~o- Water drops formed in the areas where water pools were originally present and the first ice crystals showed up around their edges. It was also noticed that when a drop was allowed to run over the surface of a filter it left a track of particles nucleating ice. This means that water-insoluble particles were present and that some small fraction of them could be hydrophobic in nature (Rosmskl, 1957). Hydrophihc particles should be preferentially present m the liquid and be deposited, mostly, m the place where the last remnant of water evaporated. The filter was kept m the chamber for 60 rain; at that time all water droplets were frozen, and ice particles grew to larger sizes and new ice crystals ceased to appear (Fsg. 4e). It is clear from Fig. 3 that I F N active at warm temperatures were found at the longitude of 150°W only in the vicinity of the equator, between 2 ° and 10 ° south latitude; a similar concentration (,,- 30 l - 1 at - 4°C) was found at 170 ° W longitude and 5 ° S latitude (21 April 1984). In Fig. 5 the 12 sampling posmons were plotted on the map of the Pacdic Ocean showing ocean currents. The high concentrations of I F N are located just over the South Equatorial Current. In the same latitude zone a large increase m the concentration ofaerosol particles with dmmeters smaller than 0.5/am was found. There Is also a sharp increase in the concentration of aerosol particles containing nitrates (Fig. 6). Submicron particles contaimng sulphates show the same trend. Production of nitrate-bearing particles is time-dependent, the maximum concentration was found around noon The peak in concentration of nitrate- and sulphate-bearing particles is probably assooated with orgamc activity within that current. Enhancement of production of these particles may play a crucial role in the formation of mixed aerosol particles which could nucleate ice at these warm temperatures; they supply the C C N for I F N and may even be supplying the necessary ~ons for activation of surfaces of

60=N

NOR

Jl

40°N

20aN ,

1

EO

41--

,oo,,.L.

EqUatorial Cuirrenl

,

'~L

I

~/

f

20°5

40°S Ocean Currenl

60"5 [ IO0=E

120°E

140°E

160°E

180°

7" , i40°W

160°W

/

- _

120°W

~,~

i

IOOUW

'

I _1

60°W

Fig_ 5 Twelveaerosol samphngpositions (blackdots) along the cruise of the NOAA ship Discoverer

Ice-forming nuclet of maritime ongm ~0

I

'

h

'

I

'

I

f

I

i

o~ Z ~J -I~

I

41 '

1

L o c o l T , m e , hr 70

a I000 o 1200 x 1400

z_ z_ 50

oZ

(,340

///

//

',~

\\/

U~ ILl ..--I <~ o_

o,-- __,z . . . .

20

-o ~_ _...,..,.._

_

hi 0

I 60N

I 40N

;)ON

0

20S

40S

60

LATITUDE,decj

F,g. 6 Percentageof particles contmningNO~ 1 ~on at differentlatitudes and times of day. hydrosol particles so they can nucleate ice. The presence of inorganic salts, e.g. sulphates, has been found to shift the process of freezing of water drops by some hydrosol particles into warmer temperature ranges (Murty and Murty, 1972). But Hoffman and Duce (1977) pointed out that most of the particulate organic matter is not associated with ocean salts; in that case organic particles smaller than 0.5/an diameter should act as CCN, and subsequently as IFN if they are responsible for our findings. The time dependence (day or night), if any, of IFN concentration could not be established from our data because each sampling period, unfortunately, overlapped with the maximum production of nitrate- and sulphate-bearing particles. The elemental chemical compositions for two samples (VI and VII) are shown in Fig. ? Size distributions are given m Fig 8 for three samples taken over the South Equatorial Current. Organic particles (X-ray non-emitting particles) seem to be more abundant in the particle population smaller than 1/~m diameter. Nearly all aerosol particles consist of mixed particles containing salts from the ocean water attached to an organic matrix. There is also a bias toward detecuon of chemical elements in larger particles. It seems that IFN nucleating ice at warm temperatures are all organic particles in the 0.1-0.3 #m diameter size range. The fraction of an aerosol particle population < 5/~m dmmeter active as IFN in the - 4 ° to - 5°C temperature range over the South Equatorial Current is ~ 10- 3. For particles in the 6-8/~m d,ameter size range nucleating ice between - 6 ° and - 7 °, it is ~ 3 x 10-2 (or ~ 3 x 10 -5 if the IFN concentrat,on in that size range is divided by the total [d > 0.1/~m] aerosol concentration). Finally, to establish the origin of aerosol particles nucleating Ice over the South Equatorial Current, a backward isentrop~c trajectory for an air parcel arriving at the sampling area at 2°10 ' S, 150°00 ' W on 6 March 1984, ,,~ 00 GMT, was constructed (Fig 9). Sampling north of the equator has shown a complete absence of IFN active at warm temperatures; the air in the boundary layer was void of IFN. The backward trajectory for the sample collected on the equator (showing low IFN concentration) suggests air transport similar to that for the sample collected south of the equator. The large concentration of IFN active m the - 4 ° to - 5°C temperature range m that IFN-free air must therefore be of local origin. The air parcel trajectory for sample X collected on 29 March 1984, ~ 00 GMT, (55007 , S, 158°07'W) is given in Fig 10. Th~s air parcel moved through this position three days prior to the sampling over the Southern Ocean Current and it did not contain any aerosol particles nucleating ice. The aerosol population present over tl'us part of the ocean differs completely from that found

42

J ROSINSKIet al IOO 2 ° I0' S 150 ° OO' W

90

Z w w

oo --

IO ° DO' S 150 ° 0 0 ' W _

8o 70

bJbJ -I

(3. L~

~-

40

3O z

zo

%)

Io

ot

ti l Ill

× No M( AI $1 P

n-e

l] ij1

S

CI

Co

Ti

Cr

Fe

X NO Mg AI $= P

Zn

$

C1 K Co Tq Cr Fe

Zn

n-e

Fig. 7 Frequency ofchenucal elements of aerosol particles. Black bars---parhcles smaller than 1/an diameter, empty bars--particles larger than 1/an diameter X n-e---X-ray non-enmtmg particles.

I00

i

i

i

i iiii

I

i

i

i

i iiir-

x 2 o l O ' S , 1 5 0 * O 0 ' W :_ • l O ° O 0 ' S i 150 ° O 0 ' W & 5 o o o ' s , 17OoO0'W

"$ IO & x

I

tE

ox

I

I

OI

DO[

J

,

OI

i ,tttJl

)

IO

PARTICLE DIAMETER, /~rn Fig- 8 S~ze d]stnbuBons of aerosol particles

over the South Equatorial Current. All the trajectory calculations (m both hermspheres) indicated transport confined to the boundary layer. 4

GENERAL

DISCUSSION

Paterson and Splllane (1968) studied the freezing of ocean water, with and without a surface film. Samples of ocean water were collected from the ocean beach at Flinders on the Victorian coast of Australia. The South Equatorial Current changes Rs name along the

Ice-forming nuclei of maritime origin

43

20°N

IO°N

EQ

IO°S

160°W

150=W

140°W

Fig. 9. Backward ,sentropic trajectories for aar parcels havmg 0 values of 298 K arnvmg on 5 March 1984 (thin line) and 6 March 1984 (thick fine), "(30 GMT, at 0°0(Y and 2° I(Y S, 150°00 ' W, respectively The time interval between dots is 12h, local time 4 and 5 March 1985---Tables 5 and 6. 28 MARCH 1984 I50OE

170=E

180°

170°W 160°W 150°W

///

40os

asmon o 4005

50°S

South Pole 50°S

60°S

70°S

60°S

Fig 10 naekward ~sentrop=c trajectory for air parcel having 0 value of 298 K arriving on 24 March 1984, -00 G M T , a t 57028 ' S and 158007 ' W. The time mterval bctwecn dots =s 12 h; local time 28 March 1984---Table I0.

Australian coast to the East Australian Coast Current and passes on the cast of the Bass Strait and Tasmania (see Fig. 5). Results of their investigation indicate that the surface-absorbed films, consisting mostly of peptides (proteins and amino acids), do not act as freezing nuclei. Warburton (1958) observed in the southeast coastal region of Australia ice crystals in clouds at temperatures just below - 2 ° C . Subsequently Mossop et ai. (1968, 1970) studied small supercooled cumulus clouds in maritime air off the Tasmanian coast. Their findings were: (1) there seems to exist a maximum concentration of ice crystals on the order of 100 I- t, which is independent of temperature over the range - 5 ° to - 15°C; (2) the concentration of ice particles is greater than the measured concentration of IFN by a factor of ~ 103 at - 15°C and correspondingly higher at warmer temperatures; and (3) the multiplication of ice crystals is due to microphysics and dynamics rather than to a chance event such as the influx of particularly effective IFN. Further examination of data reveals that concentrations of ice particles of up to ,,, 401-1 were observed on the first aircraft penetration of undisturbed clouds (see discussion in Rangno and Hobbs, 1983; Mossop, 1984; Hobbs and Rangno, 1984).

44

J ROSlNSK1et al

If the I F N produced in the South Equatorial Current are also being produced in this current when it passes along the east coast of Tasmania, then there is a sufficient number concentration of I F N active at warm temperatures to explain the above experimental findings It was also shown that, in areas where continental air can interact with maritime air, there is a formatmn o f l F N active at warm temperatures through freezing (Nagamoto et al., 1984). If this is the case, then it ~s not necessary to explain the presence of high concentrations of ice particles in those clouds by means of an ice multiplication mechanism (Hallett and Mossop, 1974) Nikandrov (1951, 1956) has shown that there is sublimation of water vapor taking place between a supercooled water droplet and an ice particle or an aerosol particle active as an I F N by sorption The direction of the flow of water vapor follows the water vapor concentration gradient (from liquid water to ice) between two approaching particles (at - 5 ° C Phqmd water = 3.163 mmHg, P~c¢ = 3.013 mmHg, and AP = 0.15 mmHg). Ice starts to grow on an ice particle towards the evaporating drop and it freezes the drop upon contact before an actual colhsion can take place In many instances Nlkandrov has shown that an ice branch actually fell off before the physical contact between two particles took place. The distance in Nlkandrov's experiments between approaching surfaces was usually ,,, 50/am, or larger. In the cloud temperature range between - 4 ° and - 6 ° C , elementary needles should form and they may separate easily from the surfaces of ice particles, or even break into fragments during separation of interacting particles at the time the heat of the phase transition is released. If this IS the case, then the experiment of Hallett and Mossop, which shows formatmn of large numbers of ~ce crystals during rotation of a metal rod at temperatures of - 4 5°C, probably demonstrates only that Ice needles are mechanically weak At other temperatures the "ice bond" is strong enough to provide freezing and capture of a cloud droplet by an ice particle. In real clouds, the residence time when two interacting particles are in the same vicinity will depend upon their size; some additional Ice particles can definitely be produced by this process If the condensation-followed-by-freezing mode of ice nucleatmn is responsible for the formation of the very first Ice particles, then they should be frozen droplets. This process is time-dependent and therefore may produce ice during different periods of the lifetimes of different clouds Frozen droplets were indeed observed as the first ice particles in Missouri (Braham, 1964) and they were nearly always present In cloud studies by Mossop et al (1970)

5_ C O N C L U S I O N S A N D C O N J E C T U R E Aerosol particles collected over the Pacific Ocean were examined for their ablhty to nucleate ice by freezing, sorption and condensation-followed-by-freezing. Conclusions drawn from this experiment are given below

Ice nucleation by freezing (a) I F N in the 6-8/~m diameter size range which are aggregates of smaller particles were active in the - 7 . 3 ° to - 11.1 °C temperature range along the western coast of North America (36 ° N, 125 ° W). Melting of frozen drops destroyed ice-nucleating ability (b) A very low concentration of I F N active around - 12°C was found over the South Pacific for aerosol particles below 0.5/zm diameter. (c) Freezing of temperatures of drops placed on aerosol particles collected over the South Pacific Ocean were generally lower than those from the North Pacific Ocean.

2. Ice nucleation by sorptlon I F N active by sorptlon In the temperature range between - 5 ° and - 1 7 ° C at supersaturatlons over ice but below saturation over liquid water were not found over the Pacific Ocean

Ice-forming nude] of maritime origin

45

3. Ice nucleatton by condensation-followed-by-freezino (a) The formation of frozen droplets by this process in clouds is time-dependent and consequently depends on the evolution of clouds. (b) IFN active at - 3.3°C were found over the South Equatorial Current (100 S, 150 ° W) in concentration of 100 m-3. (c) Concentrations of IFN active at and below - 4 ° C were up to 3 x 104 m - a (301-1) over the South Equatorial Current at 150°W and 170°W longitudes. (d) Concentrations of IFN were found to be independent of temperature for the - 4 ° to - 14°C temperature range. (e) Aerosol particles supplying IFN were below 0.5/am diameter. (f) The fraction of the aerosol population that nucleates ice between - 4 ° and - 5°C temperature range is ~ 10- 3. (g) Aerosol particles smaller than 0.5/am dmmeter seem to consist mostly of organic matter. Presence of IFN over waters of the South Equatorial Current seems to be associated with biological activity in that current.

4. Conjecture The South Equatorial Current passes along the east coast of Australia; if it produces the same concentrations of IFN as at 150°W and 170°W longitudes, then the large concentrat]ons of ice particles found by Mossop at warm temperatures in the clouds around Tasmania could be related to corresponding concentrat]ons of IFN active by condensationfollowed-by-freezing and not to the ice multiplicaUon mechanism of Hallett and Mossop. Acknowledoemems--This research could not have been performed wtthout the contmmng interest and support of Dr B A. Sdverman and Dr A. S Denms, Diws~on of Atmospheric Resources Research, Bureau of Rzclamation, Department of the Intenor, Denver, Colorado, USA.

REFERENCES

Attken, J. (1880) Trans. R. Soc Edmburoh 30, 337. Andersen, A A_ (1958) J Bac~eriol 76, 471 Braharn, R R. Jr. (1964) J. atmos. Scz. 21, 640. Clark, J. F, Clark, T L., Ching, J. S, Haagenson, P L., Husar, R B. and Patterson, D. E. (1983) Atmos. Environ. 1"1, 2449 Couher, P_ J (1875) J. Pharm. ChJm. Paris 22, 165. Crutzen, P. J, Delany, A. C., Greenberg, J, Haagenson, P. L., Hadt, L., Lueb, R., Pollock, W., Saler, W., Wartburg, A. and Zimmerman, P. (1985) J. Atmos Chem. 2, 233 Fall, R. and Schnell, R. C. (1985) J. mar Res. 43, 257. Ferek, R J, Lazrus, A. L., Haagenson, P" L. and Winchester, J. W. (1983) Environ. So. Technol. 175 315. Haagenson, P. L and Shapiro, M. A. (1979) NCAR Tech. Note, NCAR TN-149. Hallett, J. and Mossop, S. C (1974) Nature 249, 26 Hobbs, P. V and Rangno, A. L. (1984) J. Cli~ appl. Meteorol. 23, 346 Hoffman, E J and Dnce, R A. (1977) Geophys. Res. Lett. 4, 449. Junge, C. E (1952). Arch Meteorol. Geophys Biokl 5, 44 Langer, G. and Rogers, J. (1975) J. appl Meteorol. 14, 560. Lazrus, A L., Haagenson, P. L, Heubert, B J, Kok, G. L., Kreitzberg, C W., Likens, G. E., Mohnen, V. A, Wilson, W E. and Winchester, J W. (1983) Atmos. Environ. 17, 581. Mossop, S. C., Ono, A. and Wishart, E R. (1970) Q J R. meteoroi. Soc. 96, 487 Mossop, S. C. (1984) J CIwn. appl. Meteorol 23, 345. Murty, A S. R. and Murty BH, V. R. (1972) TeUus 24, 150. Nagamoto, C. T, Roslnski, J, Haagenson, P L., Michaiowska-Smak, A. and Parungo, F. (1984) J. Aerosol Scl 15, 147. Nikandrov, V. Ya (1951) Tr Gl. Geofiz. Obs. 31, 93 Nikandrov, V Ya. (1956) Tr Gl, Geofiz. Obs 57, 119. Paterson, M. P. and Spdlane, K T. (1968) Nature 218, 864 Pod~mek, J (1982) 10th International Conf. on Condensation and Ice Nuclei, 26--28 Aug 1981, Hamburg, F. R, G. J. Hung Met Serv g6, 179. Pruppacher, H. R. and Klett, J. D (1978) M~crophystcs o f Clouds and Precip, tatwn. D Reidcl, Dordrceht. Rangno, A L. and Hobbs, P. V. (1983) J. CIml. appi. Meteorol. 22, 214. Rosinskl, J (1957) Trans. Am. Geophys. Umon ~ 847. Rosinskl, J and Leanskl, A. (1983) J Aerosol Sc~. 14, 49.

46

J. ROSINSKIet al

Rosmskt, J., Morgan, G., Nagamoto, C T., l.,anger, G., Yamate, G. and Paxungo, F. (1980) Meteorol. Rdsch 33, 97 SchnelL R C (1979) Preprints, 7th Conf. on Inadvertent and Planned Weather Moddication, Banff, Canada. AMS, !10. Wegener, A. (1911) Thermodynanuk der Atmosphare. Barth, Letpzag. Warburton, J. (1958) Report on South Australian Weather Features RPL 134, CSIRO