Ice-forming nuclei in air masses over the Gulf of Mexico

J. Aerosol Sci., Vol. 19, No. 5, pp. 539-551, 1988. Printed in Great Britain.

0021-8502/88 $3.00+0.00 © 1988 Pergamon Press plc

I C E - F O R M I N G NUCLEI IN AIR MASSES OVER THE G U L F OF MEXICO J. ROSINSKI,* P. L. HAAGENSON,* C. T. NAGAMOTO,~B. QUINTANA,~ F. PARUNGO~ a n d S. D. HOYT~ * National Center for Atmospheric Research, § P.O. Box 3000, Boulder, CO 80307, U.S.A. t National Oceanic and Atmospheric Administration, Boulder, CO 80303, U.S.A. ~/Environmental Analytical Service, San Luis Obispo, CA 93401, U.S.A.

(Received 31 July 1987; and in final form 20 October 1987) Abstract--Aerosol particles collected over the Gulf of Mexico during the period from 20 July to 30 August 1986 were examined for their ability to nucleate ice by condensation-followed-by-freezing. Ice-forming nuclei (IFN) in the 0.1~).4 pro-diameter size range nucleated ice at a temperature of - 4 ° C; their concentrations were between 2 and 10 m-3. Fractions of aerosol particles in that size range nucleating ice at the initial (the highest) temperatures were between 10- a and 10- 7. Peaks in the concentration of dimethyl sulfide (DMS) (0800 h) preceded peaks in ice-nucleating temperatures (1300 h) by 5 h; this is sufficient time for DMS molecules to be oxidized to sulfates and to produce mixed aerosol particles through coagulation of different-sized aerosol particles and absorption of sulfur-bearing gas molecules. Fractions ofaerosol particles larger than 0.2 #m in diameter containing SO~- ions were larger than 0.90; most of the time they were 0.99-1.00. All IFN displayed characteristic features of mixed IFN, that is of marine origin (part of 1FN concentration independent of temperature) and of continental origin (part of IFN concentration dependent on temperature). 1. I N T R O D U C T I O N T w o p r e v i o u s studies (Rosinski et al., 1986, 1987) in the field o f ocean-derived ice-forming nuclei (Schnell, 1977) have s h o w n the presence o f a e r o s o l particles active as ice-forming nuclei ( I F N ) over the Pacific O c e a n between 7°N a n d 10°S latitude a n d 110 ° to 170°W longitude. C o n c e n t r a t i o n s o f I F N active by c o n d e n s a t i o n - f o l l o w e d - b y - f r e e z i n g were p a t c h y in the air over the ocean; they varied f r o m 0 to 4.5 x 104 m - 3 active at the initial (highest) icenucleation t e m p e r a t u r e o f - 4 ° C a n d they were f o u n d to be i n d e p e n d e n t o f t e m p e r a t u r e b e l o w this initial ice-nucleation t e m p e r a t u r e d o w n to - 1 7 ° C . T h e highest ice-nucleation t e m p e r a t u r e was f o u n d to be - 3 . 3 ° C ; the c o r r e s p o n d i n g c o n c e n t r a t i o n was 102 m - a . These I F N were present over patches o f the ocean where upwelling was t a k i n g place. T h e m o s t i m p o r t a n t a n d c o m p l e t e l y unexpected finding was that the sulfate ion was f o u n d to be an integral p a r t o f the ice-nucleating particle. I F N were sulfate-bearing h y d r o p h o b i c wateri n s o l u b l e a e r o s o l particles in the 0.1-0.3 p m - d i a m e t e r size range. D i m e t h y l sulfide ( D M S ) o r i g i n a t i n g in the ocean water is a p r e c u r s o r for S O ~ - ions in the a t m o s p h e r e . T h e flux o f D M S f r o m the surface o f the ocean water into the a t m o s p h e r e is p r o p o r t i o n a l to the c o n c e n t r a t i o n o f D M S in water. E x p e r i m e n t s o f a p r e l i m i n a r y n a t u r e were p e r f o r m e d in this s t u d y to explore a possible r e l a t i o n between the c o n c e n t r a t i o n s o f I F N a n d o f D M S . M e a s u r e m e n t s were m a d e d u r i n g the j o i n t U . S . A . - M e x i c a n cruise a b o a r d the M e x i c a n research vessel H-02 f r o m 20 July to 30 A u g u s t 1986 in the G u l f o f Mexico.

2. E X P E R I M E N T A L

SECTION

a. Measurements o f I F N population T h e s a m p l i n g technique used to collect different-sized a e r o s o l particles was similar to one described in o u r p r e v i o u s studies (Rosinski et al., 1987). Basically, (1) the two u p p e r stages (1 a n d 2) were r e m o v e d a n d the stages 3 - 6 were only used in the A n d e r s e n sampler, a n d (2) three § The National Center for Atmospheric Research is sponsored by the National Science Foundation. 539

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J. ROSINSKIet al.

backup filters were used. The backup filters and filters used in the Andersen sampler were 47 mm-diameter membrane filters with a 0.22 #m nominal pore diameter (Millipore, MA). The number of jets impinging on an area of 17.3 cm 2 of filler per stage was 149. The area outside a filter mounted on each 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. The sampling area of each backup filter was 9.6 cm 2. The filters were mounted by means of a flat Teflon ring; the area under the ring (7.7 cm 2) was not exposed to the sampled air. The cutoff diameters for the stages of an Andersen sampler were: 3: > 4.5, 4: 3.1-5.0, 5: 1.0-3.6, and 6: 0.4-1.2 #m. Particles collected on the surface of the backup filters were smaller than 0.4/am in diameter; the diameter size range for aerosol particles separated by filtration and active as IFN was assumed here to be 0.1-0.4 #m; the lower size limit was found generally to be 0.1/~m in diameter for the separated natural IFN, but the smallest organic droplet nucleating ice was reported to be 0.017 #m in diameter (Rosinski et at., 1980). A dynamic developing chamber (Langer and Rodgers, 1975) was used to detect and to determine the concentration of IFN active by condensation-followed-by-freezing. Filter holders were cleaned using an ultrasonic bath and were subsequently rinsed with filtered distilled water. Filters were mounted in a particle free hood and filter holders were sealed. Ten per cent of loaded filter holders were used to determine the IFN background on filters. The background was zero on all the filters. Consequently every ice crystal detected on the surface of a filter corresponds to an aerosol particle active as an IFN. Concentrations of IFN active by condensation-followed-by-freezing were determined at a water vapor supersaturation of 2 ~o + 0.1. The temperature of a filter was changed continuously from - 4 ° to -24°C. The cooling rate of 0.3°C min- 1 was used. b. Measurements o f D M S seawater and air

The DMS measurements were made using a fused silica capillary column and an F P D detector. The detector was specially modified for increased sensitivity. The system is constructed from Teflon and silanized glass to minimize adsorption losses. The seawater samples were collected in 50 ml glass syringes and filtered with disposable 25 mm-diameter filters with 0.45/~m pore diameter. The DMS in seawater was concentrated by stripping a 20 ml sample with an inert gas and passing the gas through a potassium carbonate dryer to remove the water vapor. The DMS is cryogenically trapped on silanized glass beads using liquid oxygen.The sample was desorbed from the glass beads using hot water and cryofocused on a section of the capillary column. The analysis was run at room temperature and the DM S concentration was measured by a high-sensitivity F P D detector. The peak area was measured by an H P 3393A integrator. The air samples were collected from an air-sampling tower located above the bridge of the ship. The air was continuously pumped down to the gas chromatograph through Teflon tubing. The DMS measurements in air were made by passing a 1000 ml air sample through a Nation dryer to a glass bead trap where it was concentrated and then desorbed to the cryo-focus loop. c. Backward isentropic trajectories for sampled air parcels

Isentropic trajectory analysis (constant potential temperature, 0) was used to determine the transport history of the sampled air. The isentropic transport model used in this study is described by Haagenson et al. (1979). It has been used successfully in boundary-layer transport application (Clark et al., 1983; Ferek et al., 1983; Lazrus et al., 1983). Application of the model to transport in equatorial regions is discussed by Crutzen et al. (1985). 3. R E S U L T S AND D I S C U S S I O N Dates of sampling of aerosol particles along the cruise and concentrations of DMS in nanograms per litre of seawater are given in Fig. 1. Concentrations of IFN active through condensation-followed-by-freezing as a function of aerosol particle diameter and tempera-

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ture were determined during four sampling periods: 21-22.7, 25-26.7, 6-7.8 and 8.8.1986. During the first sampling period it was found that the highest temperature of ice nucleation by aerosol particles in the 0.1-0.4 pm-diameter size range changed with time of day; it was - 11°C from 12130to 1400, - 17°C from 1655 to 1855, - 17°C from 2155 to 0010, - 1 0 ° C from 0300 to 0515, and - 1 4 ° C from 0815 to 1000h (Fig. 2). Previous studies over upwellingregions of the Pacific Ocean have shown that sulfate ion-bearing aerosol particles in the 0.1-0.3 pm-diameter size range already nucleated ice at a temperature of -3.3°C; at - 4 ° C , the concentrations were up to 4.5 x 104 m-3. By contrast, concentrations over that part of the waters of the Gulf of Mexico were below 10 m - a. Clearly, these concentrations of IFN do not resemble previous findings; however, they should not be similar. The upweUing regions of the Pacific Ocean are rich in biogenic activity, whereas the ecological region under investigation is of an oligotrophic character (Andreae and Raemdonck, 1983). But the IFN populations from both regions have one property in common; that is, their concentrations are independent of temperature for some temperature ranges. Over the Pacific Ocean this occurred over a temperature range of 13°C (from - 4 ° to - 17°C; this dependency was not measured below - 1 7 ° C ) and over the Gulf of Mexico it was present over different temperature ranges at different temperatures and for different aerosol particle diameter size ranges. For example (from Fig. 2), for aerosol particles in the 0.1-0.4 #xm-diameter size range the temperature range was 6°C (1200-1400), for 1.0-3.6/xm it was 7°C (1655-1855), and for 0.4-l.2 #xm it was 6°C and 5°C (2155-0010) at two different IFN concentrations. Hydrosol particles present in seawater started to nucleate ice at - 6 ° C (21 July 1830)and at - 11°C (22 July 1100). Hydrosol particles dispersed into the air could supply aerosol particles to nucleate ice in that temperature range, but IFN active at temperatures higher than - 10°C were not detected in the air. Concentrations of different-sized aerosol particles and their size distributions are shown in Figs 3 and 4. The fraction of aerosol particles f(d/xm; T°C) in the 0.1-0.4 #xm-diameter size range active as IFN at a temperature of - 1 5 ° C (temperature arbitrarily selected ) was found t o b e between 0 and 10 - s; for aerosol particles larger than 0.4 pm it was ~ 10-7 The results from the second sampling period are presented in Fig. 5. Aerosol particles in the 0.1-0.4/xm-diameter size range present between 1200 and 1400 h nucleated ice at the initial (highest) temperature of - 4 ° C ; their concentration, however, was very low (3 m-a). This time the highest temperature of ice nucleation was similar to that found over the upwelling

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regions of the Pacific Ocean. Larger aerosol particles started to nucleate ice at a temperature of -10°C. Five hours later this was reversed; larger particles nucleated ice at - 5 ° C and particles in the 0.1-0.4 #m-diameter size range at - l l ° C . During the night the highest temperature of ice nulceation dropped, reaching - 16°C in the morning hours. The IFN in all diameter size ranges nucleated ice independently of temperature over different short ranges of temperatures at different IFN concentrations. The highest temperature of ice nucleation by particulate matter present in seawater was -9°C. The larger aerosol particles (d > 0.4 #m) nucleating ice at higher temperatures ( - 5 ° to -8°C) later in the day (1705-1905 h) were probably aggregates formed through coagulation of the 0.1-0.4 #m-diameter particles with the large ones. Fractions f(0.1-0.4 #m; -4°C) and f(0.1-0.4 #m; -15°C) were about 10-a and 2 x 10 -6, respectively. The third and fourth sampling periods were over seawaters containing hydrosol particles nucleating ice at an initial temperature of - 5°C (Figs 6 and 7). Aerosol particles nucleated at initial temperatures as follows: - 9 ° C from 1205 to 1405 on 8 August, - 1 2 ° C on four occasions, and the rest between - 1 3 ° and - 1 6 ° C . Aerosol particles nucleating ice at 19:5-B

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temperatures higher than - 9 ° C were not released from seawater unless the hydrosol particles lost their ability to nucleate ice when transferred from seawater into the air or their concentrations were below detectable limits. Again I F N concentrations were sporadically independent o f temperature. Fractions f(0.1-0.4/~m; - 11 °C) and f(0.1-0.4/am; - 9°C) were a b o u t 1 0 - 7 and 5 x 10 -8, respectively; at - 1 5 ° C they were about 2 x 1 0 - 7 and 3 x 1 0 - 7 It was shown previously that SO~ - ions are an integral part o f l F N of marine origin in the 0.1-0.5 #m-diameter size range over the equatorial upwelling regions o f the Pacific Ocean. Dimethyl sulfide is the precursor o f the S O ~ - ion, and consequently it was thought that it may correlate with either I F N concentration or the initial temperature o f ice nucleation o f aerosol particles in that size range or both. Concentrations o f D M S in nan=grams per cubic meter o f air present over upwelling regions o f the Pacific Ocean (Andreae and Raemdonck, 1983) were plotted together with the highest temperatures o f ice nucleation o f aerosol particles present in air over the Gulf o f Mexico vs time o f day. Concentrations of D MS in air over the Gulf of Mexico were below the detectable limit. But since concentrations in air are proportional to the concentrations in seawater, one can insert them into the graph (Fig. 8). The data show distinct peaks in the temperature curves between 1200 and 1400 h. The concentration peak of D M S in air occurred between 0600 and 1000 h. The time difference between the midpoints o f sampling times is ~ 5 h; this time is sufficient to oxidize S ( - I I ) in (CH3)2S to S ( + V I ) in S O ~ - ion and to coagulate with aerosol particles, thus producing SO,~--bearing mixed aerosol particles active as I F N through condensation-followed-byfreezing. Another finding supporting the relation between the initial temperature o f ice

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must be produced during the exchange of matter at the air-sea interface. The fractions of aerosol particles larger than 0.2 pm in diameter containing SO~- ions were larger than 0.90, and most of the time they were 0.99-1.00. Consequently, we are dealing with IFN consisting of mixed aerosol particles, that is, product s of local biogenic activity or aerosol particles from a distant source coated or coagulated with sulfur-bearing chemical compounds. IFN of marine origin were found to nucleate ice independently of temperature (Rosinski et al., 1986). Data from the upwelling regions of the equatorial Pacific Ocean are shown together with data from the Gulf of Mexico (Fig. 10). The presence of large number concentrations of IFN nucleating ice at high temperatures was associated with large concentrations of DMS. The grey area in Fig. 10 represents typical temperature spectra of aerosol particles of continental origin; the entire area enveloped by X-es is typical of IFN present in marine-continental mixed-air masses. The presence of SO~- ions which are produced from'their precursor molecules (DMS) is an essential but insufficient condition for the formation of IFN of marine origin. The presence of derivatives of, for example, the C-16 hydrocarbon (used here as an example because its derivatives constitute the most abundant group in the ocean waters) is necessary for the production of IFN. The two components, SO~- ions and aerosol particles made of the derivatives of C-16 hydrocarbon, must be present at the same time in the atmosphere. The concentration of D MS is proportional to the biogenic activity (Parungo and Miller, 1987) which, if low, can lead to a situation in which SO~- ions will be relatively abundant but organic particles will be scarce. The result may be the presence of aerosol particles nucleating ice at a high temperature but at a very low concentration. This is what we found over the waters of the Gulf of Mexico. An example of the elemental chemical composition of aerosol particles collected over the sea is shown in Fig. 11. Nearly all aerosol particles are mixed particles containing salts from the seawater attached to an organic (Xn-e particles) or an inorganic matrix. There is also a bias toward detection of chemical elements in larger particles. Organic particles probably contain traces of most of the elements. The presence of different concentrations of organic particles and of trace elements (Cr, Mn, Ni, and Cu) at different times of sampling indicate different biogenic activity in and different chemical composition of the seawater along the ship's path. This explains the observed nonuniform character of the population of IFN over the sea. The sampled aerosol particles consist both of a fraction of an aerosol particle population that has originated at some distance and of particles that are produced locally. Therefore, to

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Ice-forming nuclei

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continental origin were from the Yucatan Peninsula and Cuba, and not f r o m the continental United States. Their contribution was shown in the characteristic dependency of I F N on the t e m p e r a t u r e of ice nucleation.

4. C O N C L U S I O N S

AND OBSERVATIONS

The following conclusions and observations can be made on the basis o f experimental data collected during the research cruise over the waters of the G u l f of Mexico for the purpose o f studying I F N population active by condensation-followed-by-freezing. (1) I F N of marine origin active at the highest temperature of - 4 ° C were present at extremely low concentrations (2-10 m - a ) . Fractions of aerosol particles in the 0.1-0.4/~mdiameter size range nucleating ice at the initial (the highest) temperatures were between 10- a and 10- 7 (2) Aerosol particles in the 0.1-0.4/~m-diameter size range nucleated ice at the highest temperatures during daytime hours (1200-1400). (3) Aerosol particles larger than 0.4/~m in diameter nucleated ice at higher temperatures than the smaller ones in the afternoon hours (1700-1900). (4) Peaks in the concentration o f dimethyl sulfide preceded peaks in ice-nucleating temperatures by ~ 5 h; this gives sufficient time for D M S molecules to be oxidized to sulfates and to produce mixed aerosol particles t h r o u g h coagulation of different-sized aerosol particles and a b s o r p t i o n o f sulfur-bearing gas molecules. (5) The temperature of ice nucleation by aerosol particles in the 0.1-0.4/~m-diameter size range was found to be p r o p o r t i o n a l to the D M S concentration and consequently to the biogenic activity in seawater. (6) All I F N displayed characteristic features o f mixed I F N , that is, partly of marine origin (part o f I F N concentration independent of temperature) and partly o f continental origin (part of I F N concentration dependent o f temperature). (7) H y d r o s o l particles present in seawater were nucleating ice on some occasions at temperatures higher than those of the aerosol particles, indicating that these hydrosol particles were not transferred f r o m seawater into the air unless the aerosolized particles lost their ability to nucleate ice at high temperatures or their n u m b e r concentrations were below detectable limits. would like to express our appreciation to Almirante Secretario, Miguel Angel Gomez Ortega for providing the Mexico Naval Oceanographic Research Ship H-02 for this research, to Vice-Admiral Gilberto Lopez Lira and to ship commander Captain Carlos Castro Trujiilo for making this cruise possible. Special thanks must go to Dr Humberto Bravo A., Seccion De Contaminacion Ambiental Centro De Ciencias De La Atmosfera Universidad Nacional Autonoma De Mexico for making this program possible. Acknowledgments--We

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