Temporal and spatial variations of marine aerosols over the Atlantic Ocean

Atmospheric Research, 20 (1986) 23--37

23

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

TEMPORAL AND SPATIAL VARIATIONS OF MARINE AEROSOLS OVER THE ATLANTIC OCEAN

F.P. PARUNGO, C.T. NAGAMOTO and J.M. HARRIS

U.S. Department of Commerce, Environmental Research Laboratories, NOAA, 325 Broadway, Boulder, CO 80303 (U.S.A.) (Received July 9, 1985; accepted after revision November 23, 1985)

ABSTRACT

Parungo, F.P., Nagamoto, C.T. and Harris, J.M., 1986. Temporal and spatial variations of marine aerosols over the Atlantic Ocean. Atmos. Res., 20: 23--37. Aerosol samples, collected daily were analyzed with electron microscopy for particle concentration and size distribution. The chemical compositions of individual particles were determined with X-ray energy spectrometry and specific morphological tests. The results demonstrated clear and significant dependencies of aerosol properties on air trajectories c o m p u t e d from a backtrack model. Air parcels, such as American air pollution and Sahara dust, were identified. Their physical mixing and chemical reactions with marine aerosols as well as aerosol wet deposition are discussed.

RI~SUMI~ O n a analysd par microscopie ~lectronique la concentration en particules et la distribution dimensionnelle d'~chantillons d'a~rosol collect~s quotidiennement. O n a d~termin~ individuellement la composition chimique de particules par spectrom~trie d'$nergie aux rayons X et par des tests morphologiques sp~cifiques. Les r~sultats d~montrent de fa~on nette et significative des correlations entre les propri~t~s de l'a~rosol et les trajectoires des masses d'air d~termin~es ~ l'aide d'un modble. O n a ainsi pu identifier des parcelles d'air marquees par exemple par la pollution de l'air am~ricaine ou par la poussi~re saharienne. O n discute de leur m~lange physique, des r~actions chimiques avec les a~rosols marins, et du d~pSt humide de l'a~rosol.

INTRODUCTION

In the spring of 1983, we participated in a cruise of the U.S. Navy Research Ship Lynch (Fig. 1), (1) to measure temporal and spatial variations of marine aerosol concentrations and size distributions, (2) to classify individual particle compositions according to their size ranges, (3) to correlate physicochemical variations of the aerosols with air trajectories and aerosol sources, and (4) to investigate ion concentrations of marine precipitation.

0169-8095/86/$03.50

© 1986 Elsevier Science Publishers B.V.

24

40'

"

"

~__

1 /'?

80 °

-

,~-~ . ~

lO °

~

,_

60 °

i~l,\.SX %

\

_

,

,~ .~ ~ ' ~

,

_

•

_

_~

50 °

- -

.

40'

30 °

Fig. 1. Approximate cruise track, USNS Lynch, 11 March to 17 April 1983 (courtesy of E.M. Mack, Calspan Corporation). Although much research has been devoted to marine aerosols,-as reviewed by Junge (1969, 1972), Gravenhorst (1977), and Podzimek (1980, 1982), our knowledge of marine-aerosol life cycles is far from complete. This is especially so for the spatial and temporal variations of aerosol concentration and chemical composition because conventional bulk chemical analyses represent the mass of a few giant soluble particles {such as sea salt) but neglect the insoluble and small particles that predominate numerically in the atmosphere. Consequently, aerosol sources, transport, and sinks are difficult to trace, and physical mixings and chemical reactions remain usually undetected. Combining electron microscopy and X-ray energy spectrometry (SEM-XES), our technique can determine particle morphology, concentrations, size distributions, and elemental compositions in a single analysis, and using spot tests, we identify sulfate and nitrate in individual particles. The variations of aerosol physical properties and chemical composition can then be related to atmospheric phenomena, such as air movements, air mixing, chemical reactions, and wet deposition by precipitation.

METHODS OF SAMPLING AND ANALYSIS The aerosol samples were collected on a flying bridge 1 5 m above the sea-surface. Air pumps and filterholders were placed upwind of the vessel exhaust. A Gardener condensation nucleus counter was used to detect any contamination from exhaust. Cruise speeds were > 10 kt and sampling spans were < 1 h, so local contamination was not a problem. Samples were n o t collected when the ship was at station and winds were shifting. The samples were collected in two ways: (1) on Nuclepore filters (pore size 0.1 um diameter) at constant flow rate (10 1 min -~) for 30 rain per

25

sample; (2) on electron microscope sampling screens with a four-stage Cascade impactor, which allows particles to be classified according to four aerodynamic sizes: diameter d ~ 5 #m; 2 p m ( d ( 5 pm; 0.7 p m ~ d ~ 2 #m; 0.2 p m ( d ( 0.7/~m. {Particles having d ( 0.2/~m may escape.) F o u r EM sampling screens of carbon-coated Formvar film were m o u n t e d on each stage. One screen was also coated with barium chloride (BaC12) and another with Nitron (C20H16N4) to identify sulfate and nitrate particles, respectively (Bigg et al., 1974; Mamane and de Pena, 1978). All the samples were later analyzed with an electron microscope and an X-ray energy spectrometer in the N O A A / E R L Boulder laboratory. Rain samples were collected with a polypropylene funnel (d = 25 cm) secured on t o p of a bottle (21). The pH values of the samples were measured immediately; other ion concentrations were determined later in the Boulder laboratory with an ion chromatograph and ion selectrodes. Particle concentrations and size distributions were analyzed with a scanning electron microscope (SEM) using Nuclepore filter samples. Generally, particles with d ~ 0.05 pm can be observed. However, small volatile particles such as certain organics, ammonium nitrate, and bisulfate may escape from the high-vacuum SEM specimen chamber (vapor pressure 10 -6 mm) and thus fail to be counted. The elemental composition of individual particles is determined by an X-ray energy spectrometer (XES) which is interfaced with the SEM. All elements with an atomic number Z ~ 9 in a particle (d ~ 0.1 pm) can be identified in one spectrum. However, c o m m o n light elements such as H, C, N, and O cannot be detected. To compensate for this, we used a transmission electron microscope (TEM) to perform two spot tests (Bigg et al., 1974) for identification of sulfate and nitrate particles, which are chemically important in both natural and anthropogenic aerosols. When a sulfate particle is collected on a BaC12-coated film, it is converted to BaSO4, and a unique hollow ring forms around it. When a nitrate particle is collected on a Nitroncoated film, it forms a fiber-like Nitron nitrate particle and can thus be identified morphologically with the TEM. These two techniques also fix the particles to avoid their evaporation in the high vacuum of the TEM specimen chamber. The analysis procedures have been described by Mamane and de Pena (1978) and Parungo et al. {1980).

AIR TRAJECTORY ANALYSIS

The c o m p u t e r model that produced the atmospheric trajectories for this study was developed by Heffter and Taylor (1975) and later modified by Harris (1982). The primary input to the model consists of gridded wind c o m p o n e n t s at standard pressure levels, produced at the National Meteorological Center (NMC) by a global atmospheric model. The calculated

26

trajectories indicate the general air flow pattern, rather than the exact path of a specific air parcel. The isobaric trajectories used in our case studies go backward in time no more than 10 days.

RESULTS AND DISCUSSION

Although all the samples were collected over open sea, the marine aerosols varied with time and location n o t only in particle concentration and size distribution, b u t also in chemical composition (Parungo et al., 1984). Fig. 2 summarizes the daily variation of aerosol concentrations of large (d > 1 pm) and small (d ~< 1/~m) particles. There is little correlation between surface winds and particle physico-chemical properties. However, when we examined wind direction and speed at 850, 700 and t 0 0 0 m b (Fig. 3), we found a consistent relationship between air trajectories and aerosol characteristics. Figs. 4 and 5 show the diversity of particle size distribution, a n d the frequencies of elements in four selected samples. We use four case studies to explain the wide variations in marine air masses. Case No. 1: Continental--maritime mixed air

On 14 March 1983, the ship sailed eastward from approximately 1000 km east of the Florida coast (Fig. 1). The modeled air trajectories on all three I°3

_

F ~ Rain

.

Rain-

Rain o

101

14 16 18 20 22 24 26

March

28 30

1 2

4

6

8

10 12 14

April

Fig. 2. Daily v a r i a t i o n o f aerosol c o n c e n t r a t i o n s . Major e l e m e n t s are indicated. B o t t o m curve r e p r e s e n t s surface w i n d speeds.

27

A. 14 March 1983 /

IVy,,o_/<

\

,Ma,c s 19 !i 70°W

60°W

50"N

40°W

30,%'V

70%'V

20°W

60°W

50°VV

40':'W

30°W

20°W

I

',

< 70'~V~

~-~--.~.J ~,,l~j

60~VV

r

50(M/

40~V

',

30~N

~~

t--

20~W

70"W

60"W

50'~W

40°W

30",N

20°W

Fig. 3. Isobaric air trajectories o n indicated days. L = 1 0 0 0 mb; M = 8 5 0 mb; U = 7 0 0 mb. Numerals mark days backward in time.

103

102

/

d

/

/

/

i !

101

Z

~ 10 o

• March 14 ---~.-- March 17 March 25 --,o--- March 30

10-r

0.01

0.1

1

10

Diameter (pm)

Fig. 4. Particle size distributions o f four s e l e c t e d s a m p l e s

28 100-

14 March1453-1523 29o19N,69~05W ~ 50 ~ ~

~

.

NaCtonly

~

0

. 100-

17 March 1215-1245

25°00N.57°23W ~

g

0

50-

o

100" 25

~

March 1122-1152 , o

o

50-

0

~

5 0 ~ ~ o o , oo,y

100~

Na Mg A

~

Si

P

~0°M3~a,~r ~1120~W'1200

S C ' K Ca Ti 'Ci 'Fe' I hlone

Fig. 5. Frequencies of elements' presence in particles of the four selected samples.

levels were from the northwest (Fig. 3A), indicating that the air mass could be affected by pollution from the North American industrial area. SEM-XES analysis showed submicron particle concentration, as high as 8 0 0 c m -a (Figs. 2 and 4). Aitken particles (d < 0.2 pm) generally emitted only S X-ray, and some of them sublimed under the electron beam during the analysis. A spot test with BaC12 identified them as sulfate, presumably (NH4)2SO4 or NH4HSO 4. Since the high concentration of these sulfate Aitken particles was not c o m m o n throughout the cruise (Fig. 2), one may suggest that they are the product of anthropogenic pollution, specifically SO2-gas-to-H:SO4-particle conversion, which involves a series of very complex atmospheric processes, such as oxidation of SO2 by atmospheric OH, neutralization by NH3, and homogeneous nucleation to form small ammonium sulfate particles. SEM-XES analysis showed that the particles with a diameter between 0.5 pm and 5 p m were mainly sea salt. They consisted of Na, C1, Ca, S, K, etc.; however, only 10% of them emitted solely Na and C1 X-ray, as pure NaC1. The BaC12 test showed ~ 90% of the large particles also containing SO~-, which could be formed with sea salt by sea spray or by air bubbling. Additional sulfate could also be formed by gas-to-particle conversion either by heterogeneous nucleation of H2SO 4 on sea-salt particles or by catalytical oxidation of SO2 on marine aerosols. Particles with d > 51zm were very scant ( < 1 0 -2 cm-a);

29

they were mostly sea sand (Si, Fe, S, etc.) and organic materials that did not emit any detectable X-ray. Fig. 6 shows particles collected with Nitron-coated film. The fiber-like reaction spots indicate nitrate content. A majority (80%) of the nitrate particles (NO~) were in the size range 1--5/~m. Since sea water contains very low concentration of NO~ (~ 5 pM), primary sea-salt particles should be nitrate-free and most NO~ should be formed by gas-to-particle conversion. Generally, a series of reactions similar to secondary sulfate formation is hypothesized as the mechanism, i.e., oxidation of air pollutants NO and NO2 by OH, 03, or organic peroxides to form nitric acid which may be neutralized with atmospheric NH3. Ammonium nitrate, which is very hygroscopic (deliquescence point is 62% relative humidity), should yield abundant fine droplets in the moist marine air. If this is the case, the nitrate and sulfate particles should have similar size distributions. The fact that most nitrates are supermicron and most sulfates are submicron (see e.g., Junge, 1954; Gravenhorst, 1977; Parungo and Pueschel, 1980; Savoie and Prospero,

o I

& wo~

-0:

O

U~ ~ °J

i.Q

e* •

.$ O ~

~

~

"

~

Fig. 6. Particles c o l l e c t e d o n 14 March w i t h N i t r o n - c o a t e d film at stages 1 - 4 . The fiberlike crystals i n d i c a t e nitrate c o n t e n t .

30 1982) leads us to believe that these two kinds of particles are formed by different processes, at least in the marine environment. Parungo et al. (1980) suggested that liquid-phase oxidation of NO and NO2 in saline drops or wet sea-salt particles to form nitrate particles is more probable than gas-phase oxidation. Being acidic gases, NO and NO2 would select more alkaline aerosols such as sea salt to react with. Thus, small acidic sulfate particles generally escaped from NOx attack and NO~ usually was n o t f o u n d in submicron particles. Our SEM-XES and TEM analyses showed NO~ indeed co,existed with Na, C1, Mg, or Ca and generally formed a coating on sea salt particles. High concentrations of nitrate have been found near the coast (e.g., Junge, 1954, Parungo et al., 1980, 1981, and 1982). On 14 March, air parcels were from the American continent and had traveled approximately 1000 km over sea water. It is possible that the polluted continental air mass enriched with NOx and the maritime air mass with abundant sea salt mixed to yield the high concentration of nitrate-coated large sea-salt particles that we observed.

Case No. 2: Before and after rain On 15 March, the air trajectories were from the northwest at 1000 and 8 5 0 m b and from the west at 7 0 0 m b . It would appear that the air mass constituted a mixture of maritime and North American continental aerosols. The aerosol was chemically similar to the sample collected on 14 March (Case No. 1) except that the concentration of Aitken particles was greatly reduced (~ 150 cm- 1). On 16 March the sky was covered with nimbostratus, and light rain began at ~ 0200 h GMT. For 10 h, intermittent rain was collected, equivalent to ~ 0.5 cm rainfall. The measured pH was 4.0. The 200-ml sample was sealed in a polypropylene bottle and shipped to our laboratory for analysis. The results are shown in Table I. The rain sample was very acidic compared with seawater, pH = 8.0 (the enrichment factor of H30 ÷ was 104). The major ions were Na+ (2.3 ppm), C1- (1.4 ppm), SO~- (0.72 ppm), and NO~ (0.68 ppm). Compared with the ratios for the seawater sample where SOD-/ Na + was 0.21 and NO~/Na + was 2.3.10 -4, the (SO~-) in rainwater was enriched approximately 1.5 times, and the (NO~) was enriched by more than 3 orders of magnitude. The C1-/Na + ratio was 0.61, which is lower than seawater (1.6). This could be due to the fact that gases such as SO2 and NOx in polluted air replace C1 in sea salt (Lodge et al., 1960). The results suggest that cloud drops and rain drops scavenged n o t only s e a salt particles, in clouds and below clouds, but also anthropogenic gases and aerosols. Since the air mass on 16 March was from the American continent, the high acidity of rainwater probably was the result of scavenging of the excess sulfate and nitrate in the polluted air mass. In an aerosol sample taken approximately 3 h after the rain sample, the

31 TABLE I I o n c o n c e n t r a t i o n s o f rain s a m p l e s a n d s e a w a t e r Sample No.

Date (1983)

Time (GMT)

Location (N/W)

pH

Na + (ppm)

C1(ppm)

SO 2(ppm)

1

3/16

0200-1200

26/61

4.0

2.3

1.4

0.72

2

3/25

0530-0540

21/33

4.7

--

20

3

3/25

1750-2200

21/32

4.7

25

4

4/14

1900-2200

54/06

4.7 8.0

Sea . water

.

.

.

NO~ (ppm)

SO~-/ Na +

NO~/ Na +

0.68

0.31

0.30

19.7

2.76

--

--

35

19.7

5.48

0.79

0.22

46

65

38.5

1.0

0.84

0.02

1.3 E4

2.1 E4

2.7 E3

3

0.21

2.3 E-4

total particle concentration was approximately one-fourth the concentration of the sample taken before the rain. The results suggest that wet deposition (specifically, wash-out below clouds) could play an important role as the sink mechanism of marine aerosols. In another aerosol sample taken 3 h later when the ship had sailed out of the rain zone, the particle concentration, size distribution, and chemical composition were similar to pre-rain samples, indicating that the samples probably were taken in a general air mass. Small rain samples ( ( 1 0 m l ) were collected on 25 March and 14 April during short squalls, and ion concentrations were analyzed (Table I). These samples (pH = 4.7) are less acidic than the 16 March sample (pH = 4.0), suggesting that they are under less influence of anthropogenic air pollution. However, the ratios SO~-/Na + and N O ~ / N a ÷ are still very much greater than that of the seawater, indicating enrichment of SO~- and NO~ in the precipitation. Since the sample volumes were t o o small for thorough analysis and since the sample m a y degrade during storage (Keene et al., 1983), these preliminary chemical data must be considered somewhat uncertain. For accurate marine precipitation research, samples should be analyzed on board in a clean room immediately after collection. To obtain statistical significance or general conclusions, numerous case studies are needed. In aerosol samples collected after rain, 50% of the particles with 0.5 pm d < 5 g m were pure NaC1 which emitted only Na and C1 X-rays b y XES. It appears that when drops evaporate, different c o m p o u n d s recrystallize separately according to each solubility product. The drop in Fig. 7 (left) had contained crystals o f NaC1, KC1, CaSO4 etc. before it landed on the sampling screen. (If a drop dried on the screen, we generally observed numerous similar sized particles with mixed chemical compositions through-

32

out the areas of drop impact). In nature, after drops evaporate, the particles form loosely attached conglomerates (Fig. 7, right). Such conglomerates would shatter very easily in the air, and thus pure salts and their mixtures

Fig. 7. Micrograph of particles collected on Nuclepore filters, and X-ray spectra of indicated particles.

33 would be found. Fig. 7 also shows the X-ray energy spectra of the indicated particles. The shattering of salt particles in the atmosphere was first hypothesized by Dessens (1946). Laboratory observations by Facy (1951), T w o m e y and McMaster (1955) and Radke and Hegg (1972) supported the hypothesis, but, Lodge and Baer (1954) and Blanchard and Spencer (1964) were unable to confirm it. The discrepancies are probably due to different experimental conditions in which aerosols were generated. Fractionation by recrystallization of salts in drops depends on the rate of evaporation, which is a function of relative humidity, temperature, ventilation, and chemical composition of the salt
Case No. 3: Long-range transport o f Sahara dust On 17 March the ship was cruising along the 25°N latitude approximately 2000 km east of Florida and 4000 km west of the African coast. At 0000 h GMT, the low-level winds ( 1 0 0 0 m b ) were from the east, and average speed was approximately 40 km h-1. The middle-level (850 rob) winds were from the southeast and also very strong. The air mass could have originated in Africa. Fig. 3B shows the air trajectories at 1200 h GMT, the lower winds were from the east and originated on the east coast of the United States. The middle-level winds were from the southeast and originated on the African coast. It appears likely that the two air masses mixed in the middle of the Atlantic Ocean. Aerosols collected both on filters and on impactor screens showed high concentrations of large particles (Figs. 2 and 4). The particles contained high percentages of Si, A1, and Fe (Fig. 5). Fig. 8 shows the morphology o f the irregular dust and the X-ray energy spectra of the selected particles. Some giant dust particles (d ~ 2 ~m) were attached with numerous smaller particles containing Na, C1, S, Ca, etc. The spot test showed that ~ 70% of the particles were coated with SO24- and ~ 40% were coated with NO~. The large siliceous particles of high concentrations were probably from Sahara dust. Some dust particles were attached to sea salts, or coated with sulfate and nitrate which could be anthropogenic air pollution from the North American continent. The aerosol samples collected on 23--25 March display another pulse of Sahara dust. The air trajectory (Fig. 3C) showed that the lower- and middlelevel air masses were from Africa. Concentrations of giant particles (d 2 pm) were as high as 20 c m - 3 ; the particles consisted of Si, A1, Fe, Na, Cl,

34 S, etc., but ~ 2 0 % were coated with NO~. The aerosols presented as a mixture of dust and sea salt. On the basis of aerosol size distribution (Fig. 4) and the density of the dust ( ~ 2 g cm -3) the estimated mass concentration of the dust is ~ 100 pg cm- 3. Transatlantic transport of Sahara dust was observed by Prospero and Carlson (1972), Schutz (1980) and Prospero et al. (1981). Our study of air

Fig. 8. SEM image of the particles collected on 17 March, and X-ray spectra of indicated particles.

35

trajectories along with aerosol physical and chemical properties gives evidence of such long-range transport, even of giant particles. Case No. 4: Maritime aerosols

The preceding case studies demonstrate that aerosols collected over oceans are n o t necessarily of marine origin. As the ship approached the European coast, one might have expected profound anthropogenic and continental influences. However, in samples collected on 29--31 March and 9--13 April, 75% to 90% of the particles were sea salts; as much as 50% of the particles contained nothing but Na and C1. Concentrations of Aitken particles, typically numerous in polluted air, were low ( ~ 200 cm-3). The air trajectories show that the air mass was from the North Atlantic Ocean (e.g., 30 March, Fig. 3D) and was n o t subject to European, African, or American continental influence. Therefore, the aerosols in the air mass were mainly maritime, and the major elements were Na, C1, Mg, K, Ca, and S. We frequently observed NaC1, KC1, CaSO4, etc., in pure crystal form. They were probably formed by recrystallization in evaporating sea-spray drops, cloud drops, or rain drops, followed by fractionation after drying (as in Case No. 2). SUMMARY AND CONCLUSION

Daily aerosol samples varied widely in size distribution and concentration, and in chemical composition. Aerosol characteristics showed clear and significant dependences on air trajectories. Case studies of marine aerosols produced these findings: (A) When maritime air mixed with polluted continental air, concentrations o f Aitken particles (which were predominantly sulfate) increased. The large sea-salt particles were also coated with sulfate and nitrate. (B) Sahara dust containing Si, A1 Fe, etc., was a major fraction of the aerosol collected on various days. The pulses of large dust arrivals correlated with eastern winds. Sometimes the dust was attached to maritime aerosols or coated with sulfate or nitrate. (C) The maritime aerosols less subject to continental influenced were mainly sea salts. However, individual particles did n o t have the same chemical composition as ocean water. Mechanisms other than the simple drying of sea spray could be responsible for particle formation of widely diverse chemical compounds. For example, pure crystals of NaC1, KC1, CaSO4, etc., were probably formed by subsequent crystallization of c o m p o u n d s in sea-spray drops (according to each solubility product). After evaporation, the solid residues of different c o m p o n e n t s fractionated and shattered to release pure Crystals. The high concentrations of pure crystals of NaC1, CaSO4, and KC1 were generally observed after rain, or on days when continental influence was less.

36

(D) Four rainwater samples were found to be acidic (pH 4.0 to 4.7). Chemical analyses, comparing rainwater with seawater, showed rainwater to be enriched with H 3 0 ÷, SO~- and NO~. More research is needed to understand the source and formation of acid rain over the Atlantic Ocean.

ACKNOWLEDGEMENTS

We thank Dr. Lothar Ruhnke of the Naval Research Laboratory for providing this research opportunity and financial support. Mr. Bruce Rogenwasser collected the samples; his work is greatly appreciated. Discussions with Dr. Jan Rosinski, Dr. William Hoppel, and Dr E.J. Mark have been very helpful. Teresa Davenport assisted in the production of the air trajectories. REFERENCES Bigg, E.K., Ono, A. and Williams, J.A., 1974. Chemical tests for individual submicron aerosol particles. Atmos. Environ., 8 : 1--13. Blanchard, D.C. and Spencer, A.T., 1964. Condensation nuclei and crystallization of saline drops. J. Atmos. Sci., 21: 182--186. Dessens, H., 1946. Les noyaux de condensation de l'atmosphbre. C.R. Acad. Sci. (Paris), 223: 915. Facy, L., 1951. Embruns et noyaux de condensation. J. Sci. Meteorol., 3: 86--98. Gravenhorst, G., 1977. Marine aerosol. Proc. 9th Int. Conf. Atmos. Aerosols (Galway University Press), pp. 468--476. Harris, J.M., 1982. The GMCC Atmospheric Trajectory Program. NOAA Tech. Mere. ERL-ARL-116, NOAA Environmental Research Laboratories, Boulder, Colo., 30 pp. Heffter, J.L. and Taylor, A.D., 1975. Trajectory Model, Part I. A Regional-Continental Scale Transport, Diffusion, and Deposition Model. NOAA Tech. Mere. ERL-ARL-50, NOAA Environmental Research Laboratories, Boulder, Colo., 28 pp. Junge, C.E., 1954. Recent investigations i n air chemistry. Tellus, 8 : 1 2 7 - - 1 3 9 . Junge, C.E., 1969. The physical and chemical properties of atmospheric aerosols and their relation to condensation processes. Suppl. Proc. 7th Int. Conf. Cond. Ice Nuclei~ Prague--Vienna Academia, Prague, pp. 31--51. Junge, C.E., 1972. Our knowledge of the physico-chemistry of aerosols in the undisturbed marine environment. J. Geophys. Res., 77: 5183--5200. Keene, W, Galloway, J. and Holden, J., 1983. Measurement of weak organic acid in precipitation. J. Geophys. Res., 88: 5122--5130. Lodge, J.P. and Baer, F., 1954. An experimental investigation of the shatter of salt particles on recrystallization. J. Meteorol., 11: 420--421. Lodge, J.P., MacDonald, A.J. and Vikman, E., 1960. A study of the composition of marine atmospheres. Tellus, 12 : 184--187. Mamane, Y., and de Pena, R., 1978. A quantitative method for the detection of individual submicrometer size sulfate particles. Atmos. Environ., 12 : 69--82. Parungo, F.P., and Pueschel, R.F., 1980. Conversion of nitrogen oxide to nitrate particles. J. Geophys. Res., 85: 4507--4511. Parungo, F.P., Pueschel, R.F. and Wellman, D.L., 1980. Chemical characteristics of oil refinery plumes. Atmos. Environ., 14: 509--522.

37 Parungo, F.P., Nagamoto, C., Schnell, R. and Nolt, I., 1981. Atmospheric aerosol and cloud microphysics measurements. HAMEC Project Report III. NOAA Environmental Research Laboratories, Boulder, Colo. Parungo, F., Nagamoto, C., Nolt, I., Dias, M. and Nickerson, E., 1982. Chemical analysis of cloud water collected over Hawaii. J. Geophys. Res., 87: 8805--8810. Parungo, F., Nagamoto, C., Harris, J., Rosenwasser, B. and Ruhnke, L., 1984. Analyses of aerosol and precipitation samples collected during a transatlantic research cruise. NOAA Tech. Memo. ERL-ESG-5. NOAA Environmental Research Laboratories, Boulder, Colo, 61 pp. Podzimek, J., 1980. Advances in marine aerosol research. J. Rech. Atmos., 14: 35---61. Podzimek, J., 1982. Marine aerosol research Idojaras, 86 : 179--199. Prospero, J.M. and Carlson, T.N., 1972. Vertical and areal distribution of Sahara dust over the western North Atlantic Ocean. J. Geophys. Res., 77: 5255--5265. Prospero, J.M., Glaccum, R.A. and Nees, R.T., 1981. Atmospheric transport of soil dust from Africa to South America. Nature, 289: 570--572. Radke, L.F. and Hegg, D., 1972. The shattering of saline droplets upon crystallization. J. Rech. Atmos., 6: 447--455. Savoie, D.L. and Prospero, J.M., 1982. Particle size distribution of nitrate and sulfate in the marine atmosphere. Geophys. Res. Lett., 9 : 1207--1210. Schutz, L., 1980. Long range transport of desert dust with special emphasis on the Sahara. N.Y. Acad. Sci. Ann., 338: 515--532. Twomey, S. and McMaster, K.N., 1955. Production of condensation nuclei by crystallizing salt particles. Tellus, 7 : 458--461.