Marine Chemistry, 21 (1987) 20~211 203 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Nether lands

ATMOSPHERIC TRANSPORT AND INPUT OF HYDROCARBONS TO THE SUBTROPICAL NORTH ATLANTIC

FATHALLAH BOUCHERTALL

Insti tut fi~r Meereskunde an der Universiti~t Kiel (F.R.G.)

(Received September 10, 1986; revision accepted March 17, 1987)

ABSTRACT

Bouchertall , F., 1987. Atmospheric t ranspor t and input of hydrocarbons to the subtropical North Atlantic. Mar. Chem., 21: 20~211.

Seawater sanpples and airborne part iculate mater ial were collected in the subtropical North Atlant ic during R.V. "Meteor" Cruise M60 (N34°47.2'W26°57.7'/N10°l.3'W32°58.3'). Hydrocarbon concentra t ions were estimated in the samples. For seawater the concentra t ions ranged from 0.2 #g to 3.5 pg dm ~. In the open ocean air the concentra t ions of the part iculate hydrocarbon measured at 14m above sea level ranged from 2.8ng to 133.1ngm 3. A significant increase was observed during a Saharan dust outbreak. Comparison with a luminium concentra t ions in seawater and in the air suggests input of atmospheric hydrocarbons by dry deposition to be an important trans- porta t ion pathway.

INTRODUCTION

Fluxes of atmospheric solid particles across the air-sea interface can be of geological, pedological and ecological importance. Aerosols originate from both natural and anthropogenic sources. Saharan dust originating from North Africa is a major source of natural atmospheric particles over the Atlantic and especially of the tropical North Atlantic troposphere. The eolian transport of desert aerosols is responsible for episodes of 'red rain' or 'red snow' which have been described in various areas of the European continent (Prodi and Fea, 1979; Pilot et al., 1986). Lepple (1975) estimated that during North African dust storms vast amounts (up to 480Mta -1 near the African coast) of dust are transported out to sea. Eolian dust particles in marine sediments were initially identified by Radczewski (1939). Various studies documented that soil material transported from arid regions by wind is primarily responsible for the distribu- tion of certain clay minerals in oceanic sediments (Delany et al., 1967; Windom, 1975; Chester et al., 1979; Prospero, 1981).

It has been realized that aerosols might contain organic substances. An average organic carbon content of 2.6% was determined in eolian dust (Lepple and Brine, 1976). Lipids (n-alkanes, n-alcohols and n-fatty acids) of eolian dust samples from air masses over the eastern Atlantic were analysed by Simoneit (1977). The composition of the lipids indicated that higher plant vegetation

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and desiccated lacus t r ine mud flats on the Afr ican con t inen t were the i r main sources.

Var ious pa r t i cu la t e mater ia l s der ived from te r res t r i a l sources have been identified in eol ian dusts on the surface of the ocean (Simoneit , 1977; Mar ty et al., 1979). The input of hyd roca rbons (HCs) into the sea a t t r ac t ed increas ing a t t en t ion because of the i r toxic effects on aquat ic ecosystems (Moore and Ramamoor thy , 1984). The t r anspo r t and input of pa r t i cu la t e HCs into the subt ropica l Nor th At lan t ic is the subject of this study.

EXPERIMENTAL

Dur ing a cruise of R.V. " M e t e o r " (M60) in the Nor th At lan t ic (Fig. 1), a i rborne par t ic les and seawate r samples were collected. Ten aerosol samples were t a ke n a long the cruise t r ack when the ship was moving. Three wa te r samples were t aken on each s t re tch. The wa te r samples were t ak en in t r ip l ica te in 2.8 dm 3 glass bot t les from the foredeck of the ship at a l m depth. A descrip- t ion of the method has been c i rcu la ted by UNESCO (Manuals and Guides No. 7). The t ime of co l lec t ion was shor t ly before the ship s topped at a s tat ion. Immedia te ly a f te r sampling the unf i l tered seawate r was ex t rac ted in the sam- pling bot t les twice wi th disti l led hydrocarbon- f ree me thy lene chlor ide (100 cm 3/2.6 dm 3 seawater) . The ex t rac t was dried wi th sodium sulphate and concen t r a t ed slowly unde r vacuum at 3°C to about 1 cm 3.

The c onc e n t r a t ed ex t rac t s were t r ans fe r red to a co lumn packed wi th acti- va ted sil ica gel to remove the polar components from the sample. The e lua te was evapora t ed under vacuum at 3°C to dryness and the res idue subsequent ly dissolved in 5cm 3 n-hexane. The samples were then t r ans fe r red into glass ampoules. The air in the ampoules was displaced with pure N2 and the

4o' I I

B e r m u d o lslonds o

30 ' -- N O R T H

A T L A N T I C

O C E A N

2 0 ' -

" .

Lesser ,~ Anl i l les ,

S O U T H A M E R I C A

i 7 0 ° 6 0 ° 5 0 °

• I . ~ . A z o r e s I ° %lop.. ,

O ~ "-. I

I I I 4 0 ° 3 0 ° 2 0 ° 1 0 °

Fig. 1. Sampling stretches. The winds prevailing during the sampling periods are described by meteorological symbols.

205

ampoules were flame sealed and stored at - 10°C. Three b lanks were prepared with each sample by r ins ing the sample bot t le wi th 100 cm 3 CH 2 C12. The rinses were subsequent ly t rea ted as a sample.

Atmospher ic pa r t i cu la te ma t te r was collected by fil tering the air t h rough 7 cm diameter glass fibre filters (Wha tman GF/F, pre-extracted with CH2 C12 and baked at 400°C in a muffle furnace for 36 h). Fil ters were mounted in stainless steel holders. The sampling system was located on the bow of the ship about 14 m above sea level. A control t ha t had an ident ical filter but wi thou t an air pump was placed at 40 cm dis tance from the sampling system. An au tomat ic cont ro l switch ensured tha t air pumping took place only when the ship was moving and when the wind di rec t ion did no t deviate by more than 25 ° from the ship 's forward di rec t ion to prevent con t amina t i on of the air from sources on the vessel. After sampling the filters were solvent-extracted with 100cm 3 of CH2C12 and were t rea ted in the same way as the wate r extracts.

Upon r e tu rn to the labora tory , the samples were analysed using a Fa r r and Foci Mark I f luorescence spec t rophotometer . The water ext rac ts were excited at 225 and 310nm, and the f luorescence emission was measured at 342 and 364nm, respectively. The aerosol ext rac ts were analysed by exci ta t ion at 225 nm. Concen t ra t ions of the hyd roca rbons (HCs) were est imated with AGHA J A R I crude oil ( I ran ian oil; sulphur: 1.4%; specific gravi ty 0.855 g cm 3) as a s tandard. The crude oil was subjected to the same ana ly t ica l procedures.

RESULTS AND DISCUSSION

The sampling s t re tches are shown in Fig. 1. A total of 10 aerosol samples were collected, and 96 water samples were t aken at 32 stat ions. Concen t ra t ions determined by f luorescence analysis of the seawater samples were averaged for each stretch.

Es t imates of h y d r o c a r b o n concen t r a t ions are shown in Table I. Concentra- t ions are in uni ts of I r an i an crude oil equivalents . Gordon et al. (1974) repor ted 0 .8pgdm 3 as average concen t ra t ions of hyd roca rbons est imated by fluor- escence spect roscopy in the At lan t ic Ocean at 1 m depth. This value corre-

TABLE I

Estimates of hydrocarbons in subtropical North Atlantic water and in atmospheric particulate matter, concentrations are in units of AGHA JARI Crude oil

Sample Exci- 1 2 3 4 5 6 7 8 9 10 tation (nm)

Seawater 225 0.5 0.4 0.6 1.3 2.6 0.7 0.8 1.9 3.5 2.0

(#gdm 3) 310 0.2 0.2 0.4 0.8 1.0 0.5 0.4 0.7 2.8 0.5

Aerosol 225 2.8 3.4 49.5 133.1 17.2 11.0 6.9 10.3 8.3 9.1 (ngm ~)

2U~-;

#g /dm :~ HC •

3 i- - (2a) A

1

~ J _L ....... ; ......... Z .... _L-_ I, _ _ _ ~ i

0 ~-- ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ng/dm 3

35 - /'J~\,

30 - X •

25 -

2° I 1 5 1 i

A A ,

/ , \ / .

2 3 4 5 6 7 8 9 10

S t r e t c h Number

Fig. '2. C o n c e n t r a t i o n s of hydro~ 'a rb(ms t2ai ~md a l u m i n u m (2bt m seawa te r .

sponds to the c o n c e n t r a t i o n s ob ta ined here at the si tes ], 2, 3, 6, and 7. 'The H( c o n c e n t r a t i o n s in the s e a w a t e r samples ob ta ined by the exc i t a t ions a t 225 n m 342 nm show h igher va lues t han with exc i ta t ion a t 310 nm/364 nm. This obser va t ion agrees with p rev ious resu l t s (Ahnoff and Johnson , 1977; Tervo. !978).

The f luorescence method gwes an es t ima te of hyd roca rbon concentration.~:

130 ng/r 110

90

70

50

30

10 0

13 HC

- ~0~0~0.~0_. • 1 l 1 I J I 1 J I I

207

3800 f ng/m 3

35OO

1500~

5O0

20O

0 -

A L

® r\

_t

J • 0~0~0~0~0~0 1 I I I 1 I I 1 I

2 3 4 5 6 7 8 9 10 Stretch Number

Fig. 3. Concentrations of particulate hydrocarbons (3a) and aluminum (3b) in the open ocean air.

based on the fluorescence of aromatic hydrocarbons. Because of their high sensitivity, fluorimetric techniques have been widely applied to the analysis of petroleum residues in seawater (Gordon and Keizer, 1974; Brown and Hofmann, 1976; Levy et al., 1981; Fogelquist et al., 1982; Corredor et al., 1983). Aromatic compounds have widely varying fluorescence characteristics depending upon

208

their structure. When a sample is excited at 225 nm and emission is measured at 342 nm, most of the flluorescence observed originates from benzenes, t e t r a lenes, indanes, fluorenes, naphthalenes, biphenyles, and dibenzthiophene:, (Bouchertall, 1980). Three and four ring aromatics, phenanthrenes, anthr~ cenes, pyrenes, chrysenes, etc. are measured at 310nm/360nm (Gordon a m Keizer, 1974; Bouchertall. 1980).

The advantage of using methylene chloride to extract seawater ~md gtas,~ fibre filters is its low boiling point, but due to its high dielectric constant (9.08,, it also extracts non-hydrocarbon material from seawater in additio~ i~ hydr~ carbons (Gordon and Keizer. 1974). Aliphatic and aromatic hydrocarbons m the collected samples were separated from polar substances by liquid liqmd chr~ matography_

The non-polar fractions ,~t' water extracts from the areas 4 ~n~i 7 wet~ combined, concentrated to about 100pl and analysed by combined gas chr¢~- matography mass spectrometry (GC-MS). The GC-MS was a HEWLETq PACKARD 5992A. Signals were detected by selectively monitoring i(m inter~ sities at m/e 128, 142, 156, 178~ and 202 corresponding to naphthalene. ~ aad C~-naphthalenes, an thracene/phenanthrene and pyrene/fluoranthene, Reter~ tion times of aromatic standards corroborated the GC-MS detect i(m These compounds were also identified by the GC-MS in the fourth aeroso! sample

In parallel to fluorimetric hydrocarbon determinations the concen t ra tmm of trace metals were determined in seawater samples by Kremling t1985). Th~ aluminium concentrat ions in the seawater are averaged here tbr each stretc~ (Fig. 2b). We also collected aerosols on Teflon filters (Sartorius, SM !] !8(}7) a.~- described for glass fibre filters in the experimental section. Schneider (1985) analysed trace metals in the aerosols by neutron activation. Figure 3b sbow~ the averaged atmospheric: concentrat ions of A1 for each sampling x~tretch Figures 2a and 2b show that HCs and Al in seawater have coinciding maxmm (stretches 5 and 9) indicating a clear correlat ion between high levels of HC~, and of A1 in seawater. The increase at sites 4 and 5 (Fig. 2a, b) of HCs and Ai concentrat ions in the surface water corresponded with the dramatic increas~ of the atmospheric HCs and A1 concentrat ions (Fig. 3a, b). This strongi:~, suggests that under certain conditions HCs and A1 in ocean water have pr~.~- dominantly the same origin and t ransportat ion system. Continental crust is th,,_ potential source for At in the surface ocean water (Kremling, 1985). Duce et ~d (1980) used the atmospheric A1 concentrat ion as an indicator of continentai dust produced from soil and crustal weathering processes. The most tiketv source of the extremely high AI value at sites 3 and 4 was an outbreak of Saharan dust which was transported with the trade winds. The daLa suggest that there is a significant t ransport of HCs to the ocean.

Monoaromatic compounds and lower molecular weight PAH (naphthalene to pyrene) have sufficiently high vapour pressures so that they can exist~ similar to chlorinated hydrocarbons, at significant, concentrat ions in the vapour phase (Neff, 1979). Eichmann et al. (1979) could not find aromatic hydrocarbons in air masses of predominantly marine origin, which indicates

209

that the ocean is not a source of these compounds. This would suggest that the interact ion between organic pollutants and dust is important for the distribu- tion of organic pollutants. The organic consti tuents of airborne dust and the wind erodable fraction of North African surface sediments were compared by Lepple and Brine (1976). They found that the average organic carbon content of dust particles in the atmosphere is significantly higher than the correspond- ing average for the desert surface material.

We suggest that pollutant emissions from various sources could be absorbed in the atmosphere by aerosols from African dry areas. The proport ionali ty of HCs and aerosol aluminium concentrat ions in the ocean environment indi- cates that the fine Sahara dust particles possess active surfaces capable of adsorbing atmospheric organic matter. A chemical react ion of Saharan dust and anthropogenic matter in the atmosphere was recognized by Piloe et al. (1986). They described the eolian dust neutral izat ion of rain acidity. Gagosian et al. (1981) found strong evidence for the t ransport of terrestrial n-alkanes, n-fatty alcohols and fatty acid esters 5000 km from a continental source. The concentra t ion of these lipids showed the same trend with atmospheric A1 and airborne dust concentrat ions determined as those described by Duce et al. (1980) during the same ENEWATAK experiment. Talc, which is used as a diluent and carr ier for pesticides, has been identified in eolian dusts sampled on mountains of various continents and at several oceanic sites (Windom et al., 1967). The average concentrat ions of Pb, Sn and Zn were found to be an order of magnitude higher in eolian dusts from the Atlantic ocean than crustal part iculate material (Chester and Stoner, 1973, 1974). The enrichment in atmo- spheric aerosols of the Saharan dust layer compared with clean air ranged from about tenfold for NH4 to more than a factor of 400 for NO.3 (Talbot et al., 1986).

In an ocean region ~3000km west of Africa, Lepple and Brine (1976) measured dust concentra t ion averaging 37#gm :~, with an organic carbon content of 3.3% (~ 1.2gg organic carbon m 3).

If A1 was used as a reference for aerosols produced from soil and crustal weathering processes (8% of the mass or less of crustal material), the aerosol concentra t ion at the third section would be 19.3pgm -3 and at the fourth section 48.4 pg m 3. The HCs concentrat ion obtained at these areas were 49.5 and 133.1ngm 3. This means a HC content of ~0.27% of the aerosols by weight. With an average organic carbon content of airborne dust of 3.3% (Lepple and Brine, 1976) it can be calculated that 11% of the atmospheric organic carbon transported to the ocean area was HCs. Prospero and Carlson (1972) estimated an annual dust flux into the North Atlantic of between 25 and 37 Mt. If the HC content is ~ 0.27%, the calculated annual atmospheric input of HCs via Saharan dust would be from 0.07 to 0.1 Mt. We must emphasize that our estimates represent a gross, not net, input of hydrocarbons to the ocean.

The larger particles in the dust probably deposit in nearshore waters. How- ever, the finer dust particles may still constitute, or contr ibute to, the major portion of organic material in mid-ocean aerosols (Lepple and Brine, 1976). In the tropical North Atlantic Talbot et al. (1986) found that the mass median diameter of the dust was 3.2 pm. For particles with dry radii between 0.5 and

210

about 5#m, S l inn and Sl inn (1980) predict a depos i t ion ve loc i ty of about 1 c m s ~ at wind speeds of 5 - 1 0 m s ~ From this i n f o r m a t i o n and from the results l isted in Table l, we es t imated an average dry depos i t ion of part icu late HCs during the dust s torm act iv i ty at s i tes 3 and 4 of 913.10 ~ g c m ' ....

Hof fmann and Duce (1977) and Chesse le t et al. (1981) tbund that the major mass of total organ ic carbon is present on mar ine a tmospher i c paruc l e s with radii < 1 pm. U s i n g a va lue of depos i t i on ve loc i ty from 0.05 to 0.5 cm s /'or such part ic les and apply ing them to the es t imated a tmospher i c c o n c e n t r a t i o n s at site 1, 2, 7, 8, 9 and 10 the resul ts would be dry depos i t ion va lues from 34-10 ' to 340-10 ~ g cm ~. s. In o ther words, the range o f dry depos i t ion of HCs during S a h a r a n dust s torm act iv i ty is about 270 to 2700 t imes h igher than under normal condi t ions .

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