Environ. Sci. Techno/. 1995, 29, 207-215
Air- Water Equilibrium of Hexachlorocyclohexanes and Chloromethexvbentenes in the JORN SCHREITMULLER AND KARLHEINZ BALLSCHMITER' Department of Analytical and Environmental Chemistry, University of Ulm, Albert-Einstein-Allee 11, 0-89069 Ulm, Germany
W e have measured the meridional changes of the concentrations of four semivolatile organochlorinated compounds (SOCs) in the air and the surface water of the Atlantic Ocean between 50" N and 50" S. The surface water of the North Atlantic Ocean is undersaturated with a-hexac hlorocyclohexane (a-HCH), y-hexachlorocyclohexane (y-HCH), tetrachloro-1,C dim ethoxy benzene, and penta c hIor omethoxy benzene (PCA), especially for limited inputs by continental air. The South Atlantic is close to a gas-water exchange equilibrium for these four compounds. This is indicated by the plots of the logarithm of the concentrations in air versus the inverse of the absolute temperature and by the comparison of the measured contents in air and water. Particularly under conditions of a diminuishing input of SOCs from continental sources, the air-surface water equilibrium will render the oceanic system to a global nonpoint source of anthropogenic compounds in marine air.
Introduction The general aspects of the transport and the fate of organic molecules in the global environment have recently been reviewed ( 1-3). The occurrence of semivolatile organochlorinated compounds is established in the troposphere and the ocean water and, consequently, in the marine biota all over the earth (4-9). Currently being studied by several groups are the details of the transport in the atmosphere, the removal from the atmosphere, and the function of the global oceanic system as a sink and as a global nonpoint source. Recent studies indicate the general influence temperature has on the concentrations of semivolatile organic compounds (SOCs) in the troposphere by influencing the equilibrium between the air and the surface of the globe given by the ocean water (10) and by the continents (1116). Wania and Mackay have forwarded a global model that includes the temperature dependence of the multiphase distribution as a major parameter ( 1 7). An ideal region to study equilibria between the atmosphere and the ocean water in a global scale is a meridional transect through an ocean. The land masses of the Northem Hemisphere act for most of the SOCs as a source while the oceans, particularly of the Southern Hemisphere, function more or less as a dilution and equilibration space for the overspill from the Northern Hemisphere. We have measured the meridional changes of the concentrations of four SOCs in the air and the surface water of the Atlantic Ocean between 50' N and 50' S. The four compounds are as follows: a-hexachlorocyclohexane (aHCH), and y-hexachlorocyclohexane (y-HCH), and the biotransformation products pentachloromethoxybenzene (pentachloroanisole, PCA) and tetrachloro-1,4-dimethoxybenzene (TCDMB). The sampleswere taken on the German RVPolarsternduring cruises across the Atlantic Ocean (1990, 1991) and on the Capo Verde Islands in the North Atlantic Ocean (1992) (10). The temperature profile in the surface water along the north-south-north cruise is used to study the temperature dependence of the air-surface water exchange. A temperature dependent air-water equilibrium can act as a source of the compounds. The output by removal (characterized by the chemical lifetime tchem given by reaction with OH radicals, and photons) and outflow (characterized by the physical lifetime Zphys given by wet and dry deposition) from the air has to be discussed. The measurements of the SOCswill not be averaged but discussed as spatially and temporally connected single events. A major factor will be the regional input of SOCs by inflowing air masses from the continents-from Europe and northwest Africa to the North Atlantic and from South America and South Africa to the South Atlantic. The actual synoptic weather situation is used to discuss the possible input of SOCs by continental air. One has also to consider the dilution effects of contaminant-depleted air given by descending air masses of the subtropical anticyclones, which may also include portions from the lower stratosphere. The delay of the interhemispheric exchange due * Correspondingauthor;Fax: +49-731-502-2763; e-mail address:
[email protected].
0013-936x/95/0929-0207$09.00/0
D 1994 American Chemical Society
VOL. 29, NO. 1. 1995 I ENVIRONMENTAL SCIENCE & TECHNOLOGY
207
TABLE 1
Physicochemical Properties of a=HexacMorocyclokexaae (a=HCH), y.Hexachlorocyclohexane (yHCH), Pentachloroanisole (PCA), and Tetrachlom-1,Q-dimethoxybenzene (TCDMB) CAS No. a-HCH y-HCH
sum formula
319-84-6 CeHsCle 58-89-9
C6H6Cl6
1water MM
mp ("C)
(moth+)
290.8 157-160'
8.4 x IO-' 1.9 x IO-'
290.8 112.5'
PCA 1825-21-4 C7H3C150 280.4 108-1 10" TCDMB 994-78-5 C8H6C1402 275.9 165'
b'
&O(Pa) 7.3 x 2.3 x 2.5 x 8.7 x 1.3 x
IO-' 10-1
d
IO-' ' IO-' IO-' '
Lw
3.8 x ' 6.3 x 2.7 x 10-4 e 8.3 x 7.9 x IO3 1.3 x e 2.8 xi05 k 9.0 10-4 m
koH &ob
(c"
6.0 x IO-"
6x
1.1 x
6x
s-1)
a &(seawater) =Z (distilled water) x 1.4 ( 7 8 ) . * bo= &J&. Ref 19, at 20 "C. Ref 20, at 25 "C. a At 20 "C,calculated from the temperature dependency given in ref 21. Ref 22. Ref 23, at 23 'C. Ref 24. Determined by high-resolutiongas chromatography. Ref 25. [Ref 26. Measured in theSouth Atlanticfrom airsample 17 andwater sample3 (forthissample pair, the contentsofthe HCH in air and water areclosestto theequilibrium values).
CI& lc : CI
CI
a-HCH
PCA FIGURE 1.
c& lcl:
CI y-HCH
TCDMB
to the separation of both hemispheric parts of the troposphere by the intertropical convergence zone (ITCZ) is considered.
Experimental Section The physicochemical properties and the structural formulas of the four compounds discussed here are given in Table 1 and Figure 1. Sample Collection and Sample Preparation. The air and water samples were taken during cruises of the RV Polarstern from Germany to Punta Arenas, Chile (fall 1990; Polarstern cruise ANT M/1) and from Cape Town, South Africa, to Germany (spring 1991;Polarstern cruise ANT E/ 4). The sampling locations are depicted in Figure 2, which also presents an average scheme of the global mass flow in the lower troposphere. In addition, two air samples were taken on the Capo Verde Islands in the northeast Trade Wind Region in March 1992 to verify the specific situation of the northern Trades close to the African continent. All relevant metereological and hydrospheric data were measured every 5 s by the data system of the vessel. These data were averaged over a period of 0.5 h. Some of the data (watertemperature, wind direction, wind velocity) are given in Table 2. Air Samples. The air samples were collected on the upper-most deck of the RV Polarstern (about 20 m above sea level). The experimental conditions and details of the sampling procedure for air samples are given in refs 10 and 27. We used high-volume air sampling (500- 1200 m3)with adsorption on 100 g of prepurified and preactivated 20-50 mesh silica gel. Particles larger than 0.5 pm were filtrated by a glass fiber filter. A second layer of 100 g of silica gel 208 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 1, 1995
separated by a second glass fiber filter was used to control the sampling efficiency of the f i s t silica gel layer. In most of the samples, the amount of the hexachlorocyclohexanes in the second layer was around 10% of the amount in the first layer, only in some samples from the tropics the amount in the second layer attained higher values (maximum30%). In all cases, the values of the two layers were added together. Due to the slightly lower affinity of silica gel toward the chloromethoxybenzenes, their amount in the second layer was around 20%. In some samples from the tropics, the breakthrough of these substances was too high (> 40%) for a correct quantification. Specific care was taken to exclude contamination during the sampling, sample storage, and the sample preparation steps. The adsorption material was kept before and after sampling in flame sealed glass bottles. All sample preparation steps were accomplished under a metal-glass-only clean bench with charcoal airfilters. The final quantitation was done by high-resolution gas chromatography (HRGC) with electron capture detection (ECD) (see below). Laboratory blanks and field blanks showed that the compounds of interest did not occur in the samples. The limit of quantification byHRGC/ECD is < lpg. Depending on chemical interferences in air samples, about 2 pg of these substances can be detected and quantified. The overall limit of quantification based on an aliquote of 10 m3of air is for the four SOCs 0.2 pg/m3. It can be extended by using greater aliquots of air. The recovery rate of the whole sample preparation procedure was checked four times-excluding the sampling step-with purified adsorbents that have been spiked with the SOCs. The values were [in %I as follows: a-HCH-, 91 f 7; y-HCH, 99 f 7; PCA, 94 f 6; and TCDMB, 89 & 6. The measured values were not corrected for these recovery rates. Water Samples. Surfacewater was collected from 10-m water depth using the pumping system from the ship. The organochlorines were adsorbed on prepurified XAD-2 polystyrene-divinylbenzene resin. The temperature of the surface water was measured at the same depth by the data system of the research vessel. The adsorption material was extensivelypurified by Soxhlet extraction with a succession of ultrapure solvents (methanol, acetone, hexane) and then packed in glass cartridges (volume60 mL). Due to the easy contamination of the adsorbent, every cartridge was counterchecked for blanks by a 2 x elution with hexane. The second eluate was concentrated and checked by GCECD for purity. If the eluent were to be free of blanks, the packed cartrige was eluted one more time with acetone
4
1
I
I1
60" 45"
and next with methanol. The methanol-filled glass joint capped cartridges were then sealed with Teflon for the transport. At a flow rate of about 3 bed vollmin, water volumes of 60-120 L were sucked through the cartridges. These conditionsshowed to be effective to prevent a breakthmugh of the compounds (28). After the cartridges were sampled, they were kept filled with seawater and were stored at +2 "C till the sample preparation was done. The extraction was done by elution with acetone (2bed vol) and hexane (2 bed vol). The cleanup of the sample extracts was done by adsorption chromatographywitha combinationofbasic alumina oxide (1g: 4% water) and acid alumina oxide (2 g, 4% water) (29). The first fraction eluted with 7.5 mL of hexane contains the chlorinatedbenzenes, PCBs, and other low polarcompounds. The second fraction (50mL, hexane with 4% dichloromethane) contains the more polar com-
pounds like the HCHs and the chlorinated methoxybenzenes. More details and recovery rates of the sample preparationaregiveninref28.Thelimitofdetection based on a water aliquot of 2 1 is for the four SOCs about 1 pg1L. High-Resolution Gas C h m m a t ~ a p h y .Standard solutions were prepared by weighing the individual compounds. a-HCH, y-HCH, and pentachloroanisole (purity > 99%) were obtained from Riedel-de-Haen (Germany). Tetrachloro-1,4-dimethoxybenzenewas synthesized by reaction of tetrachlorohydroquinone with methyl iodide. The purified reaction product was checked by GC-ECD and GC-MS for identity and for purity. The conditions of the high-resolution gas chromatographyarelistedinreflo. Thequantitationofthesubstances was done hy ECD, using relative response factors of the compounds to the internal standard 1,2,3,4-tetrachloronaphthalene (TCN). TCN has been added in a known VOL. 29. NO. 1.1995 I ENVIRONMENTAL SCIENCE B TECHNOLOGY m 209
TABLE 2
Concentrations of a=HCH, y-HCH, Pentackloroanisde (PCA), and Tetrachloro-1,Q-dimethoxybenzene (TCDMB) in Tropospheric Boundary Layer of the Atlantic Ocean sample no.
sampling location [mean lat. mean long.]
wind direction; wind velocity ( d s )(range)
surface water temp ("C) [mean value]
a-HCH (Ps/d)
y-HCH (Ps/d)
PCA (pdm?
TCDMB (pdm3)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29
46" N 12" W 41" N 18" W 32" N 24" W 27" N 26" W 12" N 29" W 2" N 29" W 9" S 31" W 23" S 37" W 27" S 40" W 31" S 44" W 38" S 51" W 46" S 60" W 45" S 8" E 50" S 3" W 47" s 2" w 42" S 0" 31" S 5" E 26" S 7" E 19" S 7" E 7" N 18" W 11" N 19" W 27" N 17" W 33" N 14" W 35" N 13" W 35" N 13" W 38" N 12" W 43" N 10" W 17" N 23" W 17" N 23" W
E 6-1 1; NE 3-6; SW 4-9; NW 11-17 W 12-15; NW 9-15; W 9-11 SW 6-9; W 4-9 NE 4-6; N 3-5; NE 4-8 NE 5-7 E 5; SE 4-8 E,SE 7-12 N 4-9; SE 11-14 SE 11-12; E 8-13; NE 6-10 E,NE 3-14 E 6-8; NE 6-9; N,NW 8-14 W,NW 8-14 sw 7-10, w 9-11 W,NW 6-9; N,NE 3-15; NW,W 9-16 W,NW 15-27; SW 16-26 SW 8-20 NW,N 7-10; NE 6-10 NE 3-8; S 5-7 S 4-8 NW,N 3-7 N 4-7 NE 8-17 N 10-16 N 11-15 N 10-14 N 10-14 N 7-11 NE,E 6-12 NE 3-9
16.9 18.2 23.1 25.2 28.5 28.3 27.0 24.3 22.4 21.0 16.4 7.6 8.2 4.7 7.7 11.7 20.9 21.6 22.3 27.5 24.1 19.3 16.9 16.7 16.6 15.0 13.1
93 112 143 122 113 47 15.5 5.6 5.3 6.9 5.2 4.1 1.8 3.8 2.7 2.6 4.3 4.8 5.1 36 41 109 51 70 72 65 64 26 24
33 1 22 28 33 28 34 29 11.9 14.5 11.4 11.3 7.4 1.5 2.7 4.9 6.0 7.2 6.2 9.0 21 8.9 158 27 43 21 35 215 14.6 8.5
40 8.5 13.6 14.1 8.2 a 9.3 a 13.1 9.0 6.5 1.9 1.8 3.3 6.5 a 4.6 >3 a a '6.5 16.0 8.2 9.6 9.6 8.9 21 5.3 5.8
96 36 33 31 41 a 8.1 a 12.2 14.5 8.7 7.6 2.0 3.1 3.6 3.8 7.0 9.3 5.8 >I8 10.7 48 15.4 37 27 29 49 14.9 10.9
a
Not measured.
amount to the sample solution directly before the injection into the gas chromatographic system. High-level samples were analyzed with GC-ECD (Varian 3700) with splitless injection as described in ref 10. In addition, quantitation of low-level samples was ensured with on-column injection and GC-ECD (Chrompack2001). In this case, up to 3 pL sample solution could be injected without discrimination effects. Successive on-column injections of 2-3 p L with cold column trapping did not result in a peak broadening nor in a discrimination of the compounds. The detection limit could be easily extended this way. The stationary phases used for the separation were a CP-Sil-8 (SE 54) with 10% octadecyl methyl polysiloxane and with 50% octadecyl methyl polysiloxane (Chrompack, Middleburg, The Netherlands), recently introduced by us for the advanced separation of SOCs (30,311. For confirmation purposes, a capillary column coated with OV 1701 (DB 1701, J&W, Folsom, CA) was used. The identification of the compounds based on standard references was verified by GC-MS using an ion-trap mass spectrometer (Saturn 11, Varian) as the detector. Concentrations and patterns of polychlorinated biphenyls as well as of hexachlorobenzene, dieldrin, and endosulfan were also analyzed in the air samples discussed here. These results are reported in separate papers (10, 32, 33).
Results and Discussion Tables 2 and 3 summarize the results of the measurements of the four trace compounds in the lower troposphere and in the surface water of selected areas of the Atlantic Ocean between 50' N and 50" S. 210
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 1, 1995
TABLE 3
Concentrations of mHCH, pHCH, Pentackloroanisole (PCA), and Tetrackloro-l,4-dimetboxybenzene (TCDMB) in Surface Water of the Atlantic Ocean locitioi sample [mean lat. corresponding a-HCH no. mean long.] to air sample (ng/d) 1 2 3 4 5 6 a
52" S 4'W 38"s 2"E 29" S 6 " E 24" S 8" E 12"N l 9 " W 32"N 14'W
14 16 17 18 21 23
15 17 10 6.7 26 70
-HCH PCA TCDMB (ng/m3) (ng/m3)
$g/m3)
21 27 45 33 47 75
2.9 a a a
a a
6.2 3.4 7.7 1.5 6.5 6.2
Not measured.
The hexachlorocyclohexanes are known as chemicals of anthropogenic origin whereas the polychlorinated methoxybenzenes are not used at all but are products of biotransformation. Pentachlorophenol (PCP) (34,35)and other compounds as the HCHs (361,pentachloronitrobenzene (PCNB),and hexachlorobenzene (HCB) (37) can be metabolized to the chlorinated methoxybenzenes. The reaction pathway and the rate of formation of these metabolites, especially of TCDMB, is only poorly known. Halogenated methoxybenzenes have already been detected in continental air (12,27,38-40) and in air of the Pacific Ocean ( 4 1 ) and the Indian Ocean (42) as well. Among the various possible isomers of TCDMB, only the 1,4-isomer could be identified by mass spectrometric detection.
One can discuss the meridional changes of the concentrations of the four SOCs in the global air flow in the lower troposphere of the Atlantic Ocean considering both the air and the water environment as a more or less stochastic means spatially and temporally changing nonsteady-state situation (we call it schlieren model) or as a steady-state nonequilibrium with the air acting as the primary source (fluxmodel) 01 as a temperature-dependent equilibrium of air-water exchange (TDE model). Any atmospheric input or output will disturb a given air-water equilibrium. It will depend on the lifetime of the output process and the relaxation time of the air-water equilibrium whether this disturbance will be measurable in air. The prevailing conditions in the environment depend on the intensity and point character of the input of the contaminants, the time scale of the mixing processes in air, the rates of intercompartmental distribution, the rates of interhemispheric exchange, and the rates of chemical removal and outflow from the compartment air. The most appealing model in theoretical terms is the TDE model. Sampling Locations in Relation to Mass Flow in Troposphere and in Ocean Surface Water of the Atlantic Ocean. Air and water are both mobile and equilibrating phases of the transport in the environment. Compounds in the truly molecular dispersed state will merely followthe general mass flow in the troposphere and in the oceanic system. A discussion of the occurrence of environmental chemicals in marine air under global aspects has therefore to include the impact of the mass flow in the troposphere and the ocean surface water on the transport (3). The northern westwind belt, which covers the industrialized regions on the globe, reached on the cruise ANT MI 1 to about 27" N including air samples 1-3. Due to the meteorological situation, sample 1 was influenced by continental air masses originating from Europe. The northeast Trades extended on the cruise ANT M/1 from about 27" N to 7" N, including air samples 4 and 5. On cruise ANT M/4, the transition between the northern Trades and the westwind belt was very far to the north at about 45" N due to the very northern position of the subtropic high pressure belt. Consequently, all samples from cruise ANT M/4 collected in the Northern Hemisphere (samples 20-27) are influenced bynortheasterlywinds and, therefore, by airmasses partly originatingfrom the continental regions of Europe and Africa. This is also the case for the samples taken on the Capo Verde Islands (samples 28 and 29). The intertropical convergencezone had its position at 7" N (ANT K/1) and 4" N (ANT W 4 ) . The air samples (ANT M/1: 6-10, ANT M/4: 18 and 19) can be attributed to the southern Trades. The southern westwind belt extended north during both cruises to about 38" S, which includes air samples 11- 17. The surface water of the Atlantic Ocean between 30" N and 30" S is governed in general by a stable warm water layer reaching down to 150 f 20 m as indicated by the oxygen content and the temperature profile (43,441. Shortterm mixing occurs to depths of 80-100 m. The layering is stable during the whole year. Near the west coasts of the African continent, the surface water is influenced by upwelling cold water from deeper regions of the Atlantic Ocean. Hexachlorocyclohexane Isomers in Air of the Atlantic Ocean. Hexachlorocyclohexane is produced as an insecticide by photochlorination ofbenzene (4.546). It is applied as the technical isomer mixture benzene hexachloride
P 200
1
I60
$100 060 0 -80 -50 40 -30 -20 -10 0 10 20 SOUTH latitude pl
1
alpha-HCH
0
30 40 50 60 NORTH
gamma-HCH
~
FIGURE 3. levels of a- and y-HCH in the tropospheric boundary layer of the Atlantic Ocean.
(BHC)containing about 16%y-HCH as the active ingredient, as a y-HCH enriched product (about 60%y-HCH), or finally as the nearly 100% pure y-HCH (lindane). The widelyused BHC consists of 65-70% a-HCH, 14-18% y-HCH, 7-10% B-HCH, 7% 8-HCH, and several other chlorocyclohexanes as byproducts (4547). In the Northern Hemisphere both types of the pesticide are applied, while in the Southem Hemisphere mainly lindane is used. This is reflected in the levels of the a-HCH and y-HCH in environmental samples from the Southern Hemisphere (5, 9,27, 48, 49). Recent studies also report elevated levels of a-HCH in the Southern Hemisphere (6,8,9). Whether these are regional inputs or whether a general change in the use pattern is observed remains in question. a-Hexachlorocyclohexane. The levels for a-HCH (Table 2, Figure 3) in the northern westwind belt were between 93 and 143pg/m3. The concentrations in the trade wind region were higher during the cruise ANT IX/l in fall 1990 (113 and 122 pg/m3) as compared to the cruise ANT M/4 in spring 1991 (mean 64 pg/m3;range 36-109 pg/m3)and on the Capo Verde (24 and 26 pg/m3). Iwata et al. recently measured during a cruise from Portugal to Florida a mean value of 200 pg/m3 (n= 4, range 87-320 pg/m3) in air for a-HCH (8). During a cruise from West Africa to North America in 1991, other authors measured a-HCH levels between 28 and 423 pg/m3, whereby the higher values were attributed to air masses originating in North America (50). The expert group GESAMP estimated a mean level of 260 pg/m3 for a-HCH in air of the North Atlantic (51). Concentrations of a-HCH in the air of the southern Trades were between 4.3 and 47 pg/m3and in the southern westwind belt between 1.8 and 4.1 pg/m3 (Table 2, Figure 3). The decline of the concentration of a-HCH in air from a level of 47 pg/m3 at the equator to 15 pg/m3 south of it at 9" S can be explained by the reduced meridional transport of air masses across the ITCZ. The relatively high levels of a-HCH in air directly south of the ITCZ where the southern Trades enter the ITCZ seem to reflect a temporary input from Africa by air parcels that partially crossed the ITCZ. Comparing the levels in the northern and southern westwind belts, an interhemispheric north-south ratio for a-HCH in air of about 40 is given. A critical evaluation of measured data by GESAMP yielded a mean level of 26 pg/ m3 in the South Atlantic leading to a north-south ratio of about 10 (51). y-Hexachlorocyclohexane.The levels and the meridional course of yHCH (Table 2, Figure 3) in air over the Atlantic differ quite significantly from that of a-HCH. In some of the air samples (samples 1, 22, and 27) collected VOL. 29, NO. 1 . 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
21 1
in the Northern Hemisphere, the concentrations of y-HCH are strongly increased apparently due to inflowing air masses from the European continent. In western Europe, HCH is mostly applied as lindane, the pure y-HCH isomer. The input of continental air in sample 1 is also indicated by a difference in the PCB pattern as compared to the other marine air samples (32) and by a strong increase of the carbon monooxide mixing ratio (52). With the exception of these air samples, the level of y-HCH in the northern tradewind system is much lower than the level of a-HCH (Figure 31, and the meridional changes are much less pronounced. In addition, for a-HCH a greater difference of the levels in air was measured on the cruises ANT E/ 1 and ANT E14 than for y-HCH (Figure 3). Excluding the high level samples of y-HCH in the Northern Hemisphere, the base-line level for y-HCH in the northern westwind belt was 25 pg/m3 (mean value of samples 2 and 3). It does not differ from the base-line level in the northern trades of 24 pg/m3. The range of the measured values in the Northern Hemisphere is comparable to the results from Iwata et al. (14-120 pg/m3)(8) and from Church et al. (12-85 pg/m3) (50)obtained during east-west transects through the North Atlantic. In the Southern Hemisphere, the concentrations of y-HCH in air are declining to higher latitudes. Levels in the southern Trades (mean value 15 pg/m3) are by a factor of 3 higher than the levels in the southern westwind belt (mean value 5 pg/m3). An interhemispheric ratio of about 5 is calculated from the levels in the northern and southern westwind belts. Air-Sea Exchange Equilibrium for a-HCH and y-HCH in the South Atlantic. The parameter of an air-water exchange equilibrium of hexachlorocyclohexanes have recently been discussed in detail for the Great Lakes (13) and for the global oceanic system (8). A similar discussion will be forwarded here for the meridional transect of the Atlantic Ocean. The air-water exchange can be expressed by the gaswater exchange coefficient Kw including its temperature dependancy. In the case of Kw < lo-* (dimensionless), equivalent to a Henry’s law constant H < 25 Pa m3/mol, changes in the concentration in air within 1 order of magnitude caused by the temperature dependence of Kw affect the concentration in water, c,, on a local basis in a hardly measurable way. In this case, c, is practically constant, and the concentration in air can be expressed by (33) In cg = -AHp/RT+ (AS,/R + In c,) (1) where AH, is the enthalpy of partition; ASp is the entropy of partition; cgis the concentration in air; c, is the dissolved concentration in water; R is the gas constant; and Tis the absolute temperature (K). Measurements of the concentration of a compound in air alone as the proof of a temperature-dependent equilibrium require that the concentration in water remains constant and that no temporal atmospheric input occurs. A linear correlation of In cg and the inverse of the absolute temperature (eq 1) proves that an equilibrium and its temperature dependence is given based on these assumptions. Deviations from linearity result from a not yet completely established equilibrium, from a decrease in activity in the water phase due to interactions of the molecules with other species, or from fluctuations in the concentrations in surface water. The slope in this plot corresponds to AHp/R while the intercept is equal to (AS,IR + In c,). 212
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29. NO. 1, 1995
=6
3,2
A
alpha-HCH
$3
3,4
~
.
3,5
3,6
3,7
3,8
1/T [K] (IOa)
ANTINI
0
ANTIN4
~
-4 P
gamma-HCH
B
m c
- 0
$2
3,3
3,4
3,5
3,6
$7
3,8
l/T [K](10’) ANTlXll
C
ANTIN4
FIGURE 4. (A) Correlation of the concentrations of a-HCH in air samples from the South Atlantic and the water temperature according eq 1. linear regressionyields the equation In c, = -6170T1+ 22.94, r = -0.73. (B) Correlation of the concentrations of y-HCH in air samples from the South Atlantic and the water temperature according eq 1. linear regressionyields the equation In c, = - 7 1 3 0 F 26.68, r = -0.82.
+
The slope can be used to calculate the enthalpy of partition AHp and to differentiate between a steady-state equilibrium and a constant input which has not yet reached the equilibrium. The intercept differentiates between the concentration in water as the contribution by AS,/R can be considered as constant in a first approximation (Law of Trouton). Two water bodies with different concentrations in water will thus give parallels in a plot of In cgversus 1 / T. The plot of In cg of a-HCH and of In cg of y-HCH in air of the South Atlantic versus the inverse of the absolute temperature is depicted in Figure 4. The measurements of both cruises (all air samples south of the ITCZ) were used for the calculation. Despite the possible parameters of influence and regional fluctuations, the relationship over a large region of the South Atlantic is surprisingly close to linearity. The results indicate an air-sea exchange equilibrium. We can conclude from this that the concentrations of both the HCHs must be rather homogeneous in the surface water of the South Atlantic. Without this event, a given gas-water equilibrium would not have been indicated by only the measurements of the concentrations in air. From the slopes values for AHp of 51 f 11 kJImol for a-HCH and 59 f 11 kJ/mol for y-HCH can be calculated. The ranges are given by deviations of the analytical results of approximately f 2 0 %summarizing the factors which add to the precision of the measurements. Henry’s law constants of HCHs have been determined in a study using artificial seawater for a temperature range of 0.5-23 “C (21).These measurements give enthalpies of partition AHp of 55 & 4 kJ/mol and 52.5 f 5.5 kJ/mol for a-HCH and y-HCH, respectively. Our global measurements and the laboratory results are practically identical. General Air-Water Distribution of H C H s in the Atlantic Ocean. Similar results as derived above are obtained on
the basis of the actual concentration in air (Table 2) and water (Table 3) measured during the cruise ANT M/4 (eastern part of the Atlantic Ocean) and using the temperature-corrected values of KW(V (21). From the concentrations in the air samples and the temperature depending gas-water partition coefficientK& for seawater, equilibrium levels in air at the respective surface water temperature during sampling are calculated and compared to the measured concentrations (Figure 5). The contents of a- and y-HCH in the water sample at 32" N, 14" W (70 and 75 ng/m3, respectively) and of a-HCH in the water sample at 11" N, 19"W (26 ng/m3)are significantly lower than the equilibrium values based on the air measurements (air samples 22-27: a-HCH, 200-500 ng/ m3; y-HCH, 250-500 ng/m3;air samples20 and21: a-HCH, 80-120 ng/m3). In either case, a net flux of a-HCH from air to the surface water is given. The degree of equilibration could also be discussed as proposed by Hinckley et al. (7) as the degree of saturation of the water. From the air samples in the Southern Hemisphere, equilibrium water levels of 20-60 ng/m3are calculated for a-HCH which are close to the measured values of 7-17 ng/m3 in the same region. In the Southern Hemisphere, equilibrium values for y-HCH from 25 to 100 ng/m3 are calculated for water based on our air measurements. These levels are close to the measured values of 21-45 ng/m3 in that region. These calculations support the conclusionthat an equilibrium between air and surface water with respect to a- and y-HCH in the South Atlantic is given as derived above from the plot of In cg versus 1/T. For the North Atlantic regional equilibria and temporal situations of a non-equilibrium, steady state can be assumed, which is fed by a non-continous input of y-HCH by air from higher latitudes. A schlieren model and a TDE model seem to overlap in the North Atlantic. Former measurements of hexachlorocyclohexanes in North Atlantic Oceanwater are rare. In 1985,we measured for a- and y-HCH concentrations of 580 and 240 ng/m3, respectively, at 31" N, 14" W, and of 40 and 18 ng/m3, respectively, at 12" N, 17" W (53). Iwata et al. measured mean values of 120 ng/m3 ( n = 4, range: 70-140 ng/m3) and 21 ng/m3 (range: 10-17 ng/m3),respectively, for aand y-HCH during a traverse from Portugal to Florida at about 30" N (8). For the surface water near the Bermudas (33" N, 65" W) levels of 117 and 16 ng/m3, respectively, were reported (53). Chemical and Physical Removal of HCHs from Air. The overall residence time titotal of a compound i in an environmental compartment is controlled by the process with the shortest specific residence time zil, t i 2 , or highest output rate. In reality, the overall residence time of a compound can only be partly split into specific residence times. In most cases, one can formalize however the output of a compound from the troposphere as the sum of physical and chemical processes (eq 2): output = l / Z i r o t a l = l/tPhY,l/Zchem (2) The lifetime z of a compound is defined by the decrease from the starting concentration co to co/e,which is equal to 38.8% Of CO. The geophysical situation in the Trades (little rainfall, low particle content in the atmosphere) and a particle attached portion of < 1% suggest that, beside dilution by HCH-depleted air, the outflow of HCHs from air occurs mainly by gas dry deposition and by reaction with OH
+
ITCZ
5001
. .
A
alpha-HCH
5 300
-60 50 40 3 0 -20 -10 0 10 20 30 40 50 60 NORTH SOUTH latitude pl
.
calculated
p 2500 2000
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0
gamma-HCH
-60 -50 40 3 0 -20 -10 0 10 20 30 40 50 Bo NORTH SOUTH latitude pl
I
calculated
0
measured
1
FIGURE 5. (A) Calculation of equlibrium concentrations of a-HCH in water from the air samples taken during the cruise ANT IW4 (see text) and comparison with measured concentrations in the surface water. (B) Calculation of equlibrium concentrations of y-HCH in water from the air samples taken during the cruise ANT IW4 (see text) and comparison with measured concentrations in the surface water.
radicals. In the tropics and subtropics, OH radicals have a continously high average concentration. The outflow of a-HCH in the tropics can be attributed to an additionally wet deposition by the high precipitation rates there. A possible increased output by rain will however be counterbalancedin part by the temperature dependency of Kw (21) that favors for HCHs the gase side of the equilibrium at elevated temperatures. Zetsch measured a rate constant for the reaction of y-HCHwith OH radicals of0.6 x 10-l2cm3/s,and estimated this value to be the same for a-HCH (22). A diurnal concentration of 3 x lo6 OH radicals/cm3 in the tropics (54)leads to a lifetime ZOH of 13 days for HCH in the atmosphere of the tropics. The photolysis of HCH in the atmosphere will be a process of minor importance in respect to the removal from the atmosphere as the absorption spectrum of HCH does not extend into the region of the sun light (55). In spite of the necessary decline of HCH in the air of the southern trades as a result of dry and wet deposition and degradation by OH radicals, an increase of the concentration of the HCHs in air flowing to lower latitudes is observed. It is the result of the temperature dependence of the gaswater equilibrium. Chloromethoxybenzenesin Air of the Atlantic Ocean. Pentachloromethoxybenzene (pentachloroanisole, PCA) and tetrachloro-1,4-dimethoxybenzene(TCDMB)are biotransformation products of anthropogenic precursors as previously discussed. The atmospheric concentrations of the two biotransformation products in the lower troposphere of the Atlantic Ocean will mainly be regulated by the input of continental air and the gas-water exchange as given by the KW value. The concentrations in air will VOL. 29, NO. 1, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
213
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