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Monsoon cloudwater chemistry on the Arabian Peninsula
Atmospheric Environment Vol. 26A, No. 9, pp. 1583-1587, 1992. Printed in Great Britain.
0004-6981/92 $5.00+0.00 © 1992 Pergamon Press Ltd
M O N S O O N CLOUDWATER CHEMISTRY ON THE ARABIAN PENINSULA ROBERT S. SCHEMENAUER Environment Canada, Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4
and PILAR CERECEDA Instituto de Geografla, Pontificia Universidad Cat61ica de Chile, Casilla l14-D, Santiago, Chile (First received 24 June 1991 and in final form 4 December 1991)
Abstract--The potentially catastrophic environmental consequences of the conflict in Iraq, Kuwait and Saudi Arabia, from mid 1990 to early 1991, have highlighted the need for background atmospheric chemistry measurements for the region. The only known cloudwater chemistry data obtained in the Arabian Peninsula are presented here. The samples were collected near the coast, in the Dhofar region of southern Oman, from 22 to 30 July 1990, immediately prior to the start of the conflict on 2 August. Analysis of the samples for pH, 10 major ion concentrations and 23 trace elements, demonstrates that the cloud water was very dean. Enrichment factor calculations showed the ions have oceanic and crustal origins, whereas trace elements such as B, V, Mn, Ni, Zn, Se, Sr, Mo and Ba have anthropogenic sources. In comparison with three mountain ctoudwater sampling sites in eastern North America, the Omani site has higher pH values, higher N a + and CI- concentrations, and lower SO2- and NH~ concentrations. Key word index: Cloud chemistry, fog chemistry, Arabian Peninsula, Kuwait, Oman,
I. INTRODUCTION There has been a concerted effort in recent years to develop the collection of high elevation fogs into a water resource that can be used for domestic and agricultural purposes in arid regions (Schemenauer et al., 1988; Schemenauer and Joe, 1989). The fogs are produced when marine stratus and stratocumulus cloud decks are advected over coastal mountains producing persistent fog cover on the mountains (Cereceda and Schemenauer, 1991). The meteorological conditions are particularly appropriate where cold surface ocean currents flow along continental margins (Schemenauer and Cereceda, 1991a). These conditions exist along the coasts of Chile and Peru in South America and the Sultanate of Oman (StanleyPrice et al., 1988; Cereceda et al., 1990) in the Arabian Peninsula. Building on the results of a major pilot project in Chile, fogwater collection experiments were conducted in the Dhofar region of the Sultanate of Oman (Scheme'nauer and Cereceda, 1991b) in 1989 and 1990. As part of the 1990 experiments, a set of cloudwater samples was collected for chemical analysis. The objectives of this paper are to present these data and to examine the origins of the ion and trace element species. The measurements are of particular interest because they fall within the geographical area selected AE(A) 26:9-C
by the World Meteorological Organization (WMO) for the establishment of four new Global Atmospheric Watch (GAW) atmospheric chemistry monitoring stations in Pakistan, Iran, Yemen and Egypt. The stations will be set up in late 1991 specifically to determine if the precipitation chemistry and surface concentrations of gaseous pollutants show effects from the oil fires in Kuwait. The main field site for the fog-collection experiments was at Ashinhaib (54 ° 16', 45" E, 17°15' 40" N) in the Jebel AI Qara. It is at an elevation of 900 m and is 15 km inland from the coastal city of Salalah. During July and August the southwest monsoon (Khareef) is well established and the boundary-layer winds below the inversion are predominantly southerly. This brings air offthe Arabian Sea to the field site during the periods with fog. The low clouds move north over a 10-kin wide dry coastal plain before ascending the abrupt escarpment of the jebel. The normal cloud-base heights are 200-300 m above sea level and the cloud tops are at 1000-1200 m, which is just above the maximum altitudes of the mountains. The winds above the inversion will be influenced by the westerlies and by regional-scale circulations assodated with intense surface heating in the Empty Quarter (Rub AI Khali) of the Arabian Peninsula. Haze due to the presence of dust extends to the middle or upper troposphere and aerosol particles may well
1583
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R.S. SCHEMENAUERand P. CERECEDA
settle into the top of the cloud layer in Dhofar and be incorporated into the cloud droplets. In the winter, the surface winds are from the north in the Jebel AI Qara and there may be substantial dry deposition of particulates and gases, which originate in the central or northern parts of the peninsula. This could play a role in determining the composition of the surface soils in the mountains of Dhofar. The coastal cloudwater chemistry should, therefore, reflect regional-scale chemical inputs from: oceanic emissions into the lowlevel flow directly off the northern Indian Ocean, local surface soil-dust sources, sedimentation of dust from distant sources into the upper part of the cloud decks and regional anthropogenic emissions. Local sources of pollutants are minimal since there is no industry in the area and few motor vehicles.
2.
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METHODS
The cloudwater samples were collected 1.5 m above the ground during fog events on the mountains of the Dhofar Jebel. A Teflon string passive cloudwater sampler, nearly identical to that of Mohnen and Kadlecek (1989), was used and the water contacts only Teflon, polyethylene and polypropylene surfaces. Mohnen and Kadlecek have shown that the collection efficiency of the 0.4-mm dia. collector fibers is above 50% for 5-/zm dia. droplets for 5 m s- ~ winds, above 80% for 10-/~mdia. droplets, and above 90% for 15-/~m dia. droplets. Thus, the larger droplets, which contain most of the fog liquid water, are very efficiently captured. Concentrations of ions in the final rinses of the collector with distilled deionized water, prior to sample collection, were 10-100 times lower than in the samples. Samples were collected in acid-washed high-density polyethylene bottles. No preservatives were added to the samples. The possibilities that the bottles may be sources or sinks for trace elements have been examined (Schemenauer and Cereceda, 1992) and no effects which would compromise the conclusions were seen. The samples were stored at approximately 4°C prior to analysis by HPLC (High Pressure Liquid Chromatography) for all ion concentrations, except for Ca 2 ÷ and Mg 2+, which were analysed by FAAS (Flame Atomic Absorption Spectroscopy). Analysis was by ICP-MS (Inductively Coupled Plasma-Mass Spectroscopy) for trace-element concentrations. Some trace-element concentrations were confirmed by optical ICP and FAAS. The samples were not filtered prior to trace element analysis. They were allowed to settle and an aliquot was then taken from the top, in accordance with the laboratory's protocols for drinking-water analysis.
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3. RESULTS AND DISCUSSION r~
Seven cloudwater samples were collected on 5 days at the Ashinhaib site between 22 and 30 July 1990. Cloud liquid water contents were measured with a laser-optical Forward Scattering Spectrometer Probe built by Particle Measuring Systems Inc. (PMS FSSP) and were typically 0.2 g m - a. The pH and ion concentrations for the samples are given in Table 1. At the bottom of the table the Omani or W H O maximum allowable concentrations for drinking water are shown in order to allow for an objective assessment of the cleanliness of the water. The dominant anion is
¢3
L; e~
[-
Cloudwater chemistry on the Arabian Peninsula CI- followed by HCO~ and NO3. The dominant cation is Na + followed by Ca 2+ and Mg 2+. The ion balance was good, though there was generally a 10% surplus of cations. This suggests that there may be an additional anion present that was not measured, since the analysis of cloudwater samples from Canada by the same laboratory results in discrepancies in the ion balances of + 5%. The average ion concentrations were typically only 10% of the Omani maximum allowable drinking-water concentrations. The average cloudwater conductivity was 339/zS c m - 1, which, for comparison, is as low or lower than in drinking water in southern Ontario (Canada). The average Na+/C1ratio was 0.55 _+ 0.01, which is identical to the ratio in sea water. In Table 2 the ion concentrations are compared to those from a remote site in Chile (Schemenauer and Cereceda, 1992) and from three mountain sites in eastern North America. The data from Whiteface Mountain, NY (Mohnen and Kadlecek, 1989), Whitetop Mountain, NC (Saxena et al., 1989) and Roundtop Mountain, Quebec (Schemenauer and Winston, 1988) were all collected in the summer of 1986 and are typical of areas where there are concerns about the effects of acidic cloud water on high elevation forests. The remote sites in Oman and Chile show some differences in ion concentrations. Oman has higher Na +, CI- and Ca 2 + concentrations and lower SO~concentrations. The pH values in Oman are also much higher than in Chile. The most obvious differences in the cloudwater chemistry between the remote coastal site in Oman and the mountainous sites in eastern North America are: higher pH values in Oman, lower concentrations of SO~- and NH~ and higher concentrations of Na +, CI- and Ca 2 +. There was a marked decrease in ion concentrations in the cloud water in eastern North America in 1986 as one moved from south to north (North Carolina 35°N, to New York, to Quebec, 45°N) and an increase in pH. All the North
1585
American sites reflected the continental origins of the air masses in their cloudwater ion concentrations, whereas the Omani site has a distinct maritime nature. Enrichment factors (Ahmed et al., 1990; Schemenauer and Cereceda, 1992) were calculated using standard values for sea water (Kennish, 1989) and the earth's crust (CRC Handbook, 1990). Enrichment factors are a useful guide to sources for elements found in the atmosphere. They are, however, limited in their application by the representativeness for the region of the standard values for sea water and the crust. The calculations indicate that Na ÷, SO~- and C1- (by definition) have oceanic origins, F - has a crustal origin, Mg 2+ and K + have important oceanic and crustal sources, and Ca 2+ is largely of anthropogenic origin in the fog water. Enrichment factors, relative to seawater CI, ranged from 1.0 for Na + to 17 for Ca 2+. Enrichment factors, relative to crustal AI, ranged from 0.6 for F - to 5930 for CI-. Values could not be obtained for NH2, NO3 and HCO3. An analysis of 13 winter precipitation samples (Ahmed et al., 1990), from 1987 and 1988, in Dhahran, Saudi Arabia, found that Na + and CI- had marine origins, Mg 2+ and K + had both marine and crustal sources, and that SO 2-, N O ; and Ca 2+ had anthropogenic origins. Other than the anthropogenic source for SO 2- in rain in the refinery center of Dhahran, the agreement in assessing sources for the ions is very good. The concentrations of 23 trace elements in the seven cloudwater samples from the Ashinhaib site are given in Table 3. In most cases the concentrations are below the method detection limit specified by the laboratory (shown by < sign). With the exception of Mn, the measured values for all elements are one to two orders of magnitude less than the maximum concentrations allowed by the drinking-water guidelines. The average Mn value is one-third of the guideline value. Enrichment factors for the trace elements were only calculated where the values exceeded the method
Table 2. A comparison of the concentrations of major ions in fogwater samples from coastal Oman and Chile, to those from three mountain sites in eastern North America in the summer of 1986 Species
Oman
Chile*
E. USAt
E. USA~
E. Canada§
pH SO4II NO 3 C1 F HCO3 Na NH+ K Ca
7.0-7.9 3.4 4.7 44 0.02 10.8 24 0.2 1.1 15
3.5-6.7 9.1 2.0 7.9 . . 4.8 1.2 0.3 1.0
3.3 52 22 1.4
3.6 12.5 5.1 -. . -2.0 ---
3.9 10.9 4.5 0.2
Mg
2.9
0.6
* Schemenauer and Cereceda (1992). I" Saxena et al. (1989). I/Mohnen and Kadlecek (1989). §Schemenauer and Winston (1988) uValues for all species in ppm.
. .
. . 0.6 7.3 0.4 2.3 0.4
--
0.2 2.7 0.3 ---
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R.S. SCHEMENAUERand P. CERECEDA
Table 3. Concentrations (ppb) of trace elements in fogwater samples from the AES fog collector at or near the Ashinhaib, Oman, site, 1990. Dates and times as per Table 1. The maximum allowable Omani or WHO drinking-water concentrations are also shown Sample no.
Fe
As
Cd
Pb
Be
B
AI
Ti
V
Cr
Mn
Ni
<60 <60
<1 <1 . <1 <1 <1 <1 50
<0.5 <0.5
<0.5 <0.5
98.5 48.0 . <20 64.2 142 34.7 5000
2.4 13.0
<5 <5
2.0 0.7
<5 <5
34.5 5.4
7.36 2.73
<0.5 <0.5 <0.5 <0.5 10
<0.5 <0.5 . <0.5 <0.5 <0.5 <0.5 50
<5 <5 <5 <5 NA
<0.5 1.0 2.4 0.7 100
<5 <5 <5 <5 50
7.5 10.5 6.1 19.4 50
<2 <2 <2 <2 50
Co
Cu
Zn
Se
Sr
3 6 10
<1 <1
<5 <5
11.3 10.5
<5 <5
77.5 24.3
15
<1 <1 <1 <1
<5 <5 <5 <5
25.6 <2 <2 2.9
<5 <5 7.4 <5
19.4 58.9 151 27.4
<0.5 <0.5 <0.5 <0.5
10
2000
500
3 6 10 15 18 22 24 Maximum allowed
<60 <60 <60 <60 300
Sample no.
18 22 24 MaHmum allowed
1000 1000 5000
<0.5 <0.5 <0.5 <0.5
detection limits. The calculations indicate that B, V, Mn, Ni, Zn, Se, Sr, M o and Ba had anthropogenic origins. AI had a crustal origin by definition. N o n e of the elements, for which values could be calculated, had strong indications of an oceanic or crustal origin. The enrichment factor values, relative to seawater C1, ranged from 4 for Sr to 10,000 for Mn. The values, relative to crustal A1, ranged from 227 for V to 3,000,000 for Se. Analysis was also performed (not reported here) on three surface soil samples and on extracts made from the soils. The extracts were prepared by mixing one part of soil to ten parts of distilled deionized water by weight. The solution was mixed and allowed to settle for 1 h before filtering. One sample was from the upwind coastal plain, one from the Ashinhaib mountain site and one from the interior desert (Nejd). The p H values for the extracts were similar to those in Table 1 for the fog samples. The extract from the Ashinhaib (mountain) site had the highest N a ÷ and C I - values, perhaps reflecting the input from the cloud water to the surface soils. The Ashinhaib site also had the highest N O 3 and N H ~ values of the three extracts, which is likely due to the more extensive vegetative cover, which in turn results from the inputs of water from cloud and drizzle. The highest values for the trace elements in the soils were almost always found in the sample from the Ashinhaib site. However, the trace-element concentrations in the extract from this soil sample were still consistently low, with 11 of
Mo
. 12.5 19.5 4.1 2.2 100
Ag
0.8 <0.5 0.5 <0.5
Sb
Ba
TI
<0.5 <0.5
13.0 <0.5 4.0 <0.5
<0.5 <0.5 <0.5 <0.5
<0.5 <0.5 <0.5 <0.5
1.5 8.8 22.5 4.6
50
150
1000
U <0.5 <0.5
<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 13
20
the 18 elements measured being present in concentrations below the method detection limit.
4. CONCLUSIONS The monsoon clouds, which form fog on the mountains of southern Oman, have been found to be very clean relative to drinking-water guidelines and cioudwater chemistry in eastern North America. The concentrations of ions and trace elements are similar or lower than in high elevation coastal fogs in the remote northern part of Chile. Table 4 shows a comparison of the mean ion concentrations in the summer cloud samples from O m a n to a set of winter precipitation samples (Ahmed et al., 1990) from Dhahran, Saudi Arabia, 1200km to the north. The Na ÷ and C I concentrations are higher in the cloud water and clearly result from marine aerosols. The S O ~ - concentration is lower in the cloud water than in the rain in Dhahran, where there are significant anthropogenic sources of sulfur. The N O ~ was negligible in the cloud water but fairly high in 6 of 13 precipitation samples for reasons that could not be explained by Ahmed et al. Other ion concentrations were similar in the two data sets, suggesting there may be some uniformity in the pre-conflict chemical composition of clouds and precipitation over the peninsula. The massive output of materials from the oil fires in Kuwait (Joyce and Charles, 1991) will undoubtedly affect the air, cloud-
1587
Cloudwater chemistry on the Arabian Peninsula Table 4. A comparison of the average ion concentrations (ppm) in monsoon clouds (Dhofar, Oman) and winter precipitation (Ahmed et al., 1990; Dhahran, Saudi Arabia) on the Arabian Peninsula Ion species
Monsoon clouds
Winter rain
SO 2NO~ CINO2 HCO~ Na ÷ NH~ K+ Ca 2+ Mg 2+
3.4 4.7 44.1 0.0 10.8 24.1 0.2 1.1 15.1 2.9
11.2 8.9 5.1 3.4 4.5 3.0 0.8 0.4 9.3 1.1
water a n d precipitation c o m p o s i t i o n in the region. Pre-conflict m e a s u r e m e n t s of cloud water a n d precipitation, ion a n d trace-element concentrations, are unfortunately very scarce b u t will prove vital in assessing the degree of change imposed o n the lower a t m o s p h e r e over the A r a b i a n Peninsula.
Acknowledgements--We would like to thank His Excellency Mohammed bin Faraj al Ghassani and the Planning Committee for Development and Environment in the Southern Region of Oman for financial and logistical support for the field work. We greatly appreciate the assistance of Robert Whitcombe in the conduct of the field work, Colleen Uyeda for help with the sample preparations, Raymond McVicars and Russ Bennett of the Ontario Ministry of the Environment for the trace metal analyses, and Sylvain Savoie of Les Laboratoires Savoie-Dufresne for the ion analyses.
REFERENCES
Ahmed A. F. M., Singh R. P. and Elmubarak A. H. (1990) Chemistry of atmospheric precipitation at the western Arabian Gulf Coast. Atmospheric Environment 24A, 29272934. Cereceda P. and Schemenauer R. S. (1991) The occurrence of fog in Chile. J. appl. Met., 30, 1097-1105. Cereceda P., Barros J. and Schemenauer R. S. (1990) Las neblinas costeras de Chile y Oman: similitudes y diferencias. Rev. Geogr. Chile Terra Austr. 33, 49-60. CRC (1990) CRC Handbook of Chemistry and Physics (edited by Lide D. R.), p. 14-7. CRC Press, Boca Raton, FL. Joyce C. and Charles D. (1991) The battle to stop the Gulf from choking. New Scientist, 23 March, pp. 20-21. Kennish M. J. (1989) Practical Handbook of Marine Science: Section 2. Chemical Oceanography, p. 55. CRC Press, Boca Raton, FL. Mohnen V. A. and Kadlecek J. A. (1989) Cloud chemistry research at Whiteface Mountain. Tellus 41B, 79-91. Saxena V. K., Stogner R. E., Hendler A. H., DeFelice T. P., Yeh R. J.-Y. and Lin N.-H. (1989) Monitoring the chemical climate of the Mt. Mitchell State Park for evaluation of its impact on forest decline. Tellus 41B, 92-109. Schemenauer R. S. and Cereceda P. (1991a) Fog water collection in arid coastal locations. Ambio 20, 303-308. Schemenauer R. S. and Cereceda P. (1991b) Fog as a permanent water resource in arid lands. In Proc. Vllth I W R A World Congr. Water Resources, Rabat, Morocco, 13-18 May, A2, 23-25. Schemenauer R. S. and Cereceda P. (1992) The quality of fog water collected for domestic and agricultural use in Chile. J. appl. Met. 31, 275-290. Schemenauer R. S. and Joe P. I. (1989) The collection efficiency of a massive fog collector. Atmos. Res. 24, 53-69. Schemenauer R. S. and Winston C. (1988) The 1986 Chemistry of High Elevation Fog Project. APCA 81st Annual Meeting, Dallas, 19-24 June, Ref. 88-129.6 Schemenauer R. S., Fuenzalida H. and Cereceda P. (1988) A neglected water resource: the camanchaca of South America. Bull. Am. Met. Soc. 69, 138-147. Stanley-Price M. R., Ahmed bin Hamoud al-Harthy and Whitcombe R. P. (1988) Fog moisture and its ecological effects in Oman. In Proc. Arid Lands Today and Tomorrow, 1985, pp. 69-87. Westview Press, Boulder, CO.
0004-6981/92 $5.00+0.00 © 1992 Pergamon Press Ltd
M O N S O O N CLOUDWATER CHEMISTRY ON THE ARABIAN PENINSULA ROBERT S. SCHEMENAUER Environment Canada, Atmospheric Environment Service, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4
and PILAR CERECEDA Instituto de Geografla, Pontificia Universidad Cat61ica de Chile, Casilla l14-D, Santiago, Chile (First received 24 June 1991 and in final form 4 December 1991)
Abstract--The potentially catastrophic environmental consequences of the conflict in Iraq, Kuwait and Saudi Arabia, from mid 1990 to early 1991, have highlighted the need for background atmospheric chemistry measurements for the region. The only known cloudwater chemistry data obtained in the Arabian Peninsula are presented here. The samples were collected near the coast, in the Dhofar region of southern Oman, from 22 to 30 July 1990, immediately prior to the start of the conflict on 2 August. Analysis of the samples for pH, 10 major ion concentrations and 23 trace elements, demonstrates that the cloud water was very dean. Enrichment factor calculations showed the ions have oceanic and crustal origins, whereas trace elements such as B, V, Mn, Ni, Zn, Se, Sr, Mo and Ba have anthropogenic sources. In comparison with three mountain ctoudwater sampling sites in eastern North America, the Omani site has higher pH values, higher N a + and CI- concentrations, and lower SO2- and NH~ concentrations. Key word index: Cloud chemistry, fog chemistry, Arabian Peninsula, Kuwait, Oman,
I. INTRODUCTION There has been a concerted effort in recent years to develop the collection of high elevation fogs into a water resource that can be used for domestic and agricultural purposes in arid regions (Schemenauer et al., 1988; Schemenauer and Joe, 1989). The fogs are produced when marine stratus and stratocumulus cloud decks are advected over coastal mountains producing persistent fog cover on the mountains (Cereceda and Schemenauer, 1991). The meteorological conditions are particularly appropriate where cold surface ocean currents flow along continental margins (Schemenauer and Cereceda, 1991a). These conditions exist along the coasts of Chile and Peru in South America and the Sultanate of Oman (StanleyPrice et al., 1988; Cereceda et al., 1990) in the Arabian Peninsula. Building on the results of a major pilot project in Chile, fogwater collection experiments were conducted in the Dhofar region of the Sultanate of Oman (Scheme'nauer and Cereceda, 1991b) in 1989 and 1990. As part of the 1990 experiments, a set of cloudwater samples was collected for chemical analysis. The objectives of this paper are to present these data and to examine the origins of the ion and trace element species. The measurements are of particular interest because they fall within the geographical area selected AE(A) 26:9-C
by the World Meteorological Organization (WMO) for the establishment of four new Global Atmospheric Watch (GAW) atmospheric chemistry monitoring stations in Pakistan, Iran, Yemen and Egypt. The stations will be set up in late 1991 specifically to determine if the precipitation chemistry and surface concentrations of gaseous pollutants show effects from the oil fires in Kuwait. The main field site for the fog-collection experiments was at Ashinhaib (54 ° 16', 45" E, 17°15' 40" N) in the Jebel AI Qara. It is at an elevation of 900 m and is 15 km inland from the coastal city of Salalah. During July and August the southwest monsoon (Khareef) is well established and the boundary-layer winds below the inversion are predominantly southerly. This brings air offthe Arabian Sea to the field site during the periods with fog. The low clouds move north over a 10-kin wide dry coastal plain before ascending the abrupt escarpment of the jebel. The normal cloud-base heights are 200-300 m above sea level and the cloud tops are at 1000-1200 m, which is just above the maximum altitudes of the mountains. The winds above the inversion will be influenced by the westerlies and by regional-scale circulations assodated with intense surface heating in the Empty Quarter (Rub AI Khali) of the Arabian Peninsula. Haze due to the presence of dust extends to the middle or upper troposphere and aerosol particles may well
1583
1584
R.S. SCHEMENAUERand P. CERECEDA
settle into the top of the cloud layer in Dhofar and be incorporated into the cloud droplets. In the winter, the surface winds are from the north in the Jebel AI Qara and there may be substantial dry deposition of particulates and gases, which originate in the central or northern parts of the peninsula. This could play a role in determining the composition of the surface soils in the mountains of Dhofar. The coastal cloudwater chemistry should, therefore, reflect regional-scale chemical inputs from: oceanic emissions into the lowlevel flow directly off the northern Indian Ocean, local surface soil-dust sources, sedimentation of dust from distant sources into the upper part of the cloud decks and regional anthropogenic emissions. Local sources of pollutants are minimal since there is no industry in the area and few motor vehicles.
2.
© J
[-
[~5
METHODS
The cloudwater samples were collected 1.5 m above the ground during fog events on the mountains of the Dhofar Jebel. A Teflon string passive cloudwater sampler, nearly identical to that of Mohnen and Kadlecek (1989), was used and the water contacts only Teflon, polyethylene and polypropylene surfaces. Mohnen and Kadlecek have shown that the collection efficiency of the 0.4-mm dia. collector fibers is above 50% for 5-/zm dia. droplets for 5 m s- ~ winds, above 80% for 10-/~mdia. droplets, and above 90% for 15-/~m dia. droplets. Thus, the larger droplets, which contain most of the fog liquid water, are very efficiently captured. Concentrations of ions in the final rinses of the collector with distilled deionized water, prior to sample collection, were 10-100 times lower than in the samples. Samples were collected in acid-washed high-density polyethylene bottles. No preservatives were added to the samples. The possibilities that the bottles may be sources or sinks for trace elements have been examined (Schemenauer and Cereceda, 1992) and no effects which would compromise the conclusions were seen. The samples were stored at approximately 4°C prior to analysis by HPLC (High Pressure Liquid Chromatography) for all ion concentrations, except for Ca 2 ÷ and Mg 2+, which were analysed by FAAS (Flame Atomic Absorption Spectroscopy). Analysis was by ICP-MS (Inductively Coupled Plasma-Mass Spectroscopy) for trace-element concentrations. Some trace-element concentrations were confirmed by optical ICP and FAAS. The samples were not filtered prior to trace element analysis. They were allowed to settle and an aliquot was then taken from the top, in accordance with the laboratory's protocols for drinking-water analysis.
©
Z ©
Z
U
._q
Z
3. RESULTS AND DISCUSSION r~
Seven cloudwater samples were collected on 5 days at the Ashinhaib site between 22 and 30 July 1990. Cloud liquid water contents were measured with a laser-optical Forward Scattering Spectrometer Probe built by Particle Measuring Systems Inc. (PMS FSSP) and were typically 0.2 g m - a. The pH and ion concentrations for the samples are given in Table 1. At the bottom of the table the Omani or W H O maximum allowable concentrations for drinking water are shown in order to allow for an objective assessment of the cleanliness of the water. The dominant anion is
¢3
L; e~
[-
Cloudwater chemistry on the Arabian Peninsula CI- followed by HCO~ and NO3. The dominant cation is Na + followed by Ca 2+ and Mg 2+. The ion balance was good, though there was generally a 10% surplus of cations. This suggests that there may be an additional anion present that was not measured, since the analysis of cloudwater samples from Canada by the same laboratory results in discrepancies in the ion balances of + 5%. The average ion concentrations were typically only 10% of the Omani maximum allowable drinking-water concentrations. The average cloudwater conductivity was 339/zS c m - 1, which, for comparison, is as low or lower than in drinking water in southern Ontario (Canada). The average Na+/C1ratio was 0.55 _+ 0.01, which is identical to the ratio in sea water. In Table 2 the ion concentrations are compared to those from a remote site in Chile (Schemenauer and Cereceda, 1992) and from three mountain sites in eastern North America. The data from Whiteface Mountain, NY (Mohnen and Kadlecek, 1989), Whitetop Mountain, NC (Saxena et al., 1989) and Roundtop Mountain, Quebec (Schemenauer and Winston, 1988) were all collected in the summer of 1986 and are typical of areas where there are concerns about the effects of acidic cloud water on high elevation forests. The remote sites in Oman and Chile show some differences in ion concentrations. Oman has higher Na +, CI- and Ca 2 + concentrations and lower SO~concentrations. The pH values in Oman are also much higher than in Chile. The most obvious differences in the cloudwater chemistry between the remote coastal site in Oman and the mountainous sites in eastern North America are: higher pH values in Oman, lower concentrations of SO~- and NH~ and higher concentrations of Na +, CI- and Ca 2 +. There was a marked decrease in ion concentrations in the cloud water in eastern North America in 1986 as one moved from south to north (North Carolina 35°N, to New York, to Quebec, 45°N) and an increase in pH. All the North
1585
American sites reflected the continental origins of the air masses in their cloudwater ion concentrations, whereas the Omani site has a distinct maritime nature. Enrichment factors (Ahmed et al., 1990; Schemenauer and Cereceda, 1992) were calculated using standard values for sea water (Kennish, 1989) and the earth's crust (CRC Handbook, 1990). Enrichment factors are a useful guide to sources for elements found in the atmosphere. They are, however, limited in their application by the representativeness for the region of the standard values for sea water and the crust. The calculations indicate that Na ÷, SO~- and C1- (by definition) have oceanic origins, F - has a crustal origin, Mg 2+ and K + have important oceanic and crustal sources, and Ca 2+ is largely of anthropogenic origin in the fog water. Enrichment factors, relative to seawater CI, ranged from 1.0 for Na + to 17 for Ca 2+. Enrichment factors, relative to crustal AI, ranged from 0.6 for F - to 5930 for CI-. Values could not be obtained for NH2, NO3 and HCO3. An analysis of 13 winter precipitation samples (Ahmed et al., 1990), from 1987 and 1988, in Dhahran, Saudi Arabia, found that Na + and CI- had marine origins, Mg 2+ and K + had both marine and crustal sources, and that SO 2-, N O ; and Ca 2+ had anthropogenic origins. Other than the anthropogenic source for SO 2- in rain in the refinery center of Dhahran, the agreement in assessing sources for the ions is very good. The concentrations of 23 trace elements in the seven cloudwater samples from the Ashinhaib site are given in Table 3. In most cases the concentrations are below the method detection limit specified by the laboratory (shown by < sign). With the exception of Mn, the measured values for all elements are one to two orders of magnitude less than the maximum concentrations allowed by the drinking-water guidelines. The average Mn value is one-third of the guideline value. Enrichment factors for the trace elements were only calculated where the values exceeded the method
Table 2. A comparison of the concentrations of major ions in fogwater samples from coastal Oman and Chile, to those from three mountain sites in eastern North America in the summer of 1986 Species
Oman
Chile*
E. USAt
E. USA~
E. Canada§
pH SO4II NO 3 C1 F HCO3 Na NH+ K Ca
7.0-7.9 3.4 4.7 44 0.02 10.8 24 0.2 1.1 15
3.5-6.7 9.1 2.0 7.9 . . 4.8 1.2 0.3 1.0
3.3 52 22 1.4
3.6 12.5 5.1 -. . -2.0 ---
3.9 10.9 4.5 0.2
Mg
2.9
0.6
* Schemenauer and Cereceda (1992). I" Saxena et al. (1989). I/Mohnen and Kadlecek (1989). §Schemenauer and Winston (1988) uValues for all species in ppm.
. .
. . 0.6 7.3 0.4 2.3 0.4
--
0.2 2.7 0.3 ---
1586
R.S. SCHEMENAUERand P. CERECEDA
Table 3. Concentrations (ppb) of trace elements in fogwater samples from the AES fog collector at or near the Ashinhaib, Oman, site, 1990. Dates and times as per Table 1. The maximum allowable Omani or WHO drinking-water concentrations are also shown Sample no.
Fe
As
Cd
Pb
Be
B
AI
Ti
V
Cr
Mn
Ni
<60 <60
<1 <1 . <1 <1 <1 <1 50
<0.5 <0.5
<0.5 <0.5
98.5 48.0 . <20 64.2 142 34.7 5000
2.4 13.0
<5 <5
2.0 0.7
<5 <5
34.5 5.4
7.36 2.73
<0.5 <0.5 <0.5 <0.5 10
<0.5 <0.5 . <0.5 <0.5 <0.5 <0.5 50
<5 <5 <5 <5 NA
<0.5 1.0 2.4 0.7 100
<5 <5 <5 <5 50
7.5 10.5 6.1 19.4 50
<2 <2 <2 <2 50
Co
Cu
Zn
Se
Sr
3 6 10
<1 <1
<5 <5
11.3 10.5
<5 <5
77.5 24.3
15
<1 <1 <1 <1
<5 <5 <5 <5
25.6 <2 <2 2.9
<5 <5 7.4 <5
19.4 58.9 151 27.4
<0.5 <0.5 <0.5 <0.5
10
2000
500
3 6 10 15 18 22 24 Maximum allowed
<60 <60 <60 <60 300
Sample no.
18 22 24 MaHmum allowed
1000 1000 5000
<0.5 <0.5 <0.5 <0.5
detection limits. The calculations indicate that B, V, Mn, Ni, Zn, Se, Sr, M o and Ba had anthropogenic origins. AI had a crustal origin by definition. N o n e of the elements, for which values could be calculated, had strong indications of an oceanic or crustal origin. The enrichment factor values, relative to seawater C1, ranged from 4 for Sr to 10,000 for Mn. The values, relative to crustal A1, ranged from 227 for V to 3,000,000 for Se. Analysis was also performed (not reported here) on three surface soil samples and on extracts made from the soils. The extracts were prepared by mixing one part of soil to ten parts of distilled deionized water by weight. The solution was mixed and allowed to settle for 1 h before filtering. One sample was from the upwind coastal plain, one from the Ashinhaib mountain site and one from the interior desert (Nejd). The p H values for the extracts were similar to those in Table 1 for the fog samples. The extract from the Ashinhaib (mountain) site had the highest N a ÷ and C I - values, perhaps reflecting the input from the cloud water to the surface soils. The Ashinhaib site also had the highest N O 3 and N H ~ values of the three extracts, which is likely due to the more extensive vegetative cover, which in turn results from the inputs of water from cloud and drizzle. The highest values for the trace elements in the soils were almost always found in the sample from the Ashinhaib site. However, the trace-element concentrations in the extract from this soil sample were still consistently low, with 11 of
Mo
. 12.5 19.5 4.1 2.2 100
Ag
0.8 <0.5 0.5 <0.5
Sb
Ba
TI
<0.5 <0.5
13.0 <0.5 4.0 <0.5
<0.5 <0.5 <0.5 <0.5
<0.5 <0.5 <0.5 <0.5
1.5 8.8 22.5 4.6
50
150
1000
U <0.5 <0.5
<0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 13
20
the 18 elements measured being present in concentrations below the method detection limit.
4. CONCLUSIONS The monsoon clouds, which form fog on the mountains of southern Oman, have been found to be very clean relative to drinking-water guidelines and cioudwater chemistry in eastern North America. The concentrations of ions and trace elements are similar or lower than in high elevation coastal fogs in the remote northern part of Chile. Table 4 shows a comparison of the mean ion concentrations in the summer cloud samples from O m a n to a set of winter precipitation samples (Ahmed et al., 1990) from Dhahran, Saudi Arabia, 1200km to the north. The Na ÷ and C I concentrations are higher in the cloud water and clearly result from marine aerosols. The S O ~ - concentration is lower in the cloud water than in the rain in Dhahran, where there are significant anthropogenic sources of sulfur. The N O ~ was negligible in the cloud water but fairly high in 6 of 13 precipitation samples for reasons that could not be explained by Ahmed et al. Other ion concentrations were similar in the two data sets, suggesting there may be some uniformity in the pre-conflict chemical composition of clouds and precipitation over the peninsula. The massive output of materials from the oil fires in Kuwait (Joyce and Charles, 1991) will undoubtedly affect the air, cloud-
1587
Cloudwater chemistry on the Arabian Peninsula Table 4. A comparison of the average ion concentrations (ppm) in monsoon clouds (Dhofar, Oman) and winter precipitation (Ahmed et al., 1990; Dhahran, Saudi Arabia) on the Arabian Peninsula Ion species
Monsoon clouds
Winter rain
SO 2NO~ CINO2 HCO~ Na ÷ NH~ K+ Ca 2+ Mg 2+
3.4 4.7 44.1 0.0 10.8 24.1 0.2 1.1 15.1 2.9
11.2 8.9 5.1 3.4 4.5 3.0 0.8 0.4 9.3 1.1
water a n d precipitation c o m p o s i t i o n in the region. Pre-conflict m e a s u r e m e n t s of cloud water a n d precipitation, ion a n d trace-element concentrations, are unfortunately very scarce b u t will prove vital in assessing the degree of change imposed o n the lower a t m o s p h e r e over the A r a b i a n Peninsula.
Acknowledgements--We would like to thank His Excellency Mohammed bin Faraj al Ghassani and the Planning Committee for Development and Environment in the Southern Region of Oman for financial and logistical support for the field work. We greatly appreciate the assistance of Robert Whitcombe in the conduct of the field work, Colleen Uyeda for help with the sample preparations, Raymond McVicars and Russ Bennett of the Ontario Ministry of the Environment for the trace metal analyses, and Sylvain Savoie of Les Laboratoires Savoie-Dufresne for the ion analyses.
REFERENCES
Ahmed A. F. M., Singh R. P. and Elmubarak A. H. (1990) Chemistry of atmospheric precipitation at the western Arabian Gulf Coast. Atmospheric Environment 24A, 29272934. Cereceda P. and Schemenauer R. S. (1991) The occurrence of fog in Chile. J. appl. Met., 30, 1097-1105. Cereceda P., Barros J. and Schemenauer R. S. (1990) Las neblinas costeras de Chile y Oman: similitudes y diferencias. Rev. Geogr. Chile Terra Austr. 33, 49-60. CRC (1990) CRC Handbook of Chemistry and Physics (edited by Lide D. R.), p. 14-7. CRC Press, Boca Raton, FL. Joyce C. and Charles D. (1991) The battle to stop the Gulf from choking. New Scientist, 23 March, pp. 20-21. Kennish M. J. (1989) Practical Handbook of Marine Science: Section 2. Chemical Oceanography, p. 55. CRC Press, Boca Raton, FL. Mohnen V. A. and Kadlecek J. A. (1989) Cloud chemistry research at Whiteface Mountain. Tellus 41B, 79-91. Saxena V. K., Stogner R. E., Hendler A. H., DeFelice T. P., Yeh R. J.-Y. and Lin N.-H. (1989) Monitoring the chemical climate of the Mt. Mitchell State Park for evaluation of its impact on forest decline. Tellus 41B, 92-109. Schemenauer R. S. and Cereceda P. (1991a) Fog water collection in arid coastal locations. Ambio 20, 303-308. Schemenauer R. S. and Cereceda P. (1991b) Fog as a permanent water resource in arid lands. In Proc. Vllth I W R A World Congr. Water Resources, Rabat, Morocco, 13-18 May, A2, 23-25. Schemenauer R. S. and Cereceda P. (1992) The quality of fog water collected for domestic and agricultural use in Chile. J. appl. Met. 31, 275-290. Schemenauer R. S. and Joe P. I. (1989) The collection efficiency of a massive fog collector. Atmos. Res. 24, 53-69. Schemenauer R. S. and Winston C. (1988) The 1986 Chemistry of High Elevation Fog Project. APCA 81st Annual Meeting, Dallas, 19-24 June, Ref. 88-129.6 Schemenauer R. S., Fuenzalida H. and Cereceda P. (1988) A neglected water resource: the camanchaca of South America. Bull. Am. Met. Soc. 69, 138-147. Stanley-Price M. R., Ahmed bin Hamoud al-Harthy and Whitcombe R. P. (1988) Fog moisture and its ecological effects in Oman. In Proc. Arid Lands Today and Tomorrow, 1985, pp. 69-87. Westview Press, Boulder, CO.