- Email: [email protected]
The characteristics of the shore-line sea spray aerosol and the landward transfer of radionuclides discharged to coastal sea water
Pergnmon
Atmospheric
Environment Vol. 28, No. 20, pp. 3299-3309,
1994 Elsevier Science Ltd Printed in Great Britain. 1352-2310/94 $7.00+0.00
1352-2310(94)00156-l
THE CHARACTERISTICS OF THE SHORE-LINE SEA SPRAY AEROSOL AND THE LANDWARD TRANSFER OF Ri4DIONUCLIDES DISCHARGED TO COASTAL SEA WATER W. A. McK,dy,* J. A. GARLAND,* D. LIVESLEY,~.C. M. HALLIWELL*and M. I. WALKER* * AEA Technology, National Environmental Technology Centre, Culham, Abingdon, Oxfordshire, OXh4 3DB, U.K.; and t AEA Technology, Harwell, Oxfordshire, OX11 ORA, U.K. (First received 18 September
1993 and
in finalform
28 April 1994)
Abstract-The droplet size distribution and mass of sea spray blown onto the Cumbrian coast have been measured on a beach at a wide range of wind speeds. The results indicate that the aerosol generated immediately above the surf zone has a size spectrum (plotted as dN/d log r) that peaks between 12 and 17 pm diameter and shows little variation within the wind speed range 5-10 ms-‘. The observed peak diameter is around the upper limit of the range of peak sizes observed in oceanic aerosol spectra. The concentration of airborne sea salt was substantially greater in on-shore winds over the beach than has been observed lover the open ocean, and there was some evidence that it increased more rapidly with wind speed. The Al to Na ratio, a measure of the concentration of fine sediment suspended within the droplets of sea spray, appears to increase with increasing wind speed. This finding has implications for assessing the transfer of particulate-associated pollutants such as plutonium, bacteria and some heavy metals. Key word index: Sea spray, actinides.
1. INTRODUCTION
Pollutants discharged. into coastal waters can be present in substantial concentrations in the surf zone, an area where wave energy, accumulated from the wind over substantial areas of sea, is dissipated in a narrow band, resulting in intense generation of sea spray. This aerosol is carried inland when the wind is on-shore, exposing the population to increased levels of airborne pollutants and contaminating crops and surfaces by deposition over the coastal zone. Studies have shown that substances that adsorb to suspended particles in sea water, e.g. metals (Peirson et nl., 1974) and bacteria (Blanchard, 1983), are present in spray at concentra.tions significantly greater than were present in the bulk sea water, enhancing the transfer of such substances inland. Some of the actinide elements, discharged from the Sellafield reprocessing plant in Cumbria into the Irish Sea, have accumulated in sediment close to the Cumbrian coast. Resuspension of this sediment maintains measurable actinide concentrations in near shore sea water, despite large decreases in radionuclide discharges (McKay and Walker, 1990), and transfer of these nuclides to land in searspray has been observed along the Cumbrian coast and other coastlines of the Irish Sea (McKay et al., 1990). Since the actinides have a unique source in the area, and background levels from other sources are limited, these studies provide valu-
able information, not only on the sea-to-land transfer of radionuclides along the Cumbrian coast, but also on the potential significance of sea spray transfer for other pollutants discharged into coastal waters. In modelling the transfer of particulate-associated pollutants from sea to air to land, information on the onshore flux and droplet size of the sea spray as a function of wind speed is required, as are estimates of the sediment content of the aerosol. Experimental data on sea spray flux to the Cumbrian coast near Sellafield have recently been published (McKay et al., 1990). In the present paper we incorporate additional data and describe the dependence of the characteristics of the sea spray aerosol on environmental factors.
2. EXPERIMENTAL 2.1. Field programme The work was carried out at Eskmeals on the coast of Cuibria (Fig. 1) in June 1987, November 1988. October 1989 and du&g’the last quarte; of 1991. The site,.typical of many intertidal shores, consists of an open sandy beach running approximately north to south, interrupted by occasional rocky outcrops, stretching out to N 600 m at low water and with a broad surfzoneat high water. Inland there is an undulating coastal plane, the land climbing progressively to 570 m about 8 km inland.
W. A. MCKAY et al.
3300 0,
5
‘fl
Kilometres
Cumbria
Fig. 1. Location of sampling site. The programme was designed to gather data on the droplet size, flux and particulate content of sea spray aerosol blown onshore in a wide range of wind speeds. Size and concentration measurements of sea spray droplets were carried out in 1988 and 1989 using a LISATEK Phase Doppler instrument (Brazier et al., 1988;Livesley et of., 1991). The instrument contained a 25 mW Helium Neon laser with beam splitting optics, which allow two coherent laser beams to be focussed together in the open air. The volume in which the beams intersect contains a pattern of interference fringes. When a droplet passes through this volume, the light it scatters is modulated by the interference fringes. Three photomultipliers were used to observe the scattered light, at different angles relative to the illumination system. The angular separation leads to phase differences in the modulation of the signals that are proportional to the diameter of the droplet. The nominal dynamic range of the instrument for particle diameter is 30: 1 with an upper limit of 108 pm, giving a minimum size of about 3-4pm. However, below about 10 pm, oscillations in the response curve are likely to impose an uncertainty of *2-3 pm on the size attributed to any individual droplet. Where an aerosol with a wide size distribution is counted, no significant distortion of the size distribution is expected. Every particle between about 3 and 100 pm diameter carried through the probe volume by the wind is thus counted. However, irregular signals such as those generated by non-spherical particles are not suitable for size analysis, and such particles are counted but not sized. This system records but does not collect particles. The instrument was set up so that the probe volume was normal to the wind direction and about l-l.5 m above the ground. The sensitive volume is w 30 cm outside the instrument casing, in an area where disturbance of the wind Bow by the presence of the instrument is small. Whilst there is little disturbance of wind flow and all droplet sizes should follow
the flow and be counted, it is likely that the data collected give a poor representation of very large droplets because of the small numbers present. On average, only 4% of the drops sized in each 30 min data file were t 20 pm diameter (see Table 1). However, such large droplets constitute a large fraction of the total mass of spray (see below). The data from the LISATEK Phase Doppler instrument were collected in 30 min runs which were each given a 4-figure code (Table 1). Representative samples of sea spray suitable for chemical analysis were collected using a high volume air-sampler. This draws air through an 18 x 23 cm exposed area of Whatman 41 filter at a nominal rate of -2 m3 min-‘, giving a face velocity of 0.8 m s- I. Air flows were determined regularly by measuring the pressure across a calibrated orifice fitted to the pump, enabling the volume of air sampled to bc estimated. An inlet cone, selected to set the inlet velocity to a value close to that of ambient wind, was fitted in front of the filter and the sampler directed into the wind. This provided approximately isokinetic sampling. so that that the bias in collection of large droplets due to their inertia was avoided. Four cones were constructed. The largest cone (inlet area = 111 cm’) allowed isokinetic (unbiased) sampling at a nominal wind speed of 3 m s-i, and others were designed for 6 (55.5 cm’), 9 (37 cm’) and 12 m s-’ (28 cm’) winds. For intermediate wind speeds, the cone designed to operate at the wind speed approximating closest to the measured wind speed was used. The aerosol sizing and sampling devices were operated on the beach about 1.3-1,5 m above the ground and roughly N 2 m from the high water spring tide mark. A portable meteorological station was run throughout the experiments, recording the wind speed (at the height of the sampler inlet) and direction, relative humidity and temperature at 1 min intervals. Sampling was carried out during onshore westerly winds in November 1988, October 1989 and in October, November and December 1991, generally at high water, but on one occasion at low water. The details of the aerosol sampling are given in Table 2. Sea water was collected from the shoreline during sea spray collection. Each sample was taken in 1 m deep water just below the sea surface in 4 and 2 r? polythene bottles. The 4 &’samples were filtered through 14 or 29 cm diameter 0.45 pm pore size cellulose acetate filters and the filtrate acidified to give a pH<2 with nitric acid. The filtrate and particulate loaded filters were stored for later radiochemical analysis. The 2 G samples were passed through 4.7 cm diameter 0.45 pm polyvinylide difluoride filters (PVDF) which were afterwards rinsed with distilled water to remove sea salt. The latter filters, being hydrophobic, can be readily dried to a constant weight within LO.5 mg and were used to estimate the particulate loadings. Some were later analysed for the elements Si and Al. 2.2. Analysis The mass of sea spray collected and the quantity and quality of sediment in the sea spray samples were determined by analysing for Na, Si, Al and Fe. Inductively coupled plasma atomic-emission spectrometry (ICP-AES) was used. As a liquid sample is required for the spectrometer, it was necessary to destroy the loaded filters, whether Whatman 41 or cellulose acetate, and solubilise the sediment. This involved fusion with lithium metaborate at 1000°C and extraction into HNOJHF acid solution (Chemical Analysis Department, Harwell, private communication). The acidified filter solutions and sea water (particulate fraction only in 1991) were analysed for 238Pu, 23g+240Pu and 24’Am (the last not in 1991) by alpha-spectrometry after radiochemical separation. The methodology, described in Lally and Eakins (1978) involved radiochemical separation, plating the nuclides out on a metal substrate in a thin layer, and finally carrying out alpha-spectrometry to a detection limit of 1 mBq.
Shore-line sea spray aerosol
3301
Table 1. bata from Phase Doppler measurements Start I.ime of run Date 10.11.88
11.11.88
11.11.88
11.11.88
12.11.88
17.10.89
20.10.89
20.10.89
21.10.89
21.40 22.10 22.40 23.10 23.40 00.10 00.40 01.10 10.26 10.56 11.26 11.56 12.:!6 12.56 22.06 22.:16 23.06 23.216 OOS6 00.36 01.06 01.36 1 l.!IO 12.00 12.30 13.W 09.30 10.00 10.30 11.00 11.30 12.00 12.2’0 13.00 14.00 14.2’0 15.CO 15.2’0 16.00 16.2’0 17.W 17.10 14.2.9 14.59 15.2.9 15.5’9 16.2.9 16.59 17.2.9 17.59
Mean distance (m) from surf zone 24 15 10 8 9 11 17 26 11 9 10 13 16 25 31 23 16 14 15 17 20 27 35.5 25.5 17.5 13 451 441 414 367 302 235 169 118 46 34 27.5 24.5 25.5 28 36.5 43.5 45.5 35.5 29.5 28 29.5 31 34 43
Mean wind speed (m s-l)
7.6 6.9 6.6 6.7 6.9 7.6 6.9 7.6 7.1 6.8 7.1 6.8 6.8 6.4 4.3 3.9 4.4 4.3 5.0 4.7 4.8 4.7 4.6 5.3 5.8 6.3 6.9 7.0 7.7 8.4
surfzone
Percentage of drops >2oq 5.7 5.3 7.8 7.4 10.5 8.5 4.8 3.6 5.6 7.1 6.0 5.6 3.1 1.9 0.7 1.7 2.0 5.8 5.6 4.6 3.0 1.9 5.0 3.9 3.2 3.6 0.9 0.8
6.2
3.5 1.3 2.6 2.6 4.1 2.1 1.9 2.4 3.0 2.4 1.3 0.9 1.0 5.3 4.8 5.6 4.3 4.2 5.2 4.6 3.3
2.7 3.2 2.3 2.4 1.6 2.2 2.5
due to evaporation
The data of the size of spray droplets were recorded in the form of a number distribution, with 256 equal size bands up to _’108 pm diameters, though relatively few drops greater than 20 pm in diameter were present in the aerosol (Table 1). The number median diameters for all the spectra measured ranged 5-8 pm. Changes in the size distribution were expected to occur between the surf zone and the instrument
’
2.6 2.5 3.0 3.6 4.2 4.0 3.7 3.1 3.3 3.4 3.3 3.0 2.9 2.5 1.4 1.5 1.2 1.1 1.2 0.93 1.3 1.3 1.8 2.2 2.0 1.9 0.59 0.66 0.87 0.93 1.0 1.2 1.4 1.9 3.2
E 9:6 9.2 9.0 9.0 8.7 9.2 10.0 8.6 9.0 8.2 9.3 10.2 9.9 10.2 9.6 9.8 10.3 10.4
3. RESULTS
3.1. In situ sizing of aerosol droplets close to
Number of drops (thousands)
and the gravitational
Median diameter (rm)
7 7
7 7
7 7 7 7
7 5 5 5 5 5 5 5 6 5 6 5 6 6 6 6 6 6 6 6
settling
large drops. The effect of the former was simulated
of for
drop sizes
0.2-120 pm, in 0.2 pm intervals, using the FACSIMILE numerical integration package (Curtis and Sweetman, 1985). The time of flight, assuming that all drops
were generated
at the surf zone, was estim-
ated at between 1 and 65 s by dividing the distance between surf-zone and instrument by the wind speed component perpendicular to the shore. The size distributions, corrected for evaporation, were calculated using the mean and extreme values of temperature,
W. A. MCKAYet al.
3302
Table 2. Details of aerosol sampling
Date 18.6.87 10.11.88 10-11.11.88 11.11.88 11-12.11.88 17.10.89 20.10.89 20.10.89 21.10.89 21.10.89 16.10.91 3.11.91 19.12.91
Run 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Estimated mean distance from water line (m)
Mean wind
Time of
speed (m s-‘)
high water
1.6 6.2 7.0 7.5 7.6 6.9 5.0 4.5 5.6 7.2 8.7 7.1 10.0 11.9 9.3 12.7
31 15 14 20 23 312 33 34.5 35 16 15
relative humidity and time of flight recorded during each of the runs. The corrected distributions represent the size spectra of the sea spray as generated immediately above the sea surface, before any evaporation occurred. The number median diameters for the corrected size distributions averaged over all the data sets were 14, 16, 17 pm for minimum, average and maximum evaporation, respectively, compared to that of about 7 pm in the measured distributions. For droplets less than N 15 pm, evaporation to an equilibrium diameter is predicted to be virtually complete before they reached the samplers, whilst in contrast droplets larger than 40pm would have shown relatively little change in diameter due to evaporation. It is the droplets between N 15 and 40 pm for which the corrections are most sensitive to the varying temperature, relative humidity and flight time during a run.
1801 1135 2340 1200 0015 1330 1600 1705 1800 0835 2125
Sampling times
1701-1917 1147-1319 2147-2318 2357-0050 0059-0145 1115-1315 1429-1625 2212-0215 1137-1515 0904-l 106 1400-1753 1004-1213 1500-1900 1628-1955 0710-1030 1930-2305
3.2. Chemical analysis of aerosol droplets and suspended particulate material in sea water The concentrations of Na, Al, Si and Fe in air were derived by dividing the mass of each element found on the filter by the quantity of air sampled. Calculation showed that the contribution of Na contained within suspended mineral particles to the Na concentration in airborne sea spray was negligible (a value of Na/Al of 0.3 for typical aluminosilicate minerals was used; Calvert, 1976). The Na sampled was therefore attributed entirely to sea salt. The concentrations in air of Na, Al, Si and Fe, determined using the high volume aerosol samplers, are listed in Table 3 with the %/Al and Al/Fe ratios. In Table 4 the ratios of Al, Si and Fe to Na are given. Aliquots of filters from runs 5 and 8 to 12 were analysed for 239+ 240Pu and 241Am and the actinide air concentrations are also shown in Table 3.
Table 3. Concentrations of radionuclides and stable element in air, determined using a high volume aerosol sampler Elemental ratios
Air concentrations Run 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Mean
Na (pgrn-j)
Al @gm-3,
Si @grnm3)
Fe @pm-“)
2.6 108kll 91+9 126k13 64k6 72*7 23k2 34*3 73.5f 7.4 14.2k1.4 54.8f 5.5 11.8k1.2 59.5f 6.0 74.7* 7.5 65.lk6.6 66.9f 6.8
1.3kO.l 1.2kO.l 1.9kO.2 0.7*0.1 0.8kO.l 0.16*0.02 0.26kO.03 0.9+0.1 0.15kO.02 0.6kO.l O.ll+O.Ol Q.8&-0.1 0.6 kO.2 1.2*0.1 1.1*0.1
2.6kO.3 2.5kO.3 4.1 kO.4 1.4kO.l 1.7kO.2 0.43&O&l 0.67f0.07 1.9kO.2 0.35kO.03 1.3*0.1 0.23kO.02 1.8kO.2 1.6kO.2 2.5kO.3 2.7kO.3
0.9*0.1 0.7kO.l 1.2kO.l 0.5kO.l 0.5*0.1 0.19*0.02 0.17&0.02 0.6kO.l 0.07*0.01 0.4*0&i 0.06~0.01 0.5 f 0.05 0.4*0.05 0.7*0.1 0.6kO.l
z38PU bBqm_“)
239+240pu 241Am @Bqmm3) @Bqmm3)
3.5* 0.9
15.3k1.8
27.3f 2.2
3.5io.5 ~1.8 3.0*0.4 <1.7 4.0 f 0.3 c 1.1 cl.0 2.4* 1.8
12.3*0.9 <1.8 13.7kO.8 <1.7 16.1kO.8 8.4k2.6 6.6k2.2 9.8k2.7
19.2k3.5 cl.8 23.1f 1.1 <1.7 23.9f 1.1 -
Si/Al
2.OkO.3 2.2kO.3 2.lkO.3 2.OkO.3 2.OkO.3 2.6kO.4 2.5kO.4 2.2kO.3 2.3 kO.3 2.2kO.3 2.1kO.3 2.1*0.3 2.6kO.5 2.1*0.3 2.5kO.4 2.2kO.2
Al/Fe
1.5kO.2 1.8kO.3 1.7kO.2 1.4kO.2 1.7kO.3 0.8kO.l 1.5iO.2 1.6kO.2 2.OkO.3 1.4kO.2 1.9kO.3 1.7kO.2 1.7+0.3 1.8iO.3 1.8kO.3 1.6kO.3
Shore-line sea spray aerosol
3303
Table 4. Radionuclides and trace element ratios to sodium in aerosol samples Trace element ratios to Na Run
A.l/Na (mgg-‘)
Si/Na (mpg-‘)
Fe/Na (mgg-‘)
: 3 4 5 6 7 8 9 10 11 12 13 14 15 Mean
12.2+ 1.7 128f1.8 15 3k2.2 107~1.5 11 3k1.6 7.0* 1.0 7.8kl.l 11.951.7 10.5* 1.5 11.0+1.5 9.4f 1.3 14 lk2.0 8.2+ 1.2 18 3+2.7 16 1 k2.4 11 8k3.1
27.9 k4.0 24.3k3.5 32.3 k4.6 22.0 + 3.2 23.5 + 3.3 19.6k2.9 19.7 k2.8 26.4k3.7 24.6 f 3.5 23.8 &-3.4 19.6k2.8 30.4k4.3 21.4k3.6 38.2k6.1 4O.Ok6.3 26 + 6.2
7.9*f 1.1 7.2 1.0 9.2 f 1.3 7.7 f 1.2 6.8 f 1.0 8.4+ 1.2 5.1 kO.7 7.5+ 1.1 5.3kO.7 7.6+ 1.1 4.9 + 0.7 8.3 f 1.2 4.9 + 0.8 10.2& 1.6 8.8* 1.4 7.3 + 1.6
23sPu/Na (mRq g - ‘)
23g+240Pu/Na (mRq g - ‘)
*‘I Am/Na (mRq g-‘)
55*7
238+18
424k28
47 <130 55*7 <145 67k7 <14 35&26 <17
168+15 <130 25Ok 18 <145 27Ok 18 113+36 101&36 147+44
262k24 i130 421+30 < 145 402 + 27
For individual samples, errors quoted include all sources of analytical uncertainty. For means, errors are the sample standard deviations.
The suspended particulate loadings in the nearshore surf zone waters varied by up to an order of magnitude between runs and a factor of about 4 during runs (Table 5). The Si/Al and Al/Fe ratios showed less variability.
Table 5. Concentrations
4. DISCUSSION
4.1. The source of sea spray measured on the beach In correcting for evaporation (Section 3.1) it was assumed that the sea spray measured originated prin-
of trace elements in particulate material suspended in sea water Sea water particulate
Run
Tim’s
1 Start 1 End 2 Start 3 Start 4 End 5 Start 5 End 6 7 Start 7 Middle 7 End 8 Middle 9 Middle 10 Middle 11 Middle 12 Middle 13 Start 13 Middle 14 Middle 14 End 15 Middle Means
1155 1325 2130 0010 0130 1130 1320 154!i 222ti OOlti 0207 1330 093ti 1530 1100 165fi 165fi 1810 0845 1035 2125
Salinity (G) 31.87 32.07 30.32 31.17 31.02 31.05 32.07 30.15 30.25 30.00 30.6
32.56 _ 31.47 31.3 31.2 29.9 30.4 29.0
Note: the Al,O, + Si02 + Fe,O, carbonate and organicis.
Particulate load mg d-r
Al(mgd-‘)
Si(mgd-‘)
7.3 * 0.7
49.7*5
3.8kO.4 21.3 k2.1
Fe(mge-‘)
Si/Al
Al/Fe
4.OkO.4
6.8+ 1.0
1.8kO.3
26+2.6
2.6kO.3
5.5f0.7
1.5kO.2
187k19
9.3 + 0.9
8.75 k 1.2
2.3kO.3
5.2kO.5 2.1 kO.2 4.1 kO.4 2.OkO.2 3.6kO.4
35.5k3.6 14.7+ 1.5 28.2k2.8 16.0* 1.6 10.7+ 1.1
2.8 f 0.3 1.2kO.l 2.2kO.2 1.250.1 2.OkO.2
6.8f 1.0 7.lkl.O 6.7k 1.0 7.9+ 1.1 3.OkO.4
1.8kO.3 1.7kO.2 1.8kO.3 1.8kO.3 1.8+0.3
3.3*0.3 4.9kO.5
10.4f 1.0 19.0+ 1.9
1.9kO.3 2.9kO.3
3.2 + 0.5 3.9 +0.5
1.7kO.2 1.7kO.2
2.9 + 0.3
8.6kO.9
2.2kO.2
3.OkO.4 5.7k2.1
1.3kO.2 1.7kO.2
90
168 158 88 328 433 122 129 419 650 371 188 68 97 63 58 47 45 62 60 167
accounts for only 4 to f of the particulate load; the remainder probably consists of
W. A. MCKAY et al.
3304
number measured. For the small droplets, evaporation is rapid and the assumption of equilibrium is valid whether the droplets originate from the surf zone or the distant sea surface. For large droplets, deposition limits the range of travel, and a greater concentration ratio is expected. The importance of the large droplets from the surf zone is indicated by the total concentrations observed on the beach in Cumbria, an order of magnitude greater than those observed over the open sea surface by other observers (Erikson, 1986). Some part of the increase in concentration is due to a difference in sampling heights, since the measurements over the ocean surface were typically at 10 m height, while the beach measurements were at 1.5-2 m. However, the results of Exton et al. (1985) at 2 and 10 m above a beach did not differ substantially. The contributions of large droplets from the surf zone is further illustrated by the five to tenfold concentration difference at low tide (runs 9 and 11, Table 3) and high tide (runs 2-5) at similar wind speeds. In addition, Eakins et al. (1982) found that the sea salt concentration in air 1 m above the sea surface to windward of the surf zone was an order of magnitude smaller than that measured at the same time on the beach. Thus, the spray from the broad surf zone dominates sea salt concentrations in measurements made on the beach, although at greater heights, or further inland, spray from the great area of sea surface offshore may make an important contribution.
cipally from the surf zone. This was suggested by an increase in concentration, particularly of the largest drops, at high tide, when the surf zone was close to the instruments. A similar increase was clearly demonstrated in the results of Exton et al. (1985). However, some increase in concentration would result from proximity to the source area at high tide, even if the whole sea surface were a uniform source. The significance of the surf zone can be assessed by comparing changes in the number of droplets counted (Table 1) with the results of Wilson (1982) who used a numerical modelling approach to predict the variation of concentration near surface source of varying extent. Wilson’s model takes account of dispersion of the droplets, but not of deposition. This is not a serious restriction since the numbers of droplets counted were dominated by small droplets, which deposit slowly. From Table 1, the measurements from 10 and 11 November 1989 up to 12.56 h, and the measurements of 20 and 21 October 1989 form a group with a limited range of wind speed in near high tide conditions. The mean wind speed was 8.4 m s- ‘; the mean distance of the instruments from the water’s edge was 24 m and the mean droplet count per run was 2900. We compare these with the data for 20 October 1989 up to 13.30 h, for which the wind speed was similar (8.2 m s-l) but the range from the edge of the sea averaged 3 12 m. For this latter group of runs the mean droplet count was 1070. If the surf zone (a band of width SO-200 m) was the source, the results of Wilson (1982) show that the ratio of concentrations should be about 4.5-8. On the other hand, if the entire Irish Sea was the source, (50 km wide) the concentration ratio should be 1.5. The observed ratio (2.7) is intermediate, suggesting that for the small droplets, which dominate the total count, the surf zone contributes 50% or more, to the
100
4.2. The injluence of wind speed on the size and concentration of sea spray
In Figs 2a and 2b, the number and volume distributions of seaspray droplets, corrected for evaportion, are shown for a wind speed of 7 m s- ’ at high and
I
I
I
I
r
I
10 m
1
‘E
L Er
0.1
0 z
0.01 0
0.001
0~0001
0
CI.5i
0 Low water
( Surf-sampler
distance
400m
0
( Surf -sampler
distance
9 m)
High water
I
1
1
2
I
(
5 Droplet Corrected
L
I
20 10 radius (microns) for evaporation
1’
I
50 )
Fig. 2a. Variation of number distribution with distance from high water mark.
10 0
Shore-line sea spray aerosol
o
0.01
/ t _ __. 0.5
1
o
Low
a
High water
water
2
(Surf
3305
-sampler
distance
(Surf-sampler 5
Droplet (Corrected
aistance
10
20
400m ) 9m 50
radius (microns) for evaporation
1 1 100
1
Fig. 2b. Variation of volumetric loading with distance from high water mark.
low water at distances from the instrument to the surf zone of 9 and 400 m, respectively. The average number of drops in 0.5 pm increments of radius were used to plot dN/d log r and dV/d log r where N and V are the total number and volume of droplets respectively of radius less than r. These plots show that both the size of the largest drop observed and the number in each size band increa.se from low water to high water. The combined data taken around high tide enable the effect of wind speeds up to about 12 ms-’ on the aerosol flux and droplet size blown onshore to be
examined. Figure 3 indicates that the number concentration of droplets increases with increasing wind speed throughout the aerosol size range, the size distribution showing little change in shape. The number spectra, corrected for evaporation, show a peak at 12-17 pm diameter throughout the range of wind speed from 5 to 10 m s- ‘. This is around the upper limit of the range of peak sizes observed in oceanic aerosol spectra (between 8 and 15 pm; Monahan, 1986) and most likely reflects the difference between a distributed source and a local one. Losses of large
IO *
5ms-l
o 6rnsIi 0 9 ms 0 10 ms-l
A
0
0
A
0
0 *
O~OOl 0 +
*
0~0001
I 0.5
I
I
I0
5
1
(
Droplet Corrected
10 radius (microns) for evapoiation
1
Fig. 3. Variation of number distribution with wind speed.
3306
W. A. MCKAY et
particles due to deposition will be more important in size spectra observed over the ocean. Whilst atmospheric sea salt concentrations can be estimated from the Phase Doppler data, the results are likely to be underestimates due to effects at both ends of the instrument size range (Livesley and Gillespie, 1991). At the small size end, bias is caused by threshold effects, while at the large size end, bias results from droplet distortion. A number of effects may cause large droplets to be incorrectly sized or to be omitted from the size distributions: droplet distortion may introduce such errors (Livesley and Gillespie, 1991) and the presence of suspended particles may have had a similar effect. Finally, the small number of large droplets counted effectively truncated the observed size distributions at about 60 pm diameter. The omission of large droplets may have had a substantial effect on the total mass collected due to the relatively large mass per particle (as illustrated in Fig. 2). However, the above effects would cause only minor distortion to the number spectrum of droplet sizes. The technique should thus provide a good measure of the sea spray size distributions, up to about 60 pm diameter, and give valid information on the influence of wind and tidal conditions on the size and concentration of sea spray in air. However, for mass concentration measurement, the high volume air sampler data are preferred. The relationship between wind speed, U, and the mass of atmospheric sea salt S (pgme3), on the Cumbrian coast was derived from the total aerosol samples collected at the shoreline using data collected
500
“E
400
In c.E!
where U = wind speed (m s- ‘) and S = sea salt concentration (pg mA3). This implies that at wind speeds below 2 m s- ’ the sea salt concentration blown onto the Cumbrian coast is -10~gm-3,increasingto ~50~gm-3in5ms-’ winds and -200 pg mm3 in 10 m s-l winds. A number of authors have attempted to relate the variation of airborne sea salt concentration to wind speed. A recent study by Erickson et al. (1986) summarised several studies of the concentration over the open ocean by the following relationship between U and S: lnS=0.16U+1.45 elevation lnS=0.13
U+1.89
for U>lSms-‘.
lnS=0.16U+2.92,
Uc13ms-‘at
June
0
November
2melevation.
A
October
*
November Best
1987
Ii
,*’
1989
/ ,’
I
1989 1991
/’ /
t
In S = 0,23u+
/’ #’ #’
3.05
-
,’
/ ,'
A * 200
15m
Exton et al. (1985) made measurements at the top of a beach adjacent to a narrow surf zone and found:
-
A / .’ / .’ .*
!
=‘ :: P m
for U
0’ ,’
?!
2 s
In S = 0.23 U + 3.05
sampler
0
- ----300
over the 1987-1991 period for high water conditions and sampling periods of about 2-4 h duration, extending about 90-120 min either side of high tide. This is equivalent to a range of mean distances from sampler to water line of about 15-35 m. High-tide measurements with sampling times shorter than about 1.5 h were rejected, since the close proximity of the surfzone to the samplers throughout the measurement period would result in high concentrations, incompatible with the majority of the “high-tide” samples. The data, shown in Fig. 4, can be represented by
I-
Air
-
E?
al.
0
100 __*----
____.....----0' 0
0
__-- _---
_*--
_*--
_/-
_.*-
.-’
/
,
I
I
I
2
4
6
6
Wind
speed,
/’
* *
/ ,/A
I
._
10
I ._
12
14
u (ms-‘I
Fig. 4. Variation in atmospheric sea salt concentration with wind speed for sampling periods 90-120 min either side of high tide.
Shore-line sea spray aerosol Above 13 m s-l, the concentration remained constant in the data set of Exton et al. Their data show concentrations somewhat larger than those measured over the open ocean by other workers. The concentration of sea salt observed in Cumbria close to the broad surf zone is somewhat greater at all wind speeds than that observed near a narrow surf zone by Exton et al. (1.985),and an order of magnitude greater than observed by Erickson et al. (1986) over the open ocean. The concentration increases more steeply with wind speied at the Cumbrian beach than in either of the other sets of measurements. 4.3. Enrichment factors Many studies have demonstrated that the concentration of inorganic particulate material in sea spray is greater than that in the sea water from which the spray was derived (Eakins and Lally, 1984; Fry, 1983; Walker et al., 1986; P’attenden et al., 1989). The most likely mechanism for I his enrichment is the scavenging of -particulate material from the water column by rising bubbles, which produce particulate rich droplets on bursting at the surface. Since evaporation would complicate direct assessment, all such studies have used Na as a me.asure of the amount of sea spray represented by a sample and the enrichment factor (EF) of an element X in a sea spray sample relative to the sea water from which it was ejected is defined as
The results discussed in Section 4.1 are consistent zone is the dominant with the hypothesis that the surf source of the marine aerosol determined in the measurements reported here, with aerosol being produced
3307
predominantly by the bursting of bubbles generated by breaking waves. At the range of wind speeds encountered in this study, the values of { [X]/[Na]}aeroaol vary by little more than a factor of 2, for Al, Si, Fe and the actinides (Table 4). For Al only there is a correlation between increasing Al/Na ratios and increasing wind speed, significant at the 5% level. In contrast, the values of the { [X]/[Na]},,, wB,crfor all measured elements in the surf zone appear far more variable. This is predominantly due to changes in the quantity of particulate present in the sea water rather than changes in its quality, the particulate composition being approximately constant (see elemental ratios in Table 5). The variability in spot samples of sea water on any particular occasion may be. large, due to small scale inhomogeneity in surf zone water, and may cause small numbers of samples to overestimate variations between occasions. The enrichment factors relative to the concentrations in the shoreline sea water range generally from about S-50 for Al and Fe, and about l-40 for Si (Table 6). The generally lower enrichment of Si reflects the lower Si/Al ratio in the aerosol of about 2 compared to 5-9 in shoreline sea water (compare Tables 3 and 5). Aluminosilicate minerals, which include the fine clays, have a Si/Al ratio of about 2, and the low Si/Al ratio in the aerosol probably reflects the more efficient scavenging of fine clay out of the water column, than of the other main silicon-containing mineral, quartz (SiO,), which is of generally coarser grain size. As for Al and Si, the { [X]/[Na]}._,, values for the actinides are fairly constant (Table 4). The wind speed was also relatively similar between runs where the actinides were measured. However, the increase in
Table 6. Enrichment factors [EF] in aerosol samples* Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Al 17*3 19*9 4Ok18 7+4 9*4 18&3 3.5_+1 22&4 50&9 28k4 47+11 40+5 25+9 35+_6 52+11
Si 5+1 8+3 16k5 3+1 4+1 7*1 1 kO.2 7+1 17&3 9*2 12*2 29&5 21k4 19+4 43*8
Fe 21k4 18k9 42k20 9*5 10+5 32k6 5+1 26&5 45+7 37+6 41*7 42&8 26k5 34+_6 37*7
26
29
19&4 ~25 30*4
19*3
58+5
36k5
* R.$ative to suspended particulate fraction only. EF =(CX1/CNalaerosol)l(Cxl/CNal sea water), where direct analyses were not made of the concentration of Al, Fe and Si suspended in sea water, the concentration was deduced from the particulate load and the mean composition of particulate matter for the eleven analyses reported in Table 5.
3308
W. A.
MCKAY
AI/Na ratio with wind speed suggests that the concentration of fine particulate and associated actinides in the aerosol does increase with wind speed. The actinide enrichment factors are between 20 and 40 (Table 6), and are similar to those for aluminium. A similar conclusion was reached by Walker et al. (1986) who noted that the EFs for actinides and aluminium were similar in offshore aerosol generated using a bubbling apparatus that simulated a breaking wave. However, Walker’s values of EF for actinides, Al and Si were considerably higher (100-600) than those observed here. The concentration of Al and Si suspended in offshore sea water was much smaller than that in the surf zone, but the ratio of each to Na in seaspray was similar in both areas. Thus the results suggest a saturation effect which prevents enrichments in seaspray at the surf zone achieving the values seen offshore. This explanation is supported by the 1 CAlI / CNalLosolmeasured by Walker offshore being within a factor of 2-3 of those measured adjacent to the surfzone of 11.8k3.1 pgg-’ in Table 4. The { [Al]/[Na]} ratios observed in sea spray here can also be compared with corresponding values of 25f52mgg-1indepositionand134f175mgg-1in air samples collected 200 m inland at Eskmeals by Pattenden et al. (1989). The higher ratio in the air samples at Eskmeals may relate to a propensity of fixed air samplers to under-collect particles larger than a few tens of microns in diameter (Garland and Nicholson, 1991), while the deposition collector will receive large particles preferentially. However, any attempt to make deductions about the variation of enrichment of suspended particulate material is confused by the contribution of land-derived dust to the air and rain samples, which are collected continuously regardless of wind direction. It is indicative that both air and deposition samples show higher { [Al]/[Na]} ratios than those recorded in the sea spray samples reported here.
5. CONCLUSIONS
The mass concentration of sea salt in on shore winds on the gently sloping beach in Cumbria was several times greater than observed over the sea surface, and was dominated by spray emitted from the surf zone. However, the offshore sea surface also made an important contribution to the number concentration of sea spray droplets measured on the beach. The concentration of sea salt in air increased with wind speed. There was some evidence that the rate of increase was more rapid than has been observed over the open sea. Fine suspended particulate matter was enriched in the sea spray. The concentration of Al, a tracer for this material, in spray relative to Na, increased with wind speed, but varied little with the concentration seawater. The enrichment of Fe,239+240Pu
in and
et al.
241Am was generally similar to that for Al, and it is expected that other particle-associated substances would show similar behaviour. Acknowledgements-We thank our Harwell colleagues J. M. Howorth for assistance with Facsimile and R. Gillespie and G. Burton for operating the LISATEK. The Commander of the Proof and Experimental Establishment, MOD, Eskmeals kindly allowed us access through the range to set up our aerosol equipment on Eskmeals beach. We also thank the referees for constructive suggestions. This report was funded by British Nuclear Fuels plc and the Deuartment of Trade and Industrv fDTIb Much of the data u;lised was derived from an earlier $tud; funded by the Department of the Environment and the CEC. However the views expressed are entirely those of the authors and do not necessarily reflect those of the funding organisations.
REFERENCES Blanchard D. (1983) The production, distribution and bacterial enrichment of the sea salt aerosol. In Air-Sea Exchange of Gases and Particles (edited by Liss P. S. and Slinn W. G. N.), pp. 407-454. Reidel, Dordrecht. Brazier K., Gillespie R. F., Dalze.11W. and Livesley D. M. (1988) Bias corrections to size distributions and concentrations in phase doppler particle measurement. AERE R13270. Calvert S. E. (1976) In Chemical Oceanography (edited by Riley J. P. and Chester R.), ,. Vol. 6. Academic Press. L&don. Curtis A. R. and Sweetman (1985) Facsimile release H user’s manual. HMSO. London. AERE R11771. Eakins J. D. and ially A. h. (1984) The transfer to land of actinide-bearing sediments from the Irish Sea by spray. Sci. total Enuir. 35, 23-32.
Eakins J. D., Lally A. E., Burton P. J., Kilworth D. R. and Pratley F. A. (1982a) Studies of environmental readioactivity in Cumbria. Part 5: the magnitude and mechanism of enrichment of sea spray with actinides in West Cumbria. AERE R10127. Eakins J. D., Lally A. E., Burton P. J., Kilworth D. R. and Pratley F. A. (1982b) Studies of environmental radioactivity in Cumbria Part 5. The magnitude and mechanism of enrichment of sea spray with actinides in West Cumbria. AERE R10127, HMSO. Erickson D. J., Merrill J. T. and Duce R. A. (1986) Seasonal estimates of global atmospheric sea salt distributions. J. geophys. Res 91, 1067-1072. Ewins C. P. and Spencer C. P. (1967) The annual cycle of nutrients in the Menai Straits. J. Mar. Biol. Assoc. UK. 47, 533-542. Exton R. J., Latham J., Park P. M., Perry S. J. and Smith M. H. (1985) The production and dispersal of marine aerosol. Q. J. R. met. Sot. 111, 817-837. Fry F. A. (1983) Airborne radionuclides in the vicinity of a coastal nuclear fuel reprocessing plant. J. Aerosol Sci. 14, 452-455. Garland J. A. and Nicholson K. W. (1991) A review of methods for sampling large airborne particles and associated radioactivity. 3. Aerosol Sci. 22, 479-499. Lally A. E. and Eakins J. D. (1978) Some recent advances in environmental analysis at AERE Harwell. Symposium on the determination of radionuclides in environment and biological materials. Central Electricity Generating Board, London, Paper 12, pp. l-9. Livesley D. M. and Gillespie R. F. (1991) Mass balance measurements using the phase Doppler analyser. In Sprays and Aerosols ‘91, jointly organ&d by ILASS-EUROPE
Shore-line sea spray aerosol and The Aerosols Soa,ety, U.K., pp. 32-36, University of Surrey, Guildford. McKay W. A. and Walker M. I. (1990) Plutonium and ameridium behaviour in Cumbrain near-shore waters. J. en&. Radioactivity 12,, 267-283. McKay W. A., Walker M. I. and Cloke J. (1990) Transfer of radionuclides from sea-to-air to land, DoE/RW/90.094. Department of the Environment, London. Monahan E. C. (1986) The ocean as a source of atmospheric particles. In The Role of Air-Sea Exchanae in Geochemical Cycling (edited by B&t-Menard), Reidei, Dordrecht. Pattenden N. J., Cambray R. S. and Playford K. (1981) Trace and major elements in the seasurface microlayer. Geochim. Cosmochim. Acta. 45, !)3-100.
3309
Pattenden N. J., Cambray R. S. and Playford K. (1989) Studies of environmental radioactivity in Cumbria Part 16. Trends in radionuclide concentrations in coastal airborne and deposited materials, 1978-1987. AERE R12617, HMSO. Peirson D. H., Cawse P. A. and Cambray R. S. (1974) Chemical uniformity of airborne particulate material, and maritine effect. Nature 251, 675-679. Walker M. I. (1989) The sea-to-air transfer of radionuclides. Ph.D. thesis, University of East Anglia. Wilson J. D. (1982) Turbulent dispersion in the atmospheric surface layer. Boundary-Layer Met. 22, 399-420.
Atmospheric
Environment Vol. 28, No. 20, pp. 3299-3309,
1994 Elsevier Science Ltd Printed in Great Britain. 1352-2310/94 $7.00+0.00
1352-2310(94)00156-l
THE CHARACTERISTICS OF THE SHORE-LINE SEA SPRAY AEROSOL AND THE LANDWARD TRANSFER OF Ri4DIONUCLIDES DISCHARGED TO COASTAL SEA WATER W. A. McK,dy,* J. A. GARLAND,* D. LIVESLEY,~.C. M. HALLIWELL*and M. I. WALKER* * AEA Technology, National Environmental Technology Centre, Culham, Abingdon, Oxfordshire, OXh4 3DB, U.K.; and t AEA Technology, Harwell, Oxfordshire, OX11 ORA, U.K. (First received 18 September
1993 and
in finalform
28 April 1994)
Abstract-The droplet size distribution and mass of sea spray blown onto the Cumbrian coast have been measured on a beach at a wide range of wind speeds. The results indicate that the aerosol generated immediately above the surf zone has a size spectrum (plotted as dN/d log r) that peaks between 12 and 17 pm diameter and shows little variation within the wind speed range 5-10 ms-‘. The observed peak diameter is around the upper limit of the range of peak sizes observed in oceanic aerosol spectra. The concentration of airborne sea salt was substantially greater in on-shore winds over the beach than has been observed lover the open ocean, and there was some evidence that it increased more rapidly with wind speed. The Al to Na ratio, a measure of the concentration of fine sediment suspended within the droplets of sea spray, appears to increase with increasing wind speed. This finding has implications for assessing the transfer of particulate-associated pollutants such as plutonium, bacteria and some heavy metals. Key word index: Sea spray, actinides.
1. INTRODUCTION
Pollutants discharged. into coastal waters can be present in substantial concentrations in the surf zone, an area where wave energy, accumulated from the wind over substantial areas of sea, is dissipated in a narrow band, resulting in intense generation of sea spray. This aerosol is carried inland when the wind is on-shore, exposing the population to increased levels of airborne pollutants and contaminating crops and surfaces by deposition over the coastal zone. Studies have shown that substances that adsorb to suspended particles in sea water, e.g. metals (Peirson et nl., 1974) and bacteria (Blanchard, 1983), are present in spray at concentra.tions significantly greater than were present in the bulk sea water, enhancing the transfer of such substances inland. Some of the actinide elements, discharged from the Sellafield reprocessing plant in Cumbria into the Irish Sea, have accumulated in sediment close to the Cumbrian coast. Resuspension of this sediment maintains measurable actinide concentrations in near shore sea water, despite large decreases in radionuclide discharges (McKay and Walker, 1990), and transfer of these nuclides to land in searspray has been observed along the Cumbrian coast and other coastlines of the Irish Sea (McKay et al., 1990). Since the actinides have a unique source in the area, and background levels from other sources are limited, these studies provide valu-
able information, not only on the sea-to-land transfer of radionuclides along the Cumbrian coast, but also on the potential significance of sea spray transfer for other pollutants discharged into coastal waters. In modelling the transfer of particulate-associated pollutants from sea to air to land, information on the onshore flux and droplet size of the sea spray as a function of wind speed is required, as are estimates of the sediment content of the aerosol. Experimental data on sea spray flux to the Cumbrian coast near Sellafield have recently been published (McKay et al., 1990). In the present paper we incorporate additional data and describe the dependence of the characteristics of the sea spray aerosol on environmental factors.
2. EXPERIMENTAL 2.1. Field programme The work was carried out at Eskmeals on the coast of Cuibria (Fig. 1) in June 1987, November 1988. October 1989 and du&g’the last quarte; of 1991. The site,.typical of many intertidal shores, consists of an open sandy beach running approximately north to south, interrupted by occasional rocky outcrops, stretching out to N 600 m at low water and with a broad surfzoneat high water. Inland there is an undulating coastal plane, the land climbing progressively to 570 m about 8 km inland.
W. A. MCKAY et al.
3300 0,
5
‘fl
Kilometres
Cumbria
Fig. 1. Location of sampling site. The programme was designed to gather data on the droplet size, flux and particulate content of sea spray aerosol blown onshore in a wide range of wind speeds. Size and concentration measurements of sea spray droplets were carried out in 1988 and 1989 using a LISATEK Phase Doppler instrument (Brazier et al., 1988;Livesley et of., 1991). The instrument contained a 25 mW Helium Neon laser with beam splitting optics, which allow two coherent laser beams to be focussed together in the open air. The volume in which the beams intersect contains a pattern of interference fringes. When a droplet passes through this volume, the light it scatters is modulated by the interference fringes. Three photomultipliers were used to observe the scattered light, at different angles relative to the illumination system. The angular separation leads to phase differences in the modulation of the signals that are proportional to the diameter of the droplet. The nominal dynamic range of the instrument for particle diameter is 30: 1 with an upper limit of 108 pm, giving a minimum size of about 3-4pm. However, below about 10 pm, oscillations in the response curve are likely to impose an uncertainty of *2-3 pm on the size attributed to any individual droplet. Where an aerosol with a wide size distribution is counted, no significant distortion of the size distribution is expected. Every particle between about 3 and 100 pm diameter carried through the probe volume by the wind is thus counted. However, irregular signals such as those generated by non-spherical particles are not suitable for size analysis, and such particles are counted but not sized. This system records but does not collect particles. The instrument was set up so that the probe volume was normal to the wind direction and about l-l.5 m above the ground. The sensitive volume is w 30 cm outside the instrument casing, in an area where disturbance of the wind Bow by the presence of the instrument is small. Whilst there is little disturbance of wind flow and all droplet sizes should follow
the flow and be counted, it is likely that the data collected give a poor representation of very large droplets because of the small numbers present. On average, only 4% of the drops sized in each 30 min data file were t 20 pm diameter (see Table 1). However, such large droplets constitute a large fraction of the total mass of spray (see below). The data from the LISATEK Phase Doppler instrument were collected in 30 min runs which were each given a 4-figure code (Table 1). Representative samples of sea spray suitable for chemical analysis were collected using a high volume air-sampler. This draws air through an 18 x 23 cm exposed area of Whatman 41 filter at a nominal rate of -2 m3 min-‘, giving a face velocity of 0.8 m s- I. Air flows were determined regularly by measuring the pressure across a calibrated orifice fitted to the pump, enabling the volume of air sampled to bc estimated. An inlet cone, selected to set the inlet velocity to a value close to that of ambient wind, was fitted in front of the filter and the sampler directed into the wind. This provided approximately isokinetic sampling. so that that the bias in collection of large droplets due to their inertia was avoided. Four cones were constructed. The largest cone (inlet area = 111 cm’) allowed isokinetic (unbiased) sampling at a nominal wind speed of 3 m s-i, and others were designed for 6 (55.5 cm’), 9 (37 cm’) and 12 m s-’ (28 cm’) winds. For intermediate wind speeds, the cone designed to operate at the wind speed approximating closest to the measured wind speed was used. The aerosol sizing and sampling devices were operated on the beach about 1.3-1,5 m above the ground and roughly N 2 m from the high water spring tide mark. A portable meteorological station was run throughout the experiments, recording the wind speed (at the height of the sampler inlet) and direction, relative humidity and temperature at 1 min intervals. Sampling was carried out during onshore westerly winds in November 1988, October 1989 and in October, November and December 1991, generally at high water, but on one occasion at low water. The details of the aerosol sampling are given in Table 2. Sea water was collected from the shoreline during sea spray collection. Each sample was taken in 1 m deep water just below the sea surface in 4 and 2 r? polythene bottles. The 4 &’samples were filtered through 14 or 29 cm diameter 0.45 pm pore size cellulose acetate filters and the filtrate acidified to give a pH<2 with nitric acid. The filtrate and particulate loaded filters were stored for later radiochemical analysis. The 2 G samples were passed through 4.7 cm diameter 0.45 pm polyvinylide difluoride filters (PVDF) which were afterwards rinsed with distilled water to remove sea salt. The latter filters, being hydrophobic, can be readily dried to a constant weight within LO.5 mg and were used to estimate the particulate loadings. Some were later analysed for the elements Si and Al. 2.2. Analysis The mass of sea spray collected and the quantity and quality of sediment in the sea spray samples were determined by analysing for Na, Si, Al and Fe. Inductively coupled plasma atomic-emission spectrometry (ICP-AES) was used. As a liquid sample is required for the spectrometer, it was necessary to destroy the loaded filters, whether Whatman 41 or cellulose acetate, and solubilise the sediment. This involved fusion with lithium metaborate at 1000°C and extraction into HNOJHF acid solution (Chemical Analysis Department, Harwell, private communication). The acidified filter solutions and sea water (particulate fraction only in 1991) were analysed for 238Pu, 23g+240Pu and 24’Am (the last not in 1991) by alpha-spectrometry after radiochemical separation. The methodology, described in Lally and Eakins (1978) involved radiochemical separation, plating the nuclides out on a metal substrate in a thin layer, and finally carrying out alpha-spectrometry to a detection limit of 1 mBq.
Shore-line sea spray aerosol
3301
Table 1. bata from Phase Doppler measurements Start I.ime of run Date 10.11.88
11.11.88
11.11.88
11.11.88
12.11.88
17.10.89
20.10.89
20.10.89
21.10.89
21.40 22.10 22.40 23.10 23.40 00.10 00.40 01.10 10.26 10.56 11.26 11.56 12.:!6 12.56 22.06 22.:16 23.06 23.216 OOS6 00.36 01.06 01.36 1 l.!IO 12.00 12.30 13.W 09.30 10.00 10.30 11.00 11.30 12.00 12.2’0 13.00 14.00 14.2’0 15.CO 15.2’0 16.00 16.2’0 17.W 17.10 14.2.9 14.59 15.2.9 15.5’9 16.2.9 16.59 17.2.9 17.59
Mean distance (m) from surf zone 24 15 10 8 9 11 17 26 11 9 10 13 16 25 31 23 16 14 15 17 20 27 35.5 25.5 17.5 13 451 441 414 367 302 235 169 118 46 34 27.5 24.5 25.5 28 36.5 43.5 45.5 35.5 29.5 28 29.5 31 34 43
Mean wind speed (m s-l)
7.6 6.9 6.6 6.7 6.9 7.6 6.9 7.6 7.1 6.8 7.1 6.8 6.8 6.4 4.3 3.9 4.4 4.3 5.0 4.7 4.8 4.7 4.6 5.3 5.8 6.3 6.9 7.0 7.7 8.4
surfzone
Percentage of drops >2oq 5.7 5.3 7.8 7.4 10.5 8.5 4.8 3.6 5.6 7.1 6.0 5.6 3.1 1.9 0.7 1.7 2.0 5.8 5.6 4.6 3.0 1.9 5.0 3.9 3.2 3.6 0.9 0.8
6.2
3.5 1.3 2.6 2.6 4.1 2.1 1.9 2.4 3.0 2.4 1.3 0.9 1.0 5.3 4.8 5.6 4.3 4.2 5.2 4.6 3.3
2.7 3.2 2.3 2.4 1.6 2.2 2.5
due to evaporation
The data of the size of spray droplets were recorded in the form of a number distribution, with 256 equal size bands up to _’108 pm diameters, though relatively few drops greater than 20 pm in diameter were present in the aerosol (Table 1). The number median diameters for all the spectra measured ranged 5-8 pm. Changes in the size distribution were expected to occur between the surf zone and the instrument
’
2.6 2.5 3.0 3.6 4.2 4.0 3.7 3.1 3.3 3.4 3.3 3.0 2.9 2.5 1.4 1.5 1.2 1.1 1.2 0.93 1.3 1.3 1.8 2.2 2.0 1.9 0.59 0.66 0.87 0.93 1.0 1.2 1.4 1.9 3.2
E 9:6 9.2 9.0 9.0 8.7 9.2 10.0 8.6 9.0 8.2 9.3 10.2 9.9 10.2 9.6 9.8 10.3 10.4
3. RESULTS
3.1. In situ sizing of aerosol droplets close to
Number of drops (thousands)
and the gravitational
Median diameter (rm)
7 7
7 7
7 7 7 7
7 5 5 5 5 5 5 5 6 5 6 5 6 6 6 6 6 6 6 6
settling
large drops. The effect of the former was simulated
of for
drop sizes
0.2-120 pm, in 0.2 pm intervals, using the FACSIMILE numerical integration package (Curtis and Sweetman, 1985). The time of flight, assuming that all drops
were generated
at the surf zone, was estim-
ated at between 1 and 65 s by dividing the distance between surf-zone and instrument by the wind speed component perpendicular to the shore. The size distributions, corrected for evaporation, were calculated using the mean and extreme values of temperature,
W. A. MCKAYet al.
3302
Table 2. Details of aerosol sampling
Date 18.6.87 10.11.88 10-11.11.88 11.11.88 11-12.11.88 17.10.89 20.10.89 20.10.89 21.10.89 21.10.89 16.10.91 3.11.91 19.12.91
Run 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Estimated mean distance from water line (m)
Mean wind
Time of
speed (m s-‘)
high water
1.6 6.2 7.0 7.5 7.6 6.9 5.0 4.5 5.6 7.2 8.7 7.1 10.0 11.9 9.3 12.7
31 15 14 20 23 312 33 34.5 35 16 15
relative humidity and time of flight recorded during each of the runs. The corrected distributions represent the size spectra of the sea spray as generated immediately above the sea surface, before any evaporation occurred. The number median diameters for the corrected size distributions averaged over all the data sets were 14, 16, 17 pm for minimum, average and maximum evaporation, respectively, compared to that of about 7 pm in the measured distributions. For droplets less than N 15 pm, evaporation to an equilibrium diameter is predicted to be virtually complete before they reached the samplers, whilst in contrast droplets larger than 40pm would have shown relatively little change in diameter due to evaporation. It is the droplets between N 15 and 40 pm for which the corrections are most sensitive to the varying temperature, relative humidity and flight time during a run.
1801 1135 2340 1200 0015 1330 1600 1705 1800 0835 2125
Sampling times
1701-1917 1147-1319 2147-2318 2357-0050 0059-0145 1115-1315 1429-1625 2212-0215 1137-1515 0904-l 106 1400-1753 1004-1213 1500-1900 1628-1955 0710-1030 1930-2305
3.2. Chemical analysis of aerosol droplets and suspended particulate material in sea water The concentrations of Na, Al, Si and Fe in air were derived by dividing the mass of each element found on the filter by the quantity of air sampled. Calculation showed that the contribution of Na contained within suspended mineral particles to the Na concentration in airborne sea spray was negligible (a value of Na/Al of 0.3 for typical aluminosilicate minerals was used; Calvert, 1976). The Na sampled was therefore attributed entirely to sea salt. The concentrations in air of Na, Al, Si and Fe, determined using the high volume aerosol samplers, are listed in Table 3 with the %/Al and Al/Fe ratios. In Table 4 the ratios of Al, Si and Fe to Na are given. Aliquots of filters from runs 5 and 8 to 12 were analysed for 239+ 240Pu and 241Am and the actinide air concentrations are also shown in Table 3.
Table 3. Concentrations of radionuclides and stable element in air, determined using a high volume aerosol sampler Elemental ratios
Air concentrations Run 0
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Mean
Na (pgrn-j)
Al @gm-3,
Si @grnm3)
Fe @pm-“)
2.6 108kll 91+9 126k13 64k6 72*7 23k2 34*3 73.5f 7.4 14.2k1.4 54.8f 5.5 11.8k1.2 59.5f 6.0 74.7* 7.5 65.lk6.6 66.9f 6.8
1.3kO.l 1.2kO.l 1.9kO.2 0.7*0.1 0.8kO.l 0.16*0.02 0.26kO.03 0.9+0.1 0.15kO.02 0.6kO.l O.ll+O.Ol Q.8&-0.1 0.6 kO.2 1.2*0.1 1.1*0.1
2.6kO.3 2.5kO.3 4.1 kO.4 1.4kO.l 1.7kO.2 0.43&O&l 0.67f0.07 1.9kO.2 0.35kO.03 1.3*0.1 0.23kO.02 1.8kO.2 1.6kO.2 2.5kO.3 2.7kO.3
0.9*0.1 0.7kO.l 1.2kO.l 0.5kO.l 0.5*0.1 0.19*0.02 0.17&0.02 0.6kO.l 0.07*0.01 0.4*0&i 0.06~0.01 0.5 f 0.05 0.4*0.05 0.7*0.1 0.6kO.l
z38PU bBqm_“)
239+240pu 241Am @Bqmm3) @Bqmm3)
3.5* 0.9
15.3k1.8
27.3f 2.2
3.5io.5 ~1.8 3.0*0.4 <1.7 4.0 f 0.3 c 1.1 cl.0 2.4* 1.8
12.3*0.9 <1.8 13.7kO.8 <1.7 16.1kO.8 8.4k2.6 6.6k2.2 9.8k2.7
19.2k3.5 cl.8 23.1f 1.1 <1.7 23.9f 1.1 -
Si/Al
2.OkO.3 2.2kO.3 2.lkO.3 2.OkO.3 2.OkO.3 2.6kO.4 2.5kO.4 2.2kO.3 2.3 kO.3 2.2kO.3 2.1kO.3 2.1*0.3 2.6kO.5 2.1*0.3 2.5kO.4 2.2kO.2
Al/Fe
1.5kO.2 1.8kO.3 1.7kO.2 1.4kO.2 1.7kO.3 0.8kO.l 1.5iO.2 1.6kO.2 2.OkO.3 1.4kO.2 1.9kO.3 1.7kO.2 1.7+0.3 1.8iO.3 1.8kO.3 1.6kO.3
Shore-line sea spray aerosol
3303
Table 4. Radionuclides and trace element ratios to sodium in aerosol samples Trace element ratios to Na Run
A.l/Na (mgg-‘)
Si/Na (mpg-‘)
Fe/Na (mgg-‘)
: 3 4 5 6 7 8 9 10 11 12 13 14 15 Mean
12.2+ 1.7 128f1.8 15 3k2.2 107~1.5 11 3k1.6 7.0* 1.0 7.8kl.l 11.951.7 10.5* 1.5 11.0+1.5 9.4f 1.3 14 lk2.0 8.2+ 1.2 18 3+2.7 16 1 k2.4 11 8k3.1
27.9 k4.0 24.3k3.5 32.3 k4.6 22.0 + 3.2 23.5 + 3.3 19.6k2.9 19.7 k2.8 26.4k3.7 24.6 f 3.5 23.8 &-3.4 19.6k2.8 30.4k4.3 21.4k3.6 38.2k6.1 4O.Ok6.3 26 + 6.2
7.9*f 1.1 7.2 1.0 9.2 f 1.3 7.7 f 1.2 6.8 f 1.0 8.4+ 1.2 5.1 kO.7 7.5+ 1.1 5.3kO.7 7.6+ 1.1 4.9 + 0.7 8.3 f 1.2 4.9 + 0.8 10.2& 1.6 8.8* 1.4 7.3 + 1.6
23sPu/Na (mRq g - ‘)
23g+240Pu/Na (mRq g - ‘)
*‘I Am/Na (mRq g-‘)
55*7
238+18
424k28
47 <130 55*7 <145 67k7 <14 35&26 <17
168+15 <130 25Ok 18 <145 27Ok 18 113+36 101&36 147+44
262k24 i130 421+30 < 145 402 + 27
For individual samples, errors quoted include all sources of analytical uncertainty. For means, errors are the sample standard deviations.
The suspended particulate loadings in the nearshore surf zone waters varied by up to an order of magnitude between runs and a factor of about 4 during runs (Table 5). The Si/Al and Al/Fe ratios showed less variability.
Table 5. Concentrations
4. DISCUSSION
4.1. The source of sea spray measured on the beach In correcting for evaporation (Section 3.1) it was assumed that the sea spray measured originated prin-
of trace elements in particulate material suspended in sea water Sea water particulate
Run
Tim’s
1 Start 1 End 2 Start 3 Start 4 End 5 Start 5 End 6 7 Start 7 Middle 7 End 8 Middle 9 Middle 10 Middle 11 Middle 12 Middle 13 Start 13 Middle 14 Middle 14 End 15 Middle Means
1155 1325 2130 0010 0130 1130 1320 154!i 222ti OOlti 0207 1330 093ti 1530 1100 165fi 165fi 1810 0845 1035 2125
Salinity (G) 31.87 32.07 30.32 31.17 31.02 31.05 32.07 30.15 30.25 30.00 30.6
32.56 _ 31.47 31.3 31.2 29.9 30.4 29.0
Note: the Al,O, + Si02 + Fe,O, carbonate and organicis.
Particulate load mg d-r
Al(mgd-‘)
Si(mgd-‘)
7.3 * 0.7
49.7*5
3.8kO.4 21.3 k2.1
Fe(mge-‘)
Si/Al
Al/Fe
4.OkO.4
6.8+ 1.0
1.8kO.3
26+2.6
2.6kO.3
5.5f0.7
1.5kO.2
187k19
9.3 + 0.9
8.75 k 1.2
2.3kO.3
5.2kO.5 2.1 kO.2 4.1 kO.4 2.OkO.2 3.6kO.4
35.5k3.6 14.7+ 1.5 28.2k2.8 16.0* 1.6 10.7+ 1.1
2.8 f 0.3 1.2kO.l 2.2kO.2 1.250.1 2.OkO.2
6.8f 1.0 7.lkl.O 6.7k 1.0 7.9+ 1.1 3.OkO.4
1.8kO.3 1.7kO.2 1.8kO.3 1.8kO.3 1.8+0.3
3.3*0.3 4.9kO.5
10.4f 1.0 19.0+ 1.9
1.9kO.3 2.9kO.3
3.2 + 0.5 3.9 +0.5
1.7kO.2 1.7kO.2
2.9 + 0.3
8.6kO.9
2.2kO.2
3.OkO.4 5.7k2.1
1.3kO.2 1.7kO.2
90
168 158 88 328 433 122 129 419 650 371 188 68 97 63 58 47 45 62 60 167
accounts for only 4 to f of the particulate load; the remainder probably consists of
W. A. MCKAY et al.
3304
number measured. For the small droplets, evaporation is rapid and the assumption of equilibrium is valid whether the droplets originate from the surf zone or the distant sea surface. For large droplets, deposition limits the range of travel, and a greater concentration ratio is expected. The importance of the large droplets from the surf zone is indicated by the total concentrations observed on the beach in Cumbria, an order of magnitude greater than those observed over the open sea surface by other observers (Erikson, 1986). Some part of the increase in concentration is due to a difference in sampling heights, since the measurements over the ocean surface were typically at 10 m height, while the beach measurements were at 1.5-2 m. However, the results of Exton et al. (1985) at 2 and 10 m above a beach did not differ substantially. The contributions of large droplets from the surf zone is further illustrated by the five to tenfold concentration difference at low tide (runs 9 and 11, Table 3) and high tide (runs 2-5) at similar wind speeds. In addition, Eakins et al. (1982) found that the sea salt concentration in air 1 m above the sea surface to windward of the surf zone was an order of magnitude smaller than that measured at the same time on the beach. Thus, the spray from the broad surf zone dominates sea salt concentrations in measurements made on the beach, although at greater heights, or further inland, spray from the great area of sea surface offshore may make an important contribution.
cipally from the surf zone. This was suggested by an increase in concentration, particularly of the largest drops, at high tide, when the surf zone was close to the instruments. A similar increase was clearly demonstrated in the results of Exton et al. (1985). However, some increase in concentration would result from proximity to the source area at high tide, even if the whole sea surface were a uniform source. The significance of the surf zone can be assessed by comparing changes in the number of droplets counted (Table 1) with the results of Wilson (1982) who used a numerical modelling approach to predict the variation of concentration near surface source of varying extent. Wilson’s model takes account of dispersion of the droplets, but not of deposition. This is not a serious restriction since the numbers of droplets counted were dominated by small droplets, which deposit slowly. From Table 1, the measurements from 10 and 11 November 1989 up to 12.56 h, and the measurements of 20 and 21 October 1989 form a group with a limited range of wind speed in near high tide conditions. The mean wind speed was 8.4 m s- ‘; the mean distance of the instruments from the water’s edge was 24 m and the mean droplet count per run was 2900. We compare these with the data for 20 October 1989 up to 13.30 h, for which the wind speed was similar (8.2 m s-l) but the range from the edge of the sea averaged 3 12 m. For this latter group of runs the mean droplet count was 1070. If the surf zone (a band of width SO-200 m) was the source, the results of Wilson (1982) show that the ratio of concentrations should be about 4.5-8. On the other hand, if the entire Irish Sea was the source, (50 km wide) the concentration ratio should be 1.5. The observed ratio (2.7) is intermediate, suggesting that for the small droplets, which dominate the total count, the surf zone contributes 50% or more, to the
100
4.2. The injluence of wind speed on the size and concentration of sea spray
In Figs 2a and 2b, the number and volume distributions of seaspray droplets, corrected for evaportion, are shown for a wind speed of 7 m s- ’ at high and
I
I
I
I
r
I
10 m
1
‘E
L Er
0.1
0 z
0.01 0
0.001
0~0001
0
CI.5i
0 Low water
( Surf-sampler
distance
400m
0
( Surf -sampler
distance
9 m)
High water
I
1
1
2
I
(
5 Droplet Corrected
L
I
20 10 radius (microns) for evaporation
1’
I
50 )
Fig. 2a. Variation of number distribution with distance from high water mark.
10 0
Shore-line sea spray aerosol
o
0.01
/ t _ __. 0.5
1
o
Low
a
High water
water
2
(Surf
3305
-sampler
distance
(Surf-sampler 5
Droplet (Corrected
aistance
10
20
400m ) 9m 50
radius (microns) for evaporation
1 1 100
1
Fig. 2b. Variation of volumetric loading with distance from high water mark.
low water at distances from the instrument to the surf zone of 9 and 400 m, respectively. The average number of drops in 0.5 pm increments of radius were used to plot dN/d log r and dV/d log r where N and V are the total number and volume of droplets respectively of radius less than r. These plots show that both the size of the largest drop observed and the number in each size band increa.se from low water to high water. The combined data taken around high tide enable the effect of wind speeds up to about 12 ms-’ on the aerosol flux and droplet size blown onshore to be
examined. Figure 3 indicates that the number concentration of droplets increases with increasing wind speed throughout the aerosol size range, the size distribution showing little change in shape. The number spectra, corrected for evaporation, show a peak at 12-17 pm diameter throughout the range of wind speed from 5 to 10 m s- ‘. This is around the upper limit of the range of peak sizes observed in oceanic aerosol spectra (between 8 and 15 pm; Monahan, 1986) and most likely reflects the difference between a distributed source and a local one. Losses of large
IO *
5ms-l
o 6rnsIi 0 9 ms 0 10 ms-l
A
0
0
A
0
0 *
O~OOl 0 +
*
0~0001
I 0.5
I
I
I0
5
1
(
Droplet Corrected
10 radius (microns) for evapoiation
1
Fig. 3. Variation of number distribution with wind speed.
3306
W. A. MCKAY et
particles due to deposition will be more important in size spectra observed over the ocean. Whilst atmospheric sea salt concentrations can be estimated from the Phase Doppler data, the results are likely to be underestimates due to effects at both ends of the instrument size range (Livesley and Gillespie, 1991). At the small size end, bias is caused by threshold effects, while at the large size end, bias results from droplet distortion. A number of effects may cause large droplets to be incorrectly sized or to be omitted from the size distributions: droplet distortion may introduce such errors (Livesley and Gillespie, 1991) and the presence of suspended particles may have had a similar effect. Finally, the small number of large droplets counted effectively truncated the observed size distributions at about 60 pm diameter. The omission of large droplets may have had a substantial effect on the total mass collected due to the relatively large mass per particle (as illustrated in Fig. 2). However, the above effects would cause only minor distortion to the number spectrum of droplet sizes. The technique should thus provide a good measure of the sea spray size distributions, up to about 60 pm diameter, and give valid information on the influence of wind and tidal conditions on the size and concentration of sea spray in air. However, for mass concentration measurement, the high volume air sampler data are preferred. The relationship between wind speed, U, and the mass of atmospheric sea salt S (pgme3), on the Cumbrian coast was derived from the total aerosol samples collected at the shoreline using data collected
500
“E
400
In c.E!
where U = wind speed (m s- ‘) and S = sea salt concentration (pg mA3). This implies that at wind speeds below 2 m s- ’ the sea salt concentration blown onto the Cumbrian coast is -10~gm-3,increasingto ~50~gm-3in5ms-’ winds and -200 pg mm3 in 10 m s-l winds. A number of authors have attempted to relate the variation of airborne sea salt concentration to wind speed. A recent study by Erickson et al. (1986) summarised several studies of the concentration over the open ocean by the following relationship between U and S: lnS=0.16U+1.45 elevation lnS=0.13
U+1.89
for U>lSms-‘.
lnS=0.16U+2.92,
Uc13ms-‘at
June
0
November
2melevation.
A
October
*
November Best
1987
Ii
,*’
1989
/ ,’
I
1989 1991
/’ /
t
In S = 0,23u+
/’ #’ #’
3.05
-
,’
/ ,'
A * 200
15m
Exton et al. (1985) made measurements at the top of a beach adjacent to a narrow surf zone and found:
-
A / .’ / .’ .*
!
=‘ :: P m
for U
0’ ,’
?!
2 s
In S = 0.23 U + 3.05
sampler
0
- ----300
over the 1987-1991 period for high water conditions and sampling periods of about 2-4 h duration, extending about 90-120 min either side of high tide. This is equivalent to a range of mean distances from sampler to water line of about 15-35 m. High-tide measurements with sampling times shorter than about 1.5 h were rejected, since the close proximity of the surfzone to the samplers throughout the measurement period would result in high concentrations, incompatible with the majority of the “high-tide” samples. The data, shown in Fig. 4, can be represented by
I-
Air
-
E?
al.
0
100 __*----
____.....----0' 0
0
__-- _---
_*--
_*--
_/-
_.*-
.-’
/
,
I
I
I
2
4
6
6
Wind
speed,
/’
* *
/ ,/A
I
._
10
I ._
12
14
u (ms-‘I
Fig. 4. Variation in atmospheric sea salt concentration with wind speed for sampling periods 90-120 min either side of high tide.
Shore-line sea spray aerosol Above 13 m s-l, the concentration remained constant in the data set of Exton et al. Their data show concentrations somewhat larger than those measured over the open ocean by other workers. The concentration of sea salt observed in Cumbria close to the broad surf zone is somewhat greater at all wind speeds than that observed near a narrow surf zone by Exton et al. (1.985),and an order of magnitude greater than observed by Erickson et al. (1986) over the open ocean. The concentration increases more steeply with wind speied at the Cumbrian beach than in either of the other sets of measurements. 4.3. Enrichment factors Many studies have demonstrated that the concentration of inorganic particulate material in sea spray is greater than that in the sea water from which the spray was derived (Eakins and Lally, 1984; Fry, 1983; Walker et al., 1986; P’attenden et al., 1989). The most likely mechanism for I his enrichment is the scavenging of -particulate material from the water column by rising bubbles, which produce particulate rich droplets on bursting at the surface. Since evaporation would complicate direct assessment, all such studies have used Na as a me.asure of the amount of sea spray represented by a sample and the enrichment factor (EF) of an element X in a sea spray sample relative to the sea water from which it was ejected is defined as
The results discussed in Section 4.1 are consistent zone is the dominant with the hypothesis that the surf source of the marine aerosol determined in the measurements reported here, with aerosol being produced
3307
predominantly by the bursting of bubbles generated by breaking waves. At the range of wind speeds encountered in this study, the values of { [X]/[Na]}aeroaol vary by little more than a factor of 2, for Al, Si, Fe and the actinides (Table 4). For Al only there is a correlation between increasing Al/Na ratios and increasing wind speed, significant at the 5% level. In contrast, the values of the { [X]/[Na]},,, wB,crfor all measured elements in the surf zone appear far more variable. This is predominantly due to changes in the quantity of particulate present in the sea water rather than changes in its quality, the particulate composition being approximately constant (see elemental ratios in Table 5). The variability in spot samples of sea water on any particular occasion may be. large, due to small scale inhomogeneity in surf zone water, and may cause small numbers of samples to overestimate variations between occasions. The enrichment factors relative to the concentrations in the shoreline sea water range generally from about S-50 for Al and Fe, and about l-40 for Si (Table 6). The generally lower enrichment of Si reflects the lower Si/Al ratio in the aerosol of about 2 compared to 5-9 in shoreline sea water (compare Tables 3 and 5). Aluminosilicate minerals, which include the fine clays, have a Si/Al ratio of about 2, and the low Si/Al ratio in the aerosol probably reflects the more efficient scavenging of fine clay out of the water column, than of the other main silicon-containing mineral, quartz (SiO,), which is of generally coarser grain size. As for Al and Si, the { [X]/[Na]}._,, values for the actinides are fairly constant (Table 4). The wind speed was also relatively similar between runs where the actinides were measured. However, the increase in
Table 6. Enrichment factors [EF] in aerosol samples* Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Al 17*3 19*9 4Ok18 7+4 9*4 18&3 3.5_+1 22&4 50&9 28k4 47+11 40+5 25+9 35+_6 52+11
Si 5+1 8+3 16k5 3+1 4+1 7*1 1 kO.2 7+1 17&3 9*2 12*2 29&5 21k4 19+4 43*8
Fe 21k4 18k9 42k20 9*5 10+5 32k6 5+1 26&5 45+7 37+6 41*7 42&8 26k5 34+_6 37*7
26
29
19&4 ~25 30*4
19*3
58+5
36k5
* R.$ative to suspended particulate fraction only. EF =(CX1/CNalaerosol)l(Cxl/CNal sea water), where direct analyses were not made of the concentration of Al, Fe and Si suspended in sea water, the concentration was deduced from the particulate load and the mean composition of particulate matter for the eleven analyses reported in Table 5.
3308
W. A.
MCKAY
AI/Na ratio with wind speed suggests that the concentration of fine particulate and associated actinides in the aerosol does increase with wind speed. The actinide enrichment factors are between 20 and 40 (Table 6), and are similar to those for aluminium. A similar conclusion was reached by Walker et al. (1986) who noted that the EFs for actinides and aluminium were similar in offshore aerosol generated using a bubbling apparatus that simulated a breaking wave. However, Walker’s values of EF for actinides, Al and Si were considerably higher (100-600) than those observed here. The concentration of Al and Si suspended in offshore sea water was much smaller than that in the surf zone, but the ratio of each to Na in seaspray was similar in both areas. Thus the results suggest a saturation effect which prevents enrichments in seaspray at the surf zone achieving the values seen offshore. This explanation is supported by the 1 CAlI / CNalLosolmeasured by Walker offshore being within a factor of 2-3 of those measured adjacent to the surfzone of 11.8k3.1 pgg-’ in Table 4. The { [Al]/[Na]} ratios observed in sea spray here can also be compared with corresponding values of 25f52mgg-1indepositionand134f175mgg-1in air samples collected 200 m inland at Eskmeals by Pattenden et al. (1989). The higher ratio in the air samples at Eskmeals may relate to a propensity of fixed air samplers to under-collect particles larger than a few tens of microns in diameter (Garland and Nicholson, 1991), while the deposition collector will receive large particles preferentially. However, any attempt to make deductions about the variation of enrichment of suspended particulate material is confused by the contribution of land-derived dust to the air and rain samples, which are collected continuously regardless of wind direction. It is indicative that both air and deposition samples show higher { [Al]/[Na]} ratios than those recorded in the sea spray samples reported here.
5. CONCLUSIONS
The mass concentration of sea salt in on shore winds on the gently sloping beach in Cumbria was several times greater than observed over the sea surface, and was dominated by spray emitted from the surf zone. However, the offshore sea surface also made an important contribution to the number concentration of sea spray droplets measured on the beach. The concentration of sea salt in air increased with wind speed. There was some evidence that the rate of increase was more rapid than has been observed over the open sea. Fine suspended particulate matter was enriched in the sea spray. The concentration of Al, a tracer for this material, in spray relative to Na, increased with wind speed, but varied little with the concentration seawater. The enrichment of Fe,239+240Pu
in and
et al.
241Am was generally similar to that for Al, and it is expected that other particle-associated substances would show similar behaviour. Acknowledgements-We thank our Harwell colleagues J. M. Howorth for assistance with Facsimile and R. Gillespie and G. Burton for operating the LISATEK. The Commander of the Proof and Experimental Establishment, MOD, Eskmeals kindly allowed us access through the range to set up our aerosol equipment on Eskmeals beach. We also thank the referees for constructive suggestions. This report was funded by British Nuclear Fuels plc and the Deuartment of Trade and Industrv fDTIb Much of the data u;lised was derived from an earlier $tud; funded by the Department of the Environment and the CEC. However the views expressed are entirely those of the authors and do not necessarily reflect those of the funding organisations.
REFERENCES Blanchard D. (1983) The production, distribution and bacterial enrichment of the sea salt aerosol. In Air-Sea Exchange of Gases and Particles (edited by Liss P. S. and Slinn W. G. N.), pp. 407-454. Reidel, Dordrecht. Brazier K., Gillespie R. F., Dalze.11W. and Livesley D. M. (1988) Bias corrections to size distributions and concentrations in phase doppler particle measurement. AERE R13270. Calvert S. E. (1976) In Chemical Oceanography (edited by Riley J. P. and Chester R.), ,. Vol. 6. Academic Press. L&don. Curtis A. R. and Sweetman (1985) Facsimile release H user’s manual. HMSO. London. AERE R11771. Eakins J. D. and ially A. h. (1984) The transfer to land of actinide-bearing sediments from the Irish Sea by spray. Sci. total Enuir. 35, 23-32.
Eakins J. D., Lally A. E., Burton P. J., Kilworth D. R. and Pratley F. A. (1982a) Studies of environmental readioactivity in Cumbria. Part 5: the magnitude and mechanism of enrichment of sea spray with actinides in West Cumbria. AERE R10127. Eakins J. D., Lally A. E., Burton P. J., Kilworth D. R. and Pratley F. A. (1982b) Studies of environmental radioactivity in Cumbria Part 5. The magnitude and mechanism of enrichment of sea spray with actinides in West Cumbria. AERE R10127, HMSO. Erickson D. J., Merrill J. T. and Duce R. A. (1986) Seasonal estimates of global atmospheric sea salt distributions. J. geophys. Res 91, 1067-1072. Ewins C. P. and Spencer C. P. (1967) The annual cycle of nutrients in the Menai Straits. J. Mar. Biol. Assoc. UK. 47, 533-542. Exton R. J., Latham J., Park P. M., Perry S. J. and Smith M. H. (1985) The production and dispersal of marine aerosol. Q. J. R. met. Sot. 111, 817-837. Fry F. A. (1983) Airborne radionuclides in the vicinity of a coastal nuclear fuel reprocessing plant. J. Aerosol Sci. 14, 452-455. Garland J. A. and Nicholson K. W. (1991) A review of methods for sampling large airborne particles and associated radioactivity. 3. Aerosol Sci. 22, 479-499. Lally A. E. and Eakins J. D. (1978) Some recent advances in environmental analysis at AERE Harwell. Symposium on the determination of radionuclides in environment and biological materials. Central Electricity Generating Board, London, Paper 12, pp. l-9. Livesley D. M. and Gillespie R. F. (1991) Mass balance measurements using the phase Doppler analyser. In Sprays and Aerosols ‘91, jointly organ&d by ILASS-EUROPE
Shore-line sea spray aerosol and The Aerosols Soa,ety, U.K., pp. 32-36, University of Surrey, Guildford. McKay W. A. and Walker M. I. (1990) Plutonium and ameridium behaviour in Cumbrain near-shore waters. J. en&. Radioactivity 12,, 267-283. McKay W. A., Walker M. I. and Cloke J. (1990) Transfer of radionuclides from sea-to-air to land, DoE/RW/90.094. Department of the Environment, London. Monahan E. C. (1986) The ocean as a source of atmospheric particles. In The Role of Air-Sea Exchanae in Geochemical Cycling (edited by B&t-Menard), Reidei, Dordrecht. Pattenden N. J., Cambray R. S. and Playford K. (1981) Trace and major elements in the seasurface microlayer. Geochim. Cosmochim. Acta. 45, !)3-100.
3309
Pattenden N. J., Cambray R. S. and Playford K. (1989) Studies of environmental radioactivity in Cumbria Part 16. Trends in radionuclide concentrations in coastal airborne and deposited materials, 1978-1987. AERE R12617, HMSO. Peirson D. H., Cawse P. A. and Cambray R. S. (1974) Chemical uniformity of airborne particulate material, and maritine effect. Nature 251, 675-679. Walker M. I. (1989) The sea-to-air transfer of radionuclides. Ph.D. thesis, University of East Anglia. Wilson J. D. (1982) Turbulent dispersion in the atmospheric surface layer. Boundary-Layer Met. 22, 399-420.