Wet deposition of 210 Pb aerosols over two areas of contrasting topography

Deposition ﬂuxes of 210 Pb on low and moderately high-elevation sites of Edinburgh (Scotland) and mid-Wales, respectively, have been measured. The excess 210 Pb ﬂuxes in moorland Edinburgh soils did not vary signiﬁcantly and ranged from 71 to 92 Bq m − 2 y − 1 with a mean value of 78 ± 9 Bq m − 2 y − 1 , for all the measured sites where both altitude and the mean annual rainfall are similar. On the other hand, the excess 210 Pb measured in moorland soils of mid-Wales sites increased by a factor of 2.4 at or near the summit (741 m asl) relative to the coast ( ∼ 15 m asl), whereas rainfall increased by a factor of 1.8 over the same height range. On average, the summit to valley ratio of 210 Pb concentration in rainfall was a factor of 1.3 due to scavenging of the feeder clouds by the seeder rain. These results are consistent with results for both modelled and ﬁeld studies on the wet deposition of pollutants in complex terrain reported by several researchers. The long-term 210 Pb wet deposition ﬁeld data will provide an important input parameter for the modelling of wet deposition of aerosols throughout the uplands of the UK and elsewhere where the seeder–feeder process is of common occurrence.


Introduction
The fallout of naturally occurring 210 Pb provides an important tool for studying pollutant transport processes and deposition fluxes to terrestrial ecosystems. Although there is a large body of data on 210 Pb worldwide, this is mainly limited to relatively lowland areas. Aerosol deposition flux measurements over mountain terrain is complicated in several ways by the meteorological conditions that are typical of high elevations. High elevation sites induce additional deposition processes that are not major contributors to deposition in lowland sites. These processes affect the efficiency with which aerosols are scavenged from the atmosphere.
Earlier studies involving measurements of major ionic concentrations as a function of elevation have been conducted to improve the estimates of acid quantities deposited in elevated sites. Results presented by Fowler et al [1]  precipitation events measured at levels between 244 and 847 m above sea level (asl), on the slopes of Great Dun Fell (Cumbria, UK) indicate an increased ionic concentration of 2.2 to 3.1 between the valley (∼250 m asl) and the summit (∼847 m asl). The increased concentrations of ions observed were interpreted as resulting from the seeder-feeder process; a mechanism first put forward by Bergeron [2], who proposed that rain falling from the high altitude (seeder) clouds wash out small droplets within the low-level cap (feeder) clouds formed by ascent over the hills and consequently grow by accreting cloud drops. The seeder-feeder mechanism was also supported by theoretical studies of Storebø [3], Bader and Roach [4]. Hill et al [5] presented detailed case studies of orographic rain falling over the hills of south Wales using data obtained from scanning radar combined with a network of autographic raingauges. They found that the orographic rainfall enhancement depended on the low-level wind speed, and that over 80% of the total orographic enhancement occurred in the lowest 1500 m above the hill. Model studies of the seeder-feeder effect by Jones and Choularton [6] support this interpretation and also show that the rainfall enhancement and deposition are a function of topography and spatial scale. These results are valuable to improve the wet deposition map of the UK, particularly over the Pennines, the Lake District and Snowdonia where the seeder-feeder effect constitutes a large fraction of annual deposition [7]. Following the observations described above, a technique pointed out by Graustein and Turekian [8], which involves the use of fallout 210 Pb and 137 Cs accumulated in undisturbed soils to measure the rate of aerosol deposition, was employed to make inferences regarding the above mechanisms. The widely used method for determining the atmospherically derived 210 Pb fluxes in soils, given in equation (1), was employed [9][10][11][12][13]: where is the total flux, including wet and occult deposition processes, (in Bq m −2 y −1 ) of the radionuclide to the Earth's surface, λ is the 210 Pb decay constant (0.0311 y −1 ) and I is the 210 Pb inventory in soil, measured in Bq m −2 . Equation (1), however, is used based on the assumption that the soils have not been disturbed for at least 100 years to be able to distinguish between the fallout and the in situ component of 210 Pb in the soil profile, and that the 210 Pb inventory is in a steady state, with the rate of 210 Pb atoms equalling the mean deposition rate. The use of soil samples as long-term collectors of atmospheric deposition has advantages over direct collection of precipitation in that: (1) there is a greatly reduced time and expense required for sampling and (2) the deposition rates are relatively free of artefacts introduced by artificial collection apparatus. Therefore, the 210 Pb measurement in soils provides practical studies of the effect of local variables such as topography, cloud and vegetation, on the inventories of aerosols on the land surfaces.
This letter reports 210 Pb deposition fluxes measured in moorland and woodland soils collected from the UK lowland and moderately high-elevated sites of Scotland and Wales, respectively. The measurements were carried out for the following reasons.
(i) To determine whether high elevation sites have an influence on the long-term annual average wet deposition of 210 Pb isotopes.
(ii) To determine whether the presence of clouds frequently shrouding hilltops have a significant influence on the deposition of 210 Pb, assuming a constant concentration of the isotope in rainfall and negligible contributions from dry deposition processes.

Methods
Two areas of contrasting topography: lowland (<300 m asl) sampling sites located in Edinburgh, Scotland and moderately high-elevated sites in mid-Wales were selected for this study.

Edinburgh sites
Soils were obtained from five different sites, each with relatively insignificant undulations, and four of which were well-established golf courses. The sites were selected on the reasonable assumption that the soils have not been subjected to erosion or other forms of disturbance in recent decades. From the four golf courses, the rough was chosen since intense perturbation from human activity was expected to be minimal. Furthermore, according to the information gathered from the greenkeepers, grass that was cut on the rough was always left on the site. It is therefore expected that the radioactivity that had deposited onto the rough was not transported elsewhere. Detailed site description and sampling techniques are given elsewhere [14].  [14] were followed. Rainfall data was obtained from the UK Meteorological Office Map [15]. Information on site history was obtained from the Centre for Ecology and Hydrology staff whereas that of soil characteristics was by visual inspection. Figure 1 shows approximate locations of sites sampled in mid-Wales. Table 1 gives a detailed summary of information about the sampled sites.

YH.
Ynys Hir, situated ∼7.5 km from the coast, was selected on the basis that it will provide the 210 Pb flux representative of low altitude sites. Soil samples were collected from both the open grassland and beneath the adjacent moderately dense 10-15 m high oak woodland canopy. The soils were rich in mineral content and contained rocks (mostly slate).

BM.
The choice for Bryn Mawr was based on its enhanced rainfall due to the extent of land mass from the coast and rise in altitude relative to YH. Thus, this site was expected to provide information on the effect of increased annual average rainfall on the deposition of 210 Pb relative to that at the coast.

PL.
The summit of Plynlimon was selected for sampling with the aim of investigating the effect of meteorological conditions on the deposition of aerosols by comparing the measured 210 Pb flux at the site with those of relatively low elevation sites. The soil at this site was mainly organic matter at the topmost and mineral matter at depths >10 cm. The grass at the top was relatively short with features suggesting that the area was almost certainly grazed. Adjacent to the site, approximately 200 m eastward, was a Sitka spruce plantation. The samples obtained under the Sitka canopy were disregarded since the soils looked heavily disturbed.

TN.
Tanllwyth was selected because of its position on a leeward side of Plynlimon mountain, so as to provide information on the 'shadow effect' in the deposition of 210 Pb aerosols. The open grassland soils were poorly drained.

210 Pb deposition-Edinburgh
Fluxes of excess 210 Pb measured from the five (I-V) Edinburgh sites are given in table 2. Since more than one sample was obtained from each site, statistical means and sample standard deviations relative to the mean values were calculated to estimate the deposition fluxes.
Deposition fluxes of 210 Pb measured in moorland soils of Edinburgh varied from 71 to 92 Bq m −2 y −1 , with a mean value of 78 ± 9 Bq m −2 y −1 . Although the sample size is small, the trends in terms of the mean values are such that there is no significant spatial variability in the deposition of 210 Pb. These Table 2. Mean fluxes of excess 210 Pb in Edinburgh moorland and woodland soils. The letter n denotes the number of cores analysed for each population. The dashes (-) indicate that no woodland data were available for the sites.  [16,17]. At sites (II-IV) where both moorland and woodland data were available, the woodland mean 210 Pb deposition flux was (47 ± 7)% over that of moorland. Assuming the same precipitation input at all sites, enhanced 210 Pb input in woodland soils may be attributed to occult deposition. Furthermore, because forests possess structures that promote capture of atmospheric particulates, retained 210 Pb aerosols subsequently fall beneath woodland canopies by mechanisms such as wash-off, gravitational settling and as senescent leaves. Fowler et al [18] reported soil inventories of excess 210 Pb under woodland canopy exceeding those under grass by between 22% and 60%. Similarly, independent studies by Branford et al [19] revealed an average enhancement factor of 36% for 210 Pb deposited inside forest canopies relative to open grassland soils in the Highlands of Scotland.

210 Pb deposition-mid-Wales
Fluxes of excess 210 Pb measured from the four sites in mid-Wales (YH, BM, PL and TN) are given in table 3. It is evident from the table that the 210 Pb deposited in moorland increases significantly (r 2 = 0.90; p < 0.05) with increase in altitude. Because the removal of atmospheric 210 Pb occurs primarily through wet deposition processes and the fluxes of the isotope in soils are expected to be closely related to the rainfall pattern, it is useful to first discuss the mean annual rainfall at the sites.

Rainfall increase with altitude.
Taking the mean annual rainfall at the site of YH to represent the non-enhanced UK west coast rainfall, the enhancement factors in rain falling over BM, PL and TN due to orography are 1.38, 1.85 and 1.54, respectively. According to Pedgley [20], there are two principal reasons for heavy rains over mountain areas: (1) mountains act as barriers to moist airstreams. The moist air is forced to rise, producing clouds by the process of expanding and cooling of air; and (2) when the days are sunny,  (1) is negligible. Enhanced rainfall over moderately sized hills of western Britain has been attributed to the effect of orography on frontal systems coming from the Atlantic Ocean, mainly westerly and/or south-westerly directions [21]. Considering the enhancement factors above, it seems that the rainfall is already significantly enhanced at BM due to uplifting of the airmass by the extent of the 15 km land from the coast.  TN) indicates that the seeder-feeder process becomes less effective as the altitude decreases, moving eastward of the hill summit. In this region downwind of the hill, the water vapour content of air is reduced due to scavenging by rain as it passes through the feeder cloud. Thus, the zone of maximum 210 Pb deposition is assumed to be at or near the summit for the sampled sites along the transect. The average 210 Pb concentration in rain, C rain (in mBq m −3 ) can be deduced from the long-term 210 Pb atmospheric deposition flux, , given in table 3 and the average rainfall, R (in mm y −1 ) using the equation:

Profile of
The 210 Pb concentration obtained this way, however, assumes that all the 210 Pb atoms reaching the surface of the earth are delivered by precipitation alone. Thus, these values may be an overestimate, considering that a proportion of the 210 Pb atoms will be delivered by processes of dry and occult deposition. Considering the 210 Pb concentrations in rain, deduced from equation (2), it is possible to determine whether or not 210 Pb concentration in rain water is enhanced at the summit of Plynlimon as would be expected. These results are given in table 4. The average increase in 210 Pb fluxes from the west coast to the summit of Plynlimon was 2.4 whereas rainfall increased over the same height range by a factor of 1.8. This provides a 30% increase in the 210 Pb summit/valley concentration ratio in rainfall. For the purpose of comparison, table 5 presents enhancement factors (summit/valley) for 210 Pb conducted at Great Dun Fell [22]. Results obtained from this study when compared with other results obtained from both field measurements of SO −2 4 , NO − 3 , NH + 4 [1,7], 210 Pb [22] and model results [23,24,6] show a similar trend in increased wet deposition on hills due to the seeder-feeder scavenging process. Comparing these results with those obtained for the relatively low elevation sites of Edinburgh discussed in section 2.1, it is evident that high elevation sites have a significant influence on the deposition of aerosol-borne 210 Pb due to the presence of clouds frequently shrouding the hilltops.

Conclusion
Fluxes of excess 210 Pb were measured in presumably undisturbed soils obtained from low (<300 m asl) and moderately high-elevated sites of Scotland and mid-Wales, respectively. The results are such that the deposition flux of 210 Pb does not vary for the five sites where both rainfall and altitude are quite similar. On the other hand, excess 210 Pb measured at the summit of Plynlimon (741 m asl) increased by a factor of 2.4 greater than that of the coast (∼15 m asl) whereas rainfall increased by a factor of 1.8 over the same height range. The measurement showed that, on average, the summit/valley ratio of 210 Pb concentration in precipitation was 1.3 due to the seeder-feeder scavenging process. Results obtained from this study, together with other results reported elsewhere, are valuable to improve wet deposition maps in the uplands of the UK. This work needs to be extended, particularly over the Plynlimon hill profile with improved resolution to validate these results.