Surface air concentration and deposition of lead-210 in French Guiana: two years of continuous monitoring

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Abstract

To make up for the lack of data on 210Pb aerosol deposition in tropical regions and to use this radionuclide as an aerosol tracer, a monitoring station was run for two years at Petit-Saut, French Guiana. Lead-210 concentration in air at ground level was monitored continuously together with atmospheric total deposition. The air concentration has a mean value of 0.23±0.02 mBq m−3 during both wet and dry seasons, and it is only weakly affected by the precipitation mechanism. This result was unexpected in a wet tropical region, with a high precipitation rate. In contrast, deposition clearly correlates with precipitation for low/moderate rainfall (<15 cm per 15-day), while this correlation is masked by strong fluctuations at high rainfall. The estimated mean annual deposition over the last ten years is 163±75 Bq m−2 y−1. This provides a procedure for estimating this mean flux at other sites in French Guiana.

Introduction

Lead-210 (half-life 22.3 y), is a natural radionuclide, being produced in the atmosphere through radon decay. Due to its affinity for particles, it is rapidly fixed by submicron-size aerosols and, with its long half-life compared to the aerosol residence time (a few weeks), 210Pb is the most important tracer of long range atmospheric aerosol transport and removal processes (see Balkanski et al., 1993, Rehfeld and Heimann, 1995, Genthon and Armengaud, 1995, Lee and Feichter, 1995, Barrie et al., 2001).

This role as atmospheric aerosol tracer could now be extended to continental surface cycles: 210Pb could be used as a tracer of atmospheric aerosol deposition and transport between different environmental reservoirs of the continental surface (soils, rivers, lakes, coastal zone). This new use will require a sound knowledge of the mean annual atmospheric 210Pb deposition over recent decades, for the following reason. In this new approach, the annual 210Pb deposition in sediments (or in soils) will be calibrated by the annual atmospheric 210Pb deposition. Now, this annual sediment deposition is obtained from the profile of 210Pb activity with depth, and usually the average value over recent decades is estimated. As a consequence, the calibration has to be performed with the atmospheric deposition averaged over a similar lapse of time.

The purpose of this work is to estimate this mean annual atmospheric 210Pb deposition in French Guiana, where a wide campaign of measurement on 210Pb in soils and sediments is being developed, and where we plan to use this new approach. At these tropical latitudes the almost complete absence of data, as shown by the data base where 210Pb measurements are collected (Preiss et al., 1996, Preiss et al., 2000), is a serious problem. For this reason a 210Pb monitoring station was implemented at Petit-Saut. The purpose was to document the prevailing mechanism of aerosol deposition (which connects deposition to climatic parameters) in order to estimate the mean annual deposition over the last ten years and hence provide a procedure for estimating this mean flux at other sites in French Guiana. Here we present the results of a two-year continuous monitoring, starting from 28 June 1999.

The site (5° 4.34′ N, 53° 2.83′ W), 70 km from the coast, can be considered as representative of an area comparable with the pixel size now used in well resolved large scale models (Global Circulation Models of Atmosphere and Chemistry Transport Models of Atmosphere with a 100 km grid-step at best resolution, see for example Barrie et al., 2001). The climate is typical of French Guiana with a mean annual rainfall over the last ten years of 301 cm at that site. The climate is governed by the seasonal shift of the Inter Tropical Convergence Zone (ITCZ). The dry season, from July to December, alternates with the wet one, which extends mainly from January to June, being only briefly interrupted by the “short” dry season in March. Guiana is subject to the regular northeast trade wind influence from the ocean. In winter the aerosol plume coming from North Africa is transported by the trade winds to the Amazon basin, whereas in summer, the ITCZ pushes this plume northward to the Caribbean Sea. This shift can be seen on satellite images (Husar et al., 1997), where the signal is dominated by inorganic aerosol from Saharan sources.

Section snippets

Measurements and analysis

Air was continuously pumped and filtered in a protected area located 3 m above ground level. Vegetable fibre filters (blue Camfilm filters) of 50 mm diameter were used. These were changed every 15 days, which corresponded to a filtered air volume of about 700 m3. Simultaneously, a 400 cm2, 20 litres rain gauge continuously recorded the dry and wet deposition of atmospheric 210Pb. Rain water was collected with a sampling period of 15 days and the amount of rain thus recorded was close to the

Results

Fig. 1 shows (a) the precipitation amount, (b) 210Pb concentrations in surface air and (c) 210Pb deposition as a function of time. All radioactivity results are decay-corrected to the time of deposition. Monitoring started 28th June 1999 during the dry season. The first 12 months of monitoring clearly show the typical opposition of dry and wet seasons. The transition between the two seasons is clearly visible (December 1999), whereas the following wet season ended in around July–August 2000.

Discussion

The processes operating in the removal of radionuclides from the atmosphere are dry deposition, washout (below cloud scavenging) and rainout (in cloud scavenging). Wet deposition in the tropics occurs mainly through convective precipitation, in contrast to polar and mid latitudes, where it occurs mainly through large-scale precipitation. In convective precipitation, rainout is by far the most important deposition mechanism, washout being much less efficient. Mechanisms at work during a single

Conclusions

A continuous monitoring of 210Pb in the atmosphere has been carried out in French Guiana over two years. The 15-day averaged concentration of 210Pb in the surface air appears to be seasonally independent, with a mean value of 0.23 mBq m−3, indicating a persistent Atlantic origin of the air masses during both seasons. Similar concentrations are found at tropical sites subjected to oceanic influence during the monsoon season (Lamto, Africa and Bombay, India). This invariance indicates that the

Acknowledgements

This project was supported by the Programme Mercure en Guyane together with the INSU-CNRS and the University of Grenoble.

References (14)

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