# Anthropogenic CO2 in the Dakar (Senegal) Urban Area Deduced from 14C Concentration in Tree Leaves

Maurice Ndeye, Matar Sène, Doudou Diop, Jean-François Saliège

► To cite this version:

Maurice Ndeye, Matar Sène, Doudou Diop, Jean-François Saliège. Anthropogenic CO2 in the Dakar (Senegal) Urban Area Deduced from 14C Concentration in Tree Leaves. Radiocarbon, 2017, 59 (03), pp.1009 - 1019. 10.1017/rdc.2017.48. hal-01833184

HAL Id: hal-01833184

https://hal.science/hal-01833184v1

Submitted on 26 Sep 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# ANTHROPOGENIC CO₂ IN THE DAKAR (SENEGAL) URBAN AREA DEDUCED FROM
¹⁴C CONCENTRATION IN TREE LEAVES

Maurice Ndeye<fnref n="1" />* • Matar Sène<fnref n="2" /> • Doudou Diop<fnref n="3" /> • Jean-François Saliège<fnref n="4" />

**ABSTRACT.** Radiocarbon (¹⁴C) in atmospheric CO₂ for the Dakar (Senegal) urban area was measured using tree leaves collected by botanists from 1900 to 2003. The aim of our study was to compare the local Suess effect in Dakar to the global one during the 20th century. The Δ¹⁴C of atmospheric CO₂ in this region decreased from 1900 to 1958 during the pre-bomb era (–2 ± 5‰ to –22 ± 4‰). From 1958 to 1964, nuclear bomb tests injected a large amount of artificial ¹⁴C into the atmosphere, reflected in the rise of Δ¹⁴C values. In the Dakar region, the atmospheric Δ¹⁴C reached 773 ± 8‰ in 1964, but subsequently decreased to 80 ± 5‰ by 2003, which is consistent with the global exponential decreasing trend. The Δ¹⁴C record presented here remains slightly lower than the global record. This result is attributed to the input of anthropogenic fossil carbon into the atmosphere. The amount of carbon input can be evaluated by comparing urban areas to those of clean air sites. The calculation of anthropogenic fossil carbon is deduced from a simple mathematical model.

**KEYWORDS:** anthropogenic CO₂, fossil fuel, radiocarbon, Suess effect, tree leaves.

## INTRODUCTION

Radiocarbon (¹⁴C) is produced in the atmosphere by a reaction of neutrons with atmospheric ¹⁴N that produces ¹⁴C, which is rapidly oxidized into CO₂ and then exchanges with different carbon reservoirs. Natural processes such as solar activity, Earth's magnetic field, ocean circulation, and rates of exchange between carbon reservoirs all affect the ¹⁴C content. Besides these natural variations, human activities also have an impact on atmospheric ¹⁴C concentration. Two anthropogenic effects are recorded by atmospheric ¹⁴C: first, the Suess effect, which is the addition of carbon dioxide by fossil fuel combustion; and secondly, the increase in ¹⁴C concentration in the atmosphere as a result of atmospheric nuclear weapons testing. Both of these anthropogenic factors occur on a global scale, however there are also local Suess effects due to local fossil-fuel sources. So, there are regional discrepancies that are related to the emission zone proximity as well as geographic location (Hua and Barbetti 2004).

If we compare the global ¹⁴C atmospheric record to regional or local signals, then it is possible to relate this to regional and local changes in human activities such as industrial activity, traffic, domestic use, and land use. Takahashi et al. (2002) highlighted an example of traffic-derived CO₂ in the atmosphere of an urban forest. Levin et al. (2003) used a novel approach for determining the independent budgeting of fossil fuel CO₂ over Europe. Using the high-altitude Jungfraujoch sampling station results as the reference level, they were able to show a measured CO₂ mixing ratio Cmeas at this location consists of three components: (1) a background component CBg, (2) a biospheric component Cbio, and (3) the fossil fuel component CFoss. These components are expressed respectively as Δ¹⁴Cmeas, Δ¹⁴Bg, Δ¹⁴Cbio, and Δ¹⁴CFoss. This deconvolution makes it possible to calculate the fossil fuel component.

In the present study, we gathered tree leaves that were collected by different botanists in the Dakar region during 1900–2003 and stored at the Cheikh Anta Diop University. The study

<footnote n="1">¹Laboratoire Carbone 14, Institut Fondamental d'Afrique Noire (IFAN), Université Cheikh Anta Diop de Dakar, Dakar, Senegal.</footnote>

<footnote n="2">²Faculté des Sciences et Techniques, Université Cheikh Anta Diop de Dakar, Dakar, Senegal.</footnote>

<footnote n="3">³Laboratoire de Botanique, Institut Fondamental d'Afrique Noire (IFAN), Université Cheikh Anta Diop de Dakar, Dakar, Senegal.</footnote>

<footnote n="4">⁴Laboratoire d'Océanographie (LOCEAN), Université Pierre et Marie Curie, Paris VI, France (deceased 1 June 2012).</footnote>

<footnote>*Corresponding author. Email: maurice.ndeye@ucad.edu.sn.</footnote>
region is situated in the area of Presqu'île du Cap Vert, a heavily urban and densely populated area of Dakar that is 550 km² in area.

To the east lies the region of Thies, and the Atlantic Ocean surrounds the areas to the north, west, and south of Dakar (Figure 1). All leaves were sampled from deciduous trees. We thus can assume their ¹⁴C content represents a single year of atmospheric ¹⁴C activity. In this study, the tree leaves are used to determine the amount of atmospheric Δ¹⁴C, and from these results the local Suess effect can be indirectly estimated using a mathematical model (Oeschger et al. 1975).

The maximum production of bomb ¹⁴C occurred in the Northern Hemisphere (NH), where the majority of atmospheric nuclear tests were conducted (Nydal et al. 1979; Hua et al. 2013). The maximum concentrations occurred in 1963–1964 and levels of almost two times the concentration levels before 1950 were observed (Nydal et al. 1979). Since 1964, ¹⁴C concentrations have decreased and this trend can be attributed to the transfer of ¹⁴C into surface and deep oceans, terrestrial biosphere, as well as the dilution by fossil fuel CO₂ (Nydal and Lovseth 1983; Hua and Barbetti 2004). Based on the amplitude and the occurrence of the bomb ¹⁴C peak, Hua and Barbetti (2004) and Hua et al. (2013) classified global tropospheric ¹⁴C records into four zones—three in the Northern Hemisphere, and one for the Southern Hemisphere (SH).

A peak bomb ¹⁴C value of 816 ± 23‰ in 1964 was measured by Nydal and Lovseth (1996). At some other locations in NH zone 2, the Δ¹⁴C peak values ranges from 794 ± 4‰ to 862 ± 21‰ (Table 3).

## Study Area

The study area is situated in the region of Presqu'île du Cap Vert, a heavily urban and densely populated part of Dakar that is 550 km² in area. Dakar (14°40'20''N, 17°25'54''W is the capital city of Senegal with a population of about 3.5 million people and a density of 4646 inhabitants/m². The rate of population increase is 2.96% per year. Dakar accounts for only 0.28% of the national territory but 80% of the economic activities of Senegal are concentrated in this area (http://www.ansd.sn/).

To the east lies the region of Thies and the Atlantic Ocean surrounds the areas to the north, west, and south of Dakar (Figure 1). In our present study, we gathered leaves stored at the Cheikh Anta Diop University and collected from 1900 to 2003 by different botanists in the Dakar region. Figure 1 shows the locations of the collection sites. All leaves were sampled from deciduous trees. We thus can assume their ¹⁴C content represents a single year of atmospheric ¹⁴C activity. In this study, the tree leaves are used to determine the amount of atmospheric Δ¹⁴C and from these results the local Suess effect can be indirectly estimated using a mathematical model (Oeschger et al. 1975).

## Population Growth

The region of Dakar developed quickly and without major planning due to the natural growth of its population and immigration. It is a major city in Africa—from 400,000 inhabitants in the 1970s, its population had grown to 3,139,325 by 2003, and it is now one of the largest cities on the continent (Dieng 2007). The population of Senegal was estimated to be 10,817,844 in 2005. The rate of annual increase was 2.7% between 1976 and 1988 and 2.4% between 1988 and 2002 (Figure 2).

In the last general census (RGHAE) published in September 2014 (http://www.ansd.sn/), Dakar counted for nearly one quarter of the total population of Senegal (23.2% or 13,508,715 inhabitants) even though it encompasses only 0.3% of the total area of the country. Because of its
<figure><img src="image_2.png" /><figcaption>Figure 1 Map of the Dakar region showing the sample collection sites (circles) and the industries (squares).</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>Figure 2 Growth of the Senegalese population, according to the World Bank (2017) DataBank.</figcaption></figure>

high population density and extensive economic activities (80% of Senegal's economic activities are based in Dakar) (Figure 3), Dakar is the primary source of CO₂ emissions in this region.

Senegal's overall climate is of the sub-desert tropical type, punctuated by wet summers and dry winters. However, the region of Dakar, which is located on the Atlantic Ocean, is characterized by a coastal microclimate. This microclimate is strongly influenced by maritime trade winds and the monsoon, which occur from November until June and from July until October in the directions N-NW and S-SE, respectively. According to the Köppen-Geiger climate classification system, the climate category is BSh (hot, low-latitude steppe, with a mean annual
<figure><img src="image_4.png" /><figcaption>Figure 3 Senegal GDP values, according to the World Bank (2017) DataBank.</figcaption></figure>

temperature >18°C). The average annual temperature in Dakar is 24.9°C. The hottest month of the year is October with an average temperature of 28.1°C. The average annual precipitation is approximately 469 mm.

# MATERIAL AND METHODS

The leaf samples used in this study are from herbaceous and woody species, which were stored in the herbarium of the Botanical Laboratory at IFAN CAD. Twenty samples were collected based on their good state of conservation and the year of collection (Table 1).

# METHODS

## Raw Data

Four herbaceous, dry-leaf samples (>10 g per sample) were treated at the ¹⁴C laboratory of IFAN CAD using the classic method. Each sample was washed in distilled water and treated with 0.5M HCl for 20 min; subsequently the samples were then subjected to 0.1M NaOH. Next, the samples were washed and dried at 30°C. To prepare for liquid scintillation spectrometry, dry samples were then directly added to lithium in the reactor of the synthesis bench and the reaction began at 800°C. After cooling of the reactor, the products [Li(OH)₂, Li₂C₂] synthesized were hydrolyzed to produce acetylene, which was finally transformed into 2 g of benzene by a catalytic reaction at 50°C and then analyzed through the liquid scintillation counter (Tricarb 3170/TL). Samples were numbered with the lab code prefix DKxx (see Table 2).

For the remaining 16 samples, these were chemically treated at the ¹⁴C laboratory of LOCEAN (Paris VI) using standard combustion techniques to create CO₂. The resulting CO₂ was flame-sealed into a 6-mm glass tube and shipped for physical measurement at the NSF-Arizona AMS Laboratory. At the Arizona AMS lab, CO₂ was graphitized and measured by accelerator mass spectrometry (AMS), as described by Jull et al. (2006). Samples were numbered with the lab code prefix AA-xx (Arizona AMS), as shown in Table 2.
<table><caption>Table 1 Leaf species, collection site, collection year, and geographical location.</caption><thead><tr><th>IFAN herbarium reference</th><th>Species</th><th>Nature</th><th>Collection year (AD)</th><th>Location</th><th>Geographical coordinates</th></tr></thead><tbody><tr><td>28088</td><td>Ipomoea coptica</td><td>Herbaceous</td><td>1900</td><td>Yoff district</td><td>14°44′N, 17°29′W</td></tr><tr><td>23083</td><td>Pentatropis spiralis</td><td>Herbaceous</td><td>1912</td><td>Runway of airport</td><td>14°44′N, 17°29′W</td></tr><tr><td>03684</td><td>Dalbergia melanoxylon</td><td>Woody</td><td>1939</td><td>District of Parcelles Assainies</td><td>14°45′N, 17°26′W</td></tr><tr><td>11646</td><td>Phyllanthus rotundifolius</td><td>Woody</td><td>1945</td><td>DAST (urban/city)</td><td>14°46′N, 17°23′W</td></tr><tr><td>01444</td><td>Cassia occidentale</td><td>Herbaceous</td><td>1948</td><td>Hann district</td><td>14°43′N, 17°25′W</td></tr><tr><td>29703</td><td>Cyperus amabilis</td><td>Herbaceous</td><td>1952</td><td>Dantec hospital</td><td>14°39′N, 17°26′W</td></tr><tr><td>29714</td><td>Cyperus bulbosus</td><td>Herbaceous</td><td>1958</td><td>Cemetery (Bel-Air)</td><td>14°41′N, 17°25′W</td></tr><tr><td>29922</td><td>Cyperus esculentus</td><td>Herbaceous</td><td>1960</td><td>Forest of Mbao1</td><td>14°45′N, 17°18′W</td></tr><tr><td>04501</td><td>Indigofera aspera</td><td>Herbaceous</td><td>1961</td><td>Highway</td><td>14°42′N, 17°26′W</td></tr><tr><td>53992</td><td>Lactuca intybacea</td><td>Herbaceous</td><td>1964</td><td>SAR factory</td><td>14°44′N, 17°20′W</td></tr><tr><td>03414</td><td>Crotalaria retusa</td><td>Herbaceous</td><td>1966</td><td>Forest of Mbao2</td><td>14°46′N, 17°18′W</td></tr><tr><td>04738</td><td>Indigofera Costata</td><td>Herbaceous</td><td>1970</td><td>UCAD1 botanic garden</td><td>14°41′N, 17°27′W</td></tr><tr><td rowspan="2">20340</td><td>Terminalia catapa</td><td>Woody</td><td>1976</td><td>District of Fass</td><td>14°41′N, 17°27′W</td></tr><tr><td>Indigofera Costata</td><td>Herbaceous</td><td>1979</td><td>UCAD2 botanic garden</td><td>14°41′N, 17°28′W</td></tr><tr><td>04739</td><td>Indigofera capitata</td><td>Herbaceous</td><td>1981</td><td>Beach 1</td><td>14°46′N, 17°23′W</td></tr><tr><td>04664</td><td>Nerium oleander</td><td>Woody</td><td>1990</td><td>UCAD3 botanic garden</td><td>14°41′N, 17°30′W</td></tr><tr><td>54162</td><td>Ipomoea coptica</td><td>Herbaceous</td><td>1998</td><td>Beach 2</td><td>14°46′N, 17°23′W</td></tr><tr><td>28114</td><td>Nérium Oleander</td><td>Woody</td><td>2000</td><td>UCAD4 botanic garden</td><td>14°41′N, 17°32′W</td></tr><tr><td>54163</td><td>Ficus umbellata</td><td>Woody</td><td>2003</td><td>Soumbédioune market</td><td>14°40′N, 17°27′W</td></tr></tbody></table>

<table><caption>Table 2 <sup>14</sup>C concentrations in tree leaves from the Dakar region, gathered AD 1900–2003.</caption><thead><tr><th>Lab code</th><th>IFAN herbarium reference</th><th>Collection year (AD)</th><th>δ<sup>13</sup>C (‰)</th><th>F</th><th><sup>14</sup>C (yr BP)</th><th>Δ<sup>14</sup>C (‰)</th></tr></thead><tbody><tr><td>AA 92990</td><td>28088</td><td>1900</td><td>-27.2</td><td>0.992 ± 0.005</td><td>62 ± 37</td><td>-2 ± 5</td></tr><tr><td>AA 92991</td><td>23083</td><td>1912</td><td>-26.6</td><td>0.975 ± 0.004</td><td>205 ± 37</td><td>-21 ± 4</td></tr><tr><td>AA 92992</td><td>03684</td><td>1939</td><td>-24.7</td><td>0.973 ± 0.004</td><td>223 ± 37</td><td>-26 ± 4</td></tr><tr><td>AA 92993</td><td>11646</td><td>1945</td><td>-26.2</td><td>0.966 ± 0.004</td><td>279 ± 35</td><td>-33 ± 4</td></tr><tr><td>AA 92994</td><td>01444</td><td>1948</td><td>-26.5</td><td>0.980 ± 0.005</td><td>160 ± 38</td><td>-20 ± 5</td></tr><tr><td>AA 92995</td><td>29703</td><td>1952</td><td>-10.2</td><td>0.984 ± 0.005</td><td>131 ± 37</td><td>-16 ± 5</td></tr><tr><td>AA 92996</td><td>29714</td><td>1958</td><td>-11.3</td><td>0.977 ± 0.004</td><td>185 ± 37</td><td>-22 ± 4</td></tr><tr><td>AA 92997</td><td>29922</td><td>1960</td><td>-11.8</td><td>1.182 ± 0.005</td><td>Post-bomb</td><td>183 ± 5</td></tr><tr><td>AA 92998</td><td>04501</td><td>1961</td><td>-29.4</td><td>1.202 ± 0.005</td><td>Post-bomb</td><td>204 ± 5</td></tr><tr><td>AA 92999</td><td>53992</td><td>1964</td><td>-29.1</td><td>1.770 ± 0.008</td><td>Post-bomb</td><td>773 ± 8</td></tr><tr><td>AA 93000</td><td>03414</td><td>1966</td><td>-28.2</td><td>1.674 ± 0.006</td><td>Post-bomb</td><td>677 ± 6</td></tr><tr><td>DK39</td><td>04738</td><td>1970</td><td>-25</td><td>1.518 ± 0.006</td><td>Post-bomb</td><td>522 ± 6</td></tr><tr><td>AA 93001</td><td>20340</td><td>1976</td><td>-26.6</td><td>1.337 ± 0.005</td><td>Post-bomb</td><td>342 ± 5</td></tr><tr><td>DK40</td><td>54162</td><td>1979</td><td>-25</td><td>1.301 ± 0.005</td><td>Post-bomb</td><td>306 ± 5</td></tr><tr><td>AA 93002</td><td>04664</td><td>1981</td><td>-25.5</td><td>1.272 ± 0.051</td><td>Post-bomb</td><td>277 ± 5</td></tr><tr><td>DK41</td><td>04739</td><td>1990</td><td>-25</td><td>1.181 ± 0.005</td><td>Post-bomb</td><td>187 ± 5</td></tr><tr><td>AA 93003</td><td>28114</td><td>1998</td><td>-29.8</td><td>1.103 ± 0.006</td><td>Post-bomb</td><td>109 ± 6</td></tr><tr><td>DK42</td><td>54163</td><td>2000</td><td>-25</td><td>1.084 ± 0.006</td><td>Post-bomb</td><td>91 ± 6</td></tr><tr><td>AA 93004</td><td>37650</td><td>2003</td><td>-23.7</td><td>1.073 ± 0.005</td><td>Post-bomb</td><td>80 ± 5</td></tr></tbody></table>
The ¹⁴C results are quoted in Δ¹⁴C, using the conventional notation (Stuiver and Polach 1977) correcting for the age at collection, using Equation (1) for samples collected before AD 1950 and Equation (2) for samples collected after AD 1950, as follows:

$$ \Delta^{14}C = 1000(Fe^{\lambda(1950-t)} - 1) \quad (1) $$

$$ \Delta^{14}C = 1000(Fe^{\lambda(t-1950)} - 1) \quad (2) $$

Where λ is the decay constant $1.209 \times 10^{-4}$ based on the half-life of 5730 yr, t is the time of collection, and F is the fraction of modern carbon measured (Donahue et al. 1990).

## Processed Data

The decrease in ¹⁴C concentration due to the local Suess effect can be estimated by using the mathematical equations of Levin et al. (2003). According to Levin et al. (2003), three components can be defined: a background component (a mixing ratio in a free troposphere, observed at a station) CO₂bg, a regional biospheric component CO₂bio, and a regional fossil fuel component CO₂foss.

The ¹⁴C/C ratios of these components in Δ notation are respectively Δ¹⁴Cmeas, Δ¹⁴Cbg, Δ¹⁴Cbio, and Δ¹⁴Cfoss. For the calculation of the regional fossil fuel component CO₂foss, the following two balance equations have been used, one for the CO₂ mixing ratio and one for the ¹⁴C/C ratio [proportional to (1000 + Δ¹⁴Ci)]:

$$ \mathrm{CO}_{2\mathrm{meas}} = \mathrm{CO}_{2\mathrm{bg}} + \mathrm{CO}_{2\mathrm{bio}} + \mathrm{CO}_{2\mathrm{foss}} \quad (3) $$

$$ \begin{aligned} \mathrm{CO}_{2\mathrm{meas}}(1000\%c + \Delta^{14}\mathrm{C}_{\mathrm{meas}}) = & \mathrm{CO}_{2\mathrm{bg}}(1000\%c + \Delta^{14}\mathrm{C}_{\mathrm{bg}}) \\ & + \mathrm{CO}_{2\mathrm{bio}}(1000\%c + \Delta^{14}\mathrm{C}_{\mathrm{bio}}) \\ & + \mathrm{CO}_{2\mathrm{foss}}(1000\%c + \Delta^{14}\mathrm{C}_{\mathrm{foss}}) \end{aligned} \quad (4) $$

These equations are an approximation valid for samples with the similar δ¹³C source with Δ¹⁴Cfoss equal to -1000‰ and we can assume that Δ¹⁴Cbio is similar to Δ¹⁴Cbg. In this case, the combination of Equations (3) and (4) leads to this equation

$$ \mathrm{CO}_{2\mathrm{foss}} = \mathrm{CO}_{2\mathrm{bg}} \frac{\Delta^{14}\mathrm{C}_{\mathrm{bg}} - \Delta^{14}\mathrm{C}_{\mathrm{meas}}}{\Delta^{14}\mathrm{C}_{\mathrm{bg}} + 1000} (\mathrm{ppm}) \quad (5) $$

where CO₂bg is the concentration in clean air (ppm), Δ¹⁴Cbg is the ¹⁴C concentration in clean air (per mil), and Δ¹⁴Cmeas is the ¹⁴C concentration in study area (per mil).

For the calculation of the CO₂foss and CO₂bg, Δ¹⁴Cbg, and Δ¹⁴Cmeas values are needed.

As no background CO₂ concentration for the 20th century is available for Dakar, we used the data from Mauna Loa Station in Hawaii (see Globalview-CO₂ 2003).

The comprehensive data for the whole century are not provided, so we used two distinct periods to represent the yearly mean ¹⁴C concentration values of “clean air.” For the period 1958–1976, the data on ¹⁴C concentration in “clean air” for NH zone 2 were used (Hua and Barbetti 2004). To complete this period, the ¹⁴C concentration values in 1970 and 1976 corresponding to those for the Northern Hemisphere for the period from 1970 onward were added (Hua and Barbetti 2004). The data from Schauinsland Station in Germany (Levin and Kromer 1997; Levin and Kromer 2004) are used for the period 1976–2003.
# RESULTS AND DISCUSSION

## Dakar City

Table 2 shows the details of the sampling sites, the name of the site where the leaves were collected, and geographical location (latitude, longitude). The species sampled are given in Table 1. The δ¹³C (‰) values are calculated and referenced to the PDB standard and listed for the period 1900–2003, except for samples measured at Dakar. In those cases, we assumed a value from the literature (Stuiver and Polach 1977) for C3 plants of –25‰ to –29‰. Analyzing the measured δ¹³C values, we realized that there were two types of plant leaves: the C4 plants with the following δ¹³C: –10.2‰, –11.3‰, and –11.8‰, and the C13 plants with δ¹³C ranging from –23.7‰ to –29.8‰. All these δ¹³C values are included in the expected intervals.

Moreover, Table 2 shows variations of Δ¹⁴C in tree leaves from AD 1900 to 2003. These variations can be divided into different periods. First, we can observe that during the pre-bomb period between 1900 and 1958 that the Δ¹⁴C decreased from –2 ± 5‰ to –22 ± 4‰. This is due to the burning of fossil fuels (Druffel and Suess 2008; Suess 1955; Houtermans et al. 1973). From 1958 to 1964, the Δ¹⁴C values increased to 773 ± 8‰, corresponding to the period of the bomb testing causing a large increase in atmospheric ¹⁴CO₂.

Atmospheric data from other sites (Nydal and Lovseth 1996) show a peak of 817 ± 23‰ in 1964. Compared to our results, one can find a decrease of 50‰ relative to a “clean location” (Table 3). From 1964 to 2003, our data show a decrease in ¹⁴C activity over time (773 ± 8‰ to 80 ± 5‰). Our results generally track lower than the ¹⁴C variation curves (Figure 4) from 1960 to 2003 for region NH zone 2 (Hua and Barbetti 2004; Hua et al. 2013). This difference can be attributed to a local Suess effect as Dakar is a densely populated, industrialized area.

To estimate long-term changes in ¹⁴C concentration, an exponential decay function was fitted to the observed data from 1970 to 2003, giving a correlation coefficient R² of 0.99 with a mean life for decay of 16.1 yr (Rakowski et al. 2008). A linear function was fitted to the Δ¹⁴C data for the period 1970–2003, showing a decreasing rate of 12.3‰ (per year).

<figure><img src="image_5.png" /><figcaption>Figure 4 Δ¹⁴C in tree leaves collected in Dakar city (blue curve, starting at left). The red curve (right) represents global Δ¹⁴C for NH zone 2 (Hua and Barbetti 2004). (Colors refer to online version.)</figcaption></figure>
<table><caption>Table 3 Measurements of peak Δ¹⁴C in atmospheric CO₂ recorded from various locations in NH zone 2.</caption><thead><tr><th>Location</th><th>Sample</th><th>Peak Δ¹⁴C (‰)</th><th>Peak year (AD)</th><th>Reference</th></tr></thead><tbody><tr><td>Santiago de Compostela (43°N, 8°W)</td><td>Atm. CO₂</td><td>821 ± 18</td><td>1964</td><td>Nydal and Lovseth (1996)</td></tr><tr><td>Izana (32°N, 35°E)</td><td>Atm. CO₂</td><td>845 ± 12</td><td>1964</td><td>Nydal and Lovseth (1996)</td></tr><tr><td>Mas Palomas (28°N, 16°W)</td><td>Atm. CO₂</td><td>862 ± 21</td><td>1964</td><td>Nydal and Lovseth (1996)</td></tr><tr><td>Dakar (15°N, 17°W)</td><td>Atm. CO₂</td><td>817 ± 23</td><td>1964</td><td>Nydal and Lovseth (1996)</td></tr><tr><td>Agematsu (36°N, 138°E)</td><td>Tree rings</td><td>794 ± 4</td><td>1964</td><td>Muraki et al. (1998)</td></tr><tr><td>Mts Chiak and Kyeryong (36–37°N, 122–128°E)</td><td>Tree rings</td><td>825 ± 13</td><td>1964</td><td>Park et al. (2002)</td></tr><tr><td>Dakar (14°N, 17°W)</td><td>Tree leaves</td><td>773 ± 8</td><td>1964</td><td>This study</td></tr></tbody></table>

## Fossil Fuel Component

The fossil fuel component CO₂foss of the atmospheric CO₂ concentration from 1958 to 2003 was calculated using Equation (5) with the results shown in Table 4. We also plot the time variation of these components are shown in Figure 3.

The results showed large increase in the CO₂foss component from 1958 to 1960, leading to the high values obtained: 30.1 ppm and 13.3 ppm, respectively. We know from Nydal and Lovseth (1983) that this period corresponds to nuclear weapons testing, which caused increases in the concentration of ¹⁴C in atmosphere. It is possible that these results are due to the fact that the samples collected in 1952–1960 are from C4 plants. Hence, it is possible that the different assimilation mode of atmospheric CO₂ can account for this discrepancy, although this needs further investigation. These species are able to absorb CO₂ by another enzyme (the phosphoenolpyruvate carboxylase or PEP carboxylase) and to form acids with 4 carbons (Li et al. 1998). These acids in C4 are going to accumulate in the cells of the bundle sheath, which are impermeable to gases. As a consequence, the decarboxylation of acids in C4 increases strikingly the concentration in CO₂ within the plant. These properties can explain the differences of δ¹³C (–10.2; –11.3; –11.8) of these types of samples δ¹³C (–10.2; –11.3, –11.8) of these types of samples (*Cyperus*, herbaceous) with other samples which gave consistently lower apparent fossil CO₂ components.

Analyzing the CO₂foss variations from 1960–2003 (Figure 5), one can realize that from 1960 to 1976 the values are high (13.3 ± 1.3 ppm to 5.5 ± 1.3 ppm) with a mean value of 8.8 ppm while for the period from 1976 to 2003 the values are low (12.5 ± 1.5 ppm to 0 ppm) with a mean of 4.4 ppm. However, we noticed that a high value of 12.5 ppm was obtained in 1990.

By the formal comparison of the fossil fuel CO₂foss variation curve to the one of the World Bank Group concerning Senegal (Figure 6) one can see that the trends of the curves are similar even if the drop of the values from 1960 to 1961 is big in our case. For the period 1960–1976, we see a variability in the fossil values (Figure 5) with a mean value of 8.8 ppm. These values are still high compared with the additional fossil fuel contribution at Schauinsland, which is only 1.4 ppm (Levin and Kromer 2004; Levin et al. 2008). This shows the fossil-fuel CO₂ addition by the Dakar region. From 1961 to 1966 the values increased from 4.9 ppm to 10.7 ppm, an increase of around 120%, comparable to 133% obtained from the World Bank data during the same time period (Figure 6). From 1966 to 1970 a decrease is noted with 34% for the calculated values and 30% for estimated CO₂ emissions. From 1970 to 1976 an increase of 14% with the data of World Bank data is less than the calculated value, which is 51%. On the other hand from
<table><caption>Table 4 CO₂ fossil values (ppm) from AD 1900–2003.</caption><thead><tr><th>Lab code</th><th>Sample code</th><th>Collection year (AD)</th><th>CO₂ (ppm)<fnref n="1" /></th><th>Δ<sup>14</sup>C<sub>bgd</sub> (‰)</th><th>Δ<sup>14</sup>C<sub>meas</sub> (‰)</th><th>CO₂foss (ppm)</th></tr></thead><tbody><tr><td>AA 92990</td><td>P1017</td><td>1900</td><td>—</td><td>—</td><td>-2 ± 5</td><td>—</td></tr><tr><td>AA 92991</td><td>P1018</td><td>1912</td><td>—</td><td>—</td><td>-21 ± 4</td><td>—</td></tr><tr><td>AA 92992</td><td>P1019</td><td>1939</td><td>—</td><td>—</td><td>-26 ± 4</td><td>—</td></tr><tr><td>AA 92993</td><td>P1020</td><td>1945</td><td>—</td><td>—</td><td>-33 ± 4</td><td>—</td></tr><tr><td>AA 92994</td><td>P1021</td><td>1948</td><td>—</td><td>—</td><td>-20 ± 5</td><td>—</td></tr><tr><td>AA 92995</td><td>P1022</td><td>1952</td><td>—</td><td>—</td><td>-16 ± 5</td><td>—</td></tr><tr><td>AA 92996</td><td>P1023</td><td>1958</td><td>315</td><td>81</td><td>-22 ± 4</td><td>30.0 ± 1.2</td></tr><tr><td>AA 92997</td><td>P1024</td><td>1960</td><td>316</td><td>235</td><td>183 ± 5</td><td>13.3 ± 1.3</td></tr><tr><td>AA 92998</td><td>P1025</td><td>1961</td><td>318</td><td>223</td><td>204 ± 5</td><td>4.9 ± 1.3</td></tr><tr><td>AA 92999</td><td>P1026</td><td>1964</td><td>320</td><td>804</td><td>773 ± 8</td><td>5.5 ± 1.5</td></tr><tr><td>AA 93000</td><td>P1027</td><td>1966</td><td>321</td><td>735</td><td>677 ± 6</td><td>10.7 ± 1.1</td></tr><tr><td>DK39</td><td>—</td><td>1970</td><td>326</td><td>556</td><td>522 ± 6</td><td>7.1 ± 1.3</td></tr><tr><td>AA 93001</td><td>P1028</td><td>1976</td><td>330</td><td>387</td><td>342 ± 5</td><td>10.7 ± 1.2</td></tr><tr><td>DK40</td><td>—</td><td>1979</td><td>337</td><td>326</td><td>306 ± 5</td><td>5.1 ± 1.3</td></tr><tr><td>AA 93002</td><td>P1029</td><td>1981</td><td>340</td><td>277</td><td>277 ± 5</td><td>0 ± 1.3</td></tr><tr><td>DK41</td><td>—</td><td>1990</td><td>354</td><td>190</td><td>148 ± 5</td><td>12.5 ± 1.5</td></tr><tr><td>AA 93003</td><td>P1030</td><td>1998</td><td>366</td><td>115</td><td>109 ± 6</td><td>2.0 ± 2</td></tr><tr><td>DK42</td><td>—</td><td>2000</td><td>369</td><td>100</td><td>91 ± 6</td><td>3.0 ± 2</td></tr><tr><td>AA 93004</td><td>P1031</td><td>2003</td><td>375</td><td>92</td><td>80 ± 5</td><td>4.1 ± 1.7</td></tr></tbody></table>

<figure><img src="image_6.png" /><figcaption>Figure 5 Values of the fossil fuel component CO₂foss (ppm) calculated using Δ¹⁴C in tree leaves from the Dakar region.</figcaption></figure>

1976 to 1981 our values decrease when those of the World Bank data increase. On the contrary, concerning the period from 1981 to 1990, the values increase when those of the World Bank data decrease. From 1990 to 1998 a dimunition of 80% for the calculated values against 13% from the World Bank data is noted. From 1998 to 2003 the increase of CO₂foss values is 11% while to one obtained from the World Bank data is 20%.

One can say that the variations in general of the calculated fossil fuel CO₂ emissions are coherent with those of the World Bank data excepted in the intervals of 1976–1981 and 1981–1990.

<footnote n="1">¹Globalview CO₂ (2003).</footnote>
<figure><img src="image_7.png" /><figcaption>Figure 6 CO₂ emissions (metric tons per capita) according to the World Bank (2017) DataBank.</figcaption></figure>

## CONCLUSION

Δ¹⁴C measurements in tree leaves collected from the urban area of Dakar in 1900–2003 are systematically lower than values measured in “clean air,” which is due to anthropogenic CO₂ emissions from fossil fuel use. For the period 1970–2003, data are fitted with an exponential and linear curve of the decline of the combined ¹⁴C signal, which gave more consistent values of decreasing rate and decay constants. The fossil fuel component was estimated and the values are slightly high, with a mean of 8.8 ppm for 1960–1976. From 1976 to 2003 the values are low with a mean of 4.4 ppm. This method provides a valuable environmental monitoring tool and could be compared to other urban data. These results show that the variations of CO₂ emission from fossil fuel are consistent with the World Bank data for this region, however, for some years anomalous values occur (periods of 1976–1981 and 1981–1990) and these need to be corrected by knowing the background level of the atmospheric CO₂. For better understanding and evaluation of the effects caused by emission of CO₂ from fossil fuel, we need to develop a local carbon cycle model. In addition, new sampling should be done monthly and during the two distinct seasons (dry and rainy), and a long-term record is needed to get better estimates of changes in fossil fuel CO₂ values.

## ACKNOWLEDGMENTS

This work was supported by the Botanic Department of the Faculty of Science (Cheikh Anta Diop University, Dakar) and the radiocarbon laboratory of IFAN ChA Diop of UCAD. We wish to express our thanks to the Paris Museum team (France) and to the late J F Saliège (LOCEAN, Paris IV, France) for his constant help, and to Mitzi de Martino of the Arizona radiocarbon laboratory for assistance in ¹⁴C measurements. In particular, my thanks go to Dr Tim Jull (University of Arizona AMS-Radiocarbon Laboratory, USA) for his assistance in terms of language and of content made in this paper. Our thanks also go to associate editor Christine Hatté (LSCE, Gif sur Yvette-Paris, France) for giving a great deal of assistance for this paper, and we also thank reviewer Felix Vogel. Many thanks also go to Alpha Oumar Diallo of the IFAN ¹⁴C laboratory (Cheikh Anta Diop, Dakar) for his valuable help in ¹⁴C measurements.
REFERENCES

Dieng M. 2007. Dakar, la 'fourmilière humaine' auditée. 25 May 2007. *Sud Quotidien*. http://xfru.it/9eiTIR. In French.

Druffel EM, Suess HE. 1983. On the radiocarbon record in banded coral: exchange parameters and net transport of ¹⁴CO₂ between atmosphere and surface ocean. *Journal of Geophysical Research* 88(C2):1271–80.

Globalview-CO2. 2003. Cooperative atmospheric data intiegation project—carbon dioxide. Boulder, CO: NOAAACMDL. ftp.cmdl.noaa.gov. path:ccg/co2/globalview.

Houtermans JC, Suess HE, Oeschger H. 1973. Reservoir models and production rate variations of natural radiocarbon. *Geophysics Research* 78:1897–908.

Hua Q, Barbetti M. 2004. Review of tropospheric bomb ¹⁴C data for carbon cycle modeling and age calibration purposes. *Radiocarbon* 46(3): 1273–98.

Hua Q, Barbetti M, Rakowski A. 2013. Atmospheric radiocarbon for the period 1950–2010. *Radiocarbon* 55(4):2059–72.

Jull AJT, Burr GS, Beck JW, Hodgins GWL, Biddulph DL, Gann J, Hatheway AL, Lange TE, Lifton NA. 2006. Application of accelerator mass spectrometry to environmental and paleoclimate studies at the University of Arizona. In: Povinec P, Sanchez-Cabrera J-A, editors. *Radionuclides in the Environment*. Amsterdam: Elsevier. p 3–23.

Levin I, Kromer B. 1997. Twenty years of atmospheric ¹⁴CO₂ observations at Schauinsland station, Germany. *Radiocarbon* 39(2):205–18.

Levin I, Kromer B. 2004. The tropospheric ¹⁴CO₂ level in mid-latitudes of the Northern Hemisphere (1959–2003). *Radiocarbon* 46(3):1261–72.

Levin I, Kromer B, Schmidt M, Sartorius H. 2003. A novel approach for independent budgeting of fossil fuel CO₂ over Europe by ¹⁴CO₂ observation. *Geophysical Research Letters* 30(23):2194.

Levin I, Hamer S, Kromer B, Meinhardt F. 2008. Radiocarbon observations in atmospheric CO₂ over Europe using Jungfraujoch observations as

background. *Science of the Total Environment* 391(2–3):211–6.

Li M-R, Wedin DA, Tieszen LL. 1999. C3 and C4 photosynthesis in *Cyperus* (Cyperaceae) in temperate eastern North America. *Canadian Journal of Botany* 77:209–18.

Nydal R, Lovesth K. 1983. Tracing bomb ¹⁴C in the atmosphere, 1963–1980. *Journal of Geophysical Research* 88:3621–42.

Nydal R, Lovseth K. 1996. Carbon-14 measurements in atmospheric CO₂ from Northern and Southern Hemisphere sites, 1962–1993. *ORNL/CDIAC-93, NDP-057*. Oak Ridge, TN: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory. doi: 10.3334/CDIAC/atg. ndp057. 67 p.

Nydal R, Lovesth K, Guillicksen S. 1979. A survey of radiocarbon variation in nature since the test band treat. In: Berger R, Suess HE, editors. *Radiocarbon Dating*. Berkeley: University of California Press. p 313–23.

Oeschger H, Siegenthaler U, Schotter U, Gugelman A. 1975. A box diffusion model to study the carbon dioxide exchange in nature. *Tellus* 27(2): 168–92.

Rakowski A, Nakamura T, Pazdur A. 2008. Variations of anthropogenic CO₂ in urban area deduced by radiocarbon concentration in modern tree rings. *Journal of Environmental Radioactivity* 99:1558–65.

Stuiver M, Polach H. 1977. Discussion: reporting of ¹⁴C data. *Radiocarbon* 19(3):355–63.

Suess HE. 1955. Radiocarbon concentration in modern wood. *Science* 122:415.

Takahashi H, Konohira E, Hiyama T, Minami M, Nakamura T, Yoshida N. 2002. Diurnal variation of CO₂ concentration, Δ¹⁴C and δ¹³C in an urban forest: estimate of the anthropogenic and biogenic CO₂ contributions. *Tellus B* 54: 97–109.

World Bank Group. 2017. http://data.worldbank.org/country/senegal.