Measurement of Atmospheric Black Carbon Concentration in Rural and Urban Environments: Cases of Lamto and Abidjan

Black carbon is one of the primary aerosols directly emitted from biomass known to have strong absorbing properties. The INDAAF and PASMU ob-servational field campaigns which took place (2018) in Abidjan (urban area) and Lamto (rural area) allow the analysis of Black carbon concentration at different time scales through real-time measurements using an analyzer named Aethalometer AE-33. Results presented here show at Lamto: 1) for the diurnal scale an average of 1.71 ± 0.3 µg·m −3 (0.34 ± 0.09 µg·m −3 ) in the dry (wet) season; 2) for the monthly scale an average of 1.14 ± 0.84 µg·m −3 ; 3) on the seasonal scale, an average of 2.2 ± 0.02 µg·m −3 (0.6 ± 0.19 µg·m −3 ) in the dry (wet) season. The black carbon variation at Lamto is seasonal with an amplification factor of 85.6. Regarding the urban area of Abidjan, due to sampling concentrations at Lamto are mainly related to biomass combustion sources while those from urban areas are related to traffic sources. The latter is permanently active, unlike those in rural Lamto, which is seasonal.


Introduction
Black carbon (BC), is a component of atmospheric particulate matter (PM) and is described as a solid pure carbon material formed during combustion. It is emitted during the incomplete combustion of fossil fuels, and biomass burning [1] and absorbs at all wavelengths of solar radiation [2]. It is usually co-emitted with other organic matter. The interest in Black carbon comes from its climatic, environmental and health implications [3]. Indeed, BC contributes significantly to climate change by absorbing incoming solar radiation and has a strong absorption capacity of short-wave radiation. Thus it increases atmospheric warming, alters atmospheric stability and affects clouds by modifying the hygroscopicity of cloud condensation nuclei [4]. As far as BC is concerned, it may have a net warming impact, while other aerosols (i.e. nitrates, sulfates, organic carbon, etc.) may have a cooling effect [2]. BC directly influences the global radiation balance, and has a lifetime ranging from 1 week to 10 days in the absence of precipitation and is therefore conducive to long-range transport [5]. BC particles also have adverse health effects [6]. The very small size of BC (PM 2.5) allows access to the finest lung branches (alveoli). Though uncertainties remain on the toxicity of the graphitized carbon core, it serves as a vector for organic and metallic compounds that have proven health effects [7].
Globally, about 20% of black carbon is emitted from the combustion of biofuels, 40% from fossil fuels and 40% from open-burning biomass [8]. It is therefore essential to study the characteristics of black carbon, in order to evaluate its radiative properties and quantify its contribution to atmospheric pollution in different regions. For instance, in Africa, urban air pollution is a major issue, especially in regions with high population monitoring atmospheric pollution and air quality remains a priority for the scientific community in the context of environmental preservation in order to implement measures for reducing the emission of atmospheric pollutants. Many studies have analyzed atmospheric BC and its impacts on household air pollution and human health in Asia [9] [10] [11] [12] [13], in Latin America [14] [15] [16] and in Africa [17] [18]. Moreover, airborne measurements were made during the international observation network IDAF in 1994 which is part of the international Global At-

Measurement Sites
The sites of our study concern one of the INDAAF network measurement site of Lamto (rural area) and one PASMU project site, Abidjan (urban site). The former represents a savannah ecosystem and has one of the most important scientific databases. It is a geophysical station and also a measurement site of the  [28]. This city is considered as a cultural crossroads and has experienced rapid economic growth and high immigration in recent years. Its population is estimated at more than 7404 million inhabitants (RGPH, 2014), i.e. 20.8% of the total population in Côte d'Ivoire (INS, 2015). As a result, Abidjan is facing numerous anthropogenic emissions, due to the galloping demography and rapid urbanization. The area is characterized by a warm and humid sub-equatorial climate, with two alternating rainy and dry seasons. On the West African coasts, there is an alternation of the climatic profile, which is due to the passage of the inter tropical front [29] [30]. As a result, the seasonal climatic profile of Abidjan is a reflection of this alternation. Indeed, the climate is sub-equatorial, and defined by four major seasons: the great dry season (December to March), the great rainy season (April to July); the small dry season, (August to September); the small rainy season (October to November).  [34]. To account for the "shadowing" effect due to the filter loading (decrease in the sensitivity of the Aethalometer), a loading correction, "R" (ATN) was performed [35]. In order to avoid the effect of humidity on the Aethalometer measurements, the aerosol is pre-dried (less than 40% RRH) by diffusion before detection. Previous studies have shown that some organic aerosols from wood combustion have absorption peaks at wavelengths of 370 nm compared to 880 nm [36].  [38]. However, in order to find the total BC concentration, the wavelength of 880 nm was used in the present study as it has also been used in other studies [39], although, some studies have underlined the need and importance of additional corrections for instrumental artifacts and dust absorption [40].

Measurements and Instrumentation
The data were sampled from January 2015 to May 2017 with a frequency of 30 minutes in the wet season and 10 minutes for the dry season, at the Lamto site.

BC Diurnal Variations
BC diurnal variations were investigated by calculating daily average of BC concentrations for each month.  Table 1.

Monthly Variations
The monthly variations of BC concentrations from January 2015 to May 2017, are presented in Figure 2. Nevertheless, discontinuities were observed in the data, for the period of April 2017 and June to December 2017, which is mainly due to maintenance of the Aethalometer. Over the study period, monthly BC con-     in this area [41].
Indeed, it is well documented that in humid tropical forest areas, the presence of carbonaceous particles in the atmosphere is generally linked to emissions from biomass fires and domestic fires. In addition, in rural areas, agricultural practices such as shifting cultivation and the preparation of agricultural land through burning are practices linked to human activities. Biomass fires appear important during the dry season, as they allow for the preparation of cultivable land (clearing) before cultivation. "Reference" [42] showed that in the savanna zone about 80% of the land is burnt during the dry season, thus influencing concentration levels.     could be attributed to the low air temperatures experienced by the city of Abidjan during this period. In fact, these low air temperatures are related by the cooler ocean surface temperatures (SST) over the Gulf of Guinea coasts during this period. Indeed, these low temperatures do not favor combustion processes such as the combustion of waste, occurring around the site. However, the relatively high concentration levels in July, observed in the middle of the rainy season, can be justified by fairly high traffic emissions. Obviously, the decrease of BC concentration level after mid-July could be linked to a decrease in road traffic related to the period of major holidays. Actually, compared to biomass combustion, fossil fuels are the major sources of BC release during the wet season compare in to the dry season [45].

Weekly Variations
High BC concentrations in urban areas imply that anthropogenic activities are the major source of emissions. In order to justify our hypothesis, we analyzed the diurnal variation of urban BC concentrations according to days of the week. The diurnal concentrations of BC were averaged from Monday to Friday in order to distinguish the evolution of daytime concentrations between working days and weekend (e.g. Saturday and Sunday). The idea behind is to highlight that BC emissions are modulated by socioeconomic activities. The diurnal variation of BC concentrations on weekdays and weekend is presented in Figure 6. The weekdays are averaged over two months The analysis showed a rather similar pattern of diurnal BC concentrations for all the weekdays from Monday to Friday, and for Saturday, with the exception of Sunday. During the week, BC concentrations increase progressively in the early morning from 5 to 8 AM with values ranging from 7.11 to 14.00 µg·m −3 , then decrease until 12 PM to reach values of approximately 5.77 µg·m −3 , and remain almost constant from this time until 4 PM. Then concentrations rise again until around 9 PM before dropping back to around 5.98 µg·m −3 at midnight. The lowest values on weekdays are obtained between noon and 4 PM. This peak observed at 7 AM suggests that there is a correlation between traffic source and BC  . This could be explained by a decrease in work activities during the weekend, which is even more pronounced.
Similar results were found by [45] in the city of Bamako, Table 3

Comparison of Rural and Urban Areas
At The ratio between the maximum values is of the order of 5.86. We also note that the emission sources in urban areas are permanently active, whereas those in rural areas, are rather linked to the seasonality of the area of interest. For instance, emission sources are more active in the dry season than in the wet season. Moreover, BC concentrations of Abidjan are of the same order as those obtained in Bamako (weekend) and about half of that on weekdays [45]. Some urban concentrations are however, slightly above the daily standard set by the WHO (10 µg·m −3 ).
As a result, the levels of urban BC concentrations are alarming in contrast to rural areas, where levels remain below international daily WHO standards.
Our findings once again, underline the anthropogenic activities influence on the chemical composition of the atmosphere and air quality, particularly in large African urban centers, amplified by rapid population growth and poorly planned urbanization. Table 4 shows BC concentration for our study compared to literature for metropolises in West Africa, Asia and Europe. We have distinguished between urban and rural pollution.
Values from this study are in the same order of magnitude than those obtained in some African rural sites during intense winter biomass burning periods  such as Dakar, Cotonou, Yaounde but remains below those from Bamako according to [45]. According to study of [46], our results remain well below of thoses of Asian metropolises, in Karachi, Pakistan, [47] in Singapore, and [48] (Tripath et al.), in Kampur. In Europe, the results of [49] at Marylebone, in London and those of [50] in Paris present values largely above those from this study. It is worthy to point out the contribution of industrial pollution in the above-mentioned studies, in contrast to that of Abidjan which does not have the same industrial level as these metropolises. In rural areas, the results of our study are consistent with those of [51] in Lamto, but are above those of Banizoumou in the forest, and below that of Djougou during dry savanna [52].  As economic activities are permanent in urban areas, the sources of BC emissions will have a permanent character dominating largely the seasonal aspect.

Conclusions
This is the opposite in rural areas where anthropic action is weak (little economic activity), only seasonal aspects dominate BC emission sources. Furthermore, BC concentration levels in urban are more alarming, in contrast to those of rural areas, which remain below international standards. The ratio between the maximum values is around 5.86. The variability of BC concentrations in urban areas (Abidjan) is quite significant compared to that observed in rural areas (Lamto).
However, the levels of BC emissions in Abidjan are comparable to those of West African capitals but remain below the major metropolises of other continents.