The study was conducted in the township of Yopougon, within its industrial zone. The selected sampling site was the Abidjan Detention and Correctional Center (MACA), located in the immediate vicinity of major sources of air pollution, including heavy road traffic and surrounding industrial activities.

MACA, the largest prison in Côte d’Ivoire, is located in the northwestern part of the Abidjan district, at the geographic coordinates 5°20’56’’ N and 4°00’56’’ W. The townwship of Yopougon, with an area of 152 km² (Madina et al., 2018), is both the largest and most populated in the country, with an estimated population of 1,571,065 inhabitants (INS, 2022).

The industrial zone of Yopougon comprises over 400 companies operating in sectors such as cement manufacturing, metallurgical products, food processing, cosmetics, and chemicals. The sampling site is located on a busy road, exposed to heavy vehicular traffic and high pedestrian traffic, making it a strategic location for studying PM2.5 and black carbon concentrations.

(Fig.1) shows the study area and the air sampling point marked in red on the map. It is taken from (Popouen et al., 2021).

Air samples were collected using the LVS/LVS6-RV low-volume sampler, manufactured by Sven Leckel. The filters used for particle retention were high-retention borosilicate glass microfiber filters (Grade GF/A 1820-47), with a diameter of 47 mm and a porosity of 1.6 µm. They have a smooth, spherical surface capable of withstanding temperatures exceeding 50°C.

Figure 2 shows the installation of the sample at the sampling area. On the site, approximately 200 meters from a busy road, the LVS/LVS6-RV sampler was installed 2.5 meters above the ground on a rooftop to avoid disturbances at ground level caused by accidental local sources or various turbulences.

Before aerosol sampling, the equipment was pre-configured. During the PM2.5 particulate matter sampling campaign, conducted from May 1 to July 28, 2018, during the rainy season, samples were taken from filters placed in thoroughly cleaned Petri dishes. To prevent contamination, the filters in the Petri dishes were stored in a desiccator to protect them from moisture. A total of 19 PM2.5 samples were collected and placed in the desiccator. During the sampling campaign, meteorological conditions were recorded, including wind speed, rainfall, and relative humidity.

The collected samples are shown in (Fig. 3A & 3B) below. To determine the mass of the collected fine particles, the unused filters (Fig. 3A) and the sampled filters (Fig. 3B) were weighed using a Sartorius microbalance (Quintix 65-1S). This microbalance, with a sensitivity of 1 µg, is located in a temperature- and humidity-controlled environment in the Physics Lab at the University.

(Fig. 3A) shows a filter before sampling while (Fig. 3B) indicates a filter after sampling which contains aerosol particulates.

The mass of the aerosols is therefore the difference between the two masses obtained with the microbalance and is given by the following equation:

With: m_B, the mass of the sampled filter and m_A, the mass of the blank filter.

The concentration of particulate matter is the ratio of the mass of aerosols to the volume of air sampled. The volume of air sampled is calculated based on the flow rate (2.3 m³/h) or (38.33 L/min) and the sampling duration (24 h). The volume of air was calculated using the equation below.

Where V: sampled air volume, F: pumping flow rate (L/min), and Δt: sampling time (h)

Thus, the mass concentration of particulate matter is determined in micrograms per cubic meter (µg/m³) using the following equation :

With: C_m: Mass concentration of deposited particles (µg/m³), m: Net mass of particles collected on the sampled filters (µg), m_f: Mass of the filter after sampling (µg), m_i: Mass of the empty filter (µg), and V: Sampled air volume (m³).

The sampled filters were examined for black smoke to measure reflectance using an EEL Smoke Stain reflectometer (Model 43M diffusion system Ltd 43). The reflectometer, connected to the main unit, was placed on the white standard for at least 20 minutes. Then, to measure the reflectance of the black carbon deposited on the filters, each filter was examined using the five-point method, which considers the points most likely to be reached by the reflectometer's light beam (Fig. 4).

The concentration of black carbon in the air sample is determined by relation (4) below:

C: black carbon concentration expressed in µg/m³

ε: mass absorption coefficient for a given wavelength, equal to 5.27 m³/g

F: correction factor, equal to 1 according to (UE, 2002)

α: absorption coefficient of the collected aerosols given by the equation :

Where: RS: the average reflectance of the five points on the filter

RO: the reflectance of the white filter (by definition equal to 100)

V: the volume of air aspirated during sampling in m³

A: the active surface area of the filter loaded with particulate matter, given by:

d: the diameter of the loaded filter equals 41mm, i.e. A = 0.132 cm² = 13.2×10^-6 m² .

The PM2.5 and carbon black concentrations measured in the industrial area from May 1st to July 28th are shown in Table 1 below. The mass concentrations of PM2.5 and carbon black ranged from 27.11 to 70.68 µg/m³ and from 43.85 to 66.07 µg/m³, respectively, with average values of 46.48 ± 12.18 µg/ m³ and 55.26 ± 6.65 µg/m³, respectively. The study shows that the average carbon black concentration is higher than that of PM2.5 at the site. The PM2.5 concentration levels obtained at the site were all above the reference value established by the WHO (15 µg/m3), thus indicating an accumulated health risk for the population residing in the area (WHO, 2021). As for black carbon (BC), its concentrations were high in PM2.5 because it preferentially binds to this particle size category. This suggests that BC pollution is associated with incomplete combustion sources (Marcazzan et la., 2001), which is consistent with the significant industrial activity and road traffic that characterize the study area.

(Table 2) presents the daily mass concentrations of PM2.5 and black carbon in the PM2.5 fraction in µg/m³. The averages of these parameters are also provided.

The variation in Black Carbon (BC) concentrations in PM2.5 measured daily is shown in (Fig. 5). It also shows that during the PM2.5 particulate fraction sampling period, BC concentrations were higher than PM2.5 concentrations in several samples. (Fig. 5) indicates peak concentrations of PM2.5 and BC observed on 9 May, 2018, and 28 June, 2018.

Comparison of PM2.5 and BC concentrations obtained with other studies.

The PM2.5 and BC values obtained in this study were compared with those obtained at the national and international levels (Table 2).

It can be noted that the PM2.5 values in our study are lower than those observed in various African countries, but they remain above international standards (WHO, 2021). However, black carbon concentrations are significant at the study sites.

Furthermore, meteorological parameters are likely to influence pollutant concentrations at a study site; these include wind speed, relative humidity, and any rainfall. A time series of PM2.5 concentrations is shown in (Fig. 6).

(Fig. 6) shows that the highest PM2.5 concentrations are recorded on sunny days with an average rainfall of 86 mm, and a maximum value of 70.68 ± 0.78 µg/m³. Conversely, on rainy, cloudy days (high humidity) with wind, PM2.5 concentrations in the atmosphere of the Yopougon industrial zone are relatively low. This decrease in concentration during rainy days is explained by the wet deposition of particulate matter caused by rainfall, and by the role played by wind in dispersing the particles away from the sampling site. A negative correlation between PM2.5 and rainfall was observed during June 2018; this is well supported by the data.