Dust is an environmental factor that can accelerate the ageing and malfunction of electrical equipment. The West African region, situated within the intertropical zone, extends from 4˚N to the southern edge of the Sahara (~20˚N) and from 18˚W to 20˚E (eastern border of Lake Chad) [1] . This region is a significant source of desert aerosols, which include dust particles, sand, pollen, volcanic ash, and sea spray. The transport of these particles across West Africa is largely influenced by regional atmospheric dynamics, particularly the Harmattan and monsoon winds [1] - [4] .

The chemical composition of aerosols in West Africa varies according to soil characteristics. Typically, they are composed of oxides of aluminium, calcium, iron, and silica (Al₂O₃, CaO, Fe₂O₃, and SiO₂, respectively) [5] - [12] . Desert aerosols often mix with urban pollutants like hydrocarbons [13] [14] and accumulate on electrical equipment, particularly circuit breakers. Through vent holes, the mixture enters circuit breakers and deposits on the electrodes and within the breaking chamber [15] [16] . Figures 1-2 show dust episodes and accumulation on circuit breakers. Previous studies have demonstrated the impact of dust particles, such as silica, on circuit breaker performance. Silica significantly alters molar fractions, forming solid and liquid SiO₂ phases that condense on the gas generator surfaces. This process modifies the arc’s dynamic viscosity and speed [15] [16] . Aerosols can degrade electrical circuits and components within circuit breakers. However, prior studies have not addressed the combined effects of various species, including Fe₂O₃, CaO, Al₂O₃, and CO, which may be present in dust deposits depending on regional conditions. This study aims to investigate the effects of aerosols on the thermodynamic properties of plasma in the West African region.

The remainder of this paper is organised as follows: Section 2 outlines the methods for computing thermodynamic properties; Section 3 discusses the effects of aerosols on these properties; and Section 4 presents the conclusions.

In West Africa, the period from February to April is characterised by large emissions of desert dust from the Saharan and local areas. However, the wet period from June to September and the period from December to January are characterised by small emissions of dust [17] . The most abundant chemical species in aerosols are silicon, calcium, iron, and aluminium oxides. The carbon oxide species are derived from biomass fires and fossil-fuel production. Further, silicon oxides account for up to 60% of the total mass. After silica, the most abundant oxides are aluminium (Al₂O₃) and ferric oxides (Fe₂O₃) [9] [10] . These proportions are directly linked to the texture of the soil itself. They’re typically derived from analyses conducted in various studies across a sub-region. It’s important to understand that these proportions are not static; they can vary significantly from one area to another due to differences in geology and topography. The composition and proportion of the chemical components of aerosols vary depending on the area. The mass proportions reflect desert sandstorms, regional soil composition, and human activity, with SiO₂ as the dominant species. The red colour of the dust in Ouagadougou indicates the presence of Fe₂O₃. Moreover, CaO has remarkable proportions, but less than Fe₂O₃ in Ouagadougou. The presence of Al₂O₃ is remarkable. We have not yet encountered work that establishes a clear composition of aerosols. Studies typically focus on mass fractions and specific chemical species based on the region [1] [3] [8] - [10] .

A circuit breaker may need to operate in a polluted environment over a long duration. Our study found that aerosols were composed of silicon oxides, iron oxides, aluminium oxides, calcium oxides, and carbon monoxide. We assume that 1 g of aerosol contained the following mass percentages: 50% SiO₂, 20% Fe₂O₃, 10% Al₂O₃, 5% CO, and 15% CaO. This work is entirely theoretical because we do not have the equipment for experiments.

The mass fractions of the chemical elements of aerosol species that constitute the basic elements of the plasma were calculated using Equation (1) below:

$\text{\%}A=\text{\%}air*\text{\%}{A}_{air}+\text{\%}aero\left(\begin{array}{l}\text{\%}{A}_k{C}_l\left(\frac{k{M}_A}{M_{A_k{C}_l}}\right)+\text{\%}{A}_x{D}_z\left(\frac{x{M}_A}{M_{A_x{D}_z}}\right)+\\ \text{\%}{A}_{\gamma }{E}_{\beta }\left(\frac{\gamma M_A}{M_{A_{\gamma }{E}_{\beta }}}\right)+\text{\%}{A}_{\nu }{F}_{\theta }\left(\frac{\nu M_A}{M_{A_{\nu }{F}_{\theta }}}\right)\end{array}\right)$ (1)

where A, B, C, D, E, and F represent chemical elements; A_iB_j represents the chemical species of air of mass percentage %air; $A_{\nu }{F}_{\theta }$, $A_k{C}_l$, $A_x{D}_z$, and $A_{\gamma }{E}_{\beta }$ represent the species brought by aerosols of mass percentage %aero; $M_{A_k{C}_l}$, $M_{A_x{D}_z}$, $M_{A_{\gamma }{E}_{\beta }}$, $M_{A_{\nu }{F}_{\theta }}$, $M_A$, $M_B$, $M_C$, and $M_D$ represent the molar masses of the species considered; and i, j, k, l, x, z, 𝜃, 𝜈, β, and γ represent the numbers of atoms. From Equation (1), we obtain the mass percentages of the basic elements of our plasma, as listed in Table 1 . The initial step in calculating the thermodynamic properties and transport coefficients is determining the equilibrium composition of the gas mixture using the principle of minimisation of the Gibbs free energy of the mixture [16] [18] - [21] . To this end, the specific chemical potential of all plasma particles must be determined [20] . Smoothed and tabulated thermodynamic data from Bonnie and Bendjebbar are used to calculate specific thermodynamic properties. The specific heat capacity, specific enthalpy, and specific entropy are obtained by Equation (2) [22] [23] :

$\{\begin{array}{l}\frac{C_P^0\left(T\right)}{R}=a_1{T}^{-2}+a_2{T}^{-1}+a_3+a_{4}T+a_5{T}^2+a_6{T}^3+a_7{T}^4\\ \frac{H^0\left(T\right)}{RT}=-a_1{T}^{-2}+a_2\frac{\ln \left(T\right)}{T}+a_3+a_4\frac{T}{2}+a_5\frac{T^2}{3}+a_6\frac{T^3}{4}+a_7\frac{T^4}{5}+\frac{b_1}{T}\\ \frac{S^0\left(T\right)}{R}=-a_1\frac{T^{-2}}{2}+a_2\frac{T^{-1}}{2}+a_3\ln \left(T\right)+a_{4}T+a_5\frac{T^2}{2}+a_6\frac{T^3}{3}+a_7\frac{T^4}{4}+b_2\end{array}$ (2)

where R and T represent the constant of perfect gases and the temperature in Kelvin. The coefficients a_i and b_i for each particle are given by [22] [23] .

For air-aerosol mixtures, 34 monatomic species have been considered (C, O, N, Si, Al, Fe, Ca, C⁺, O⁺, N⁺, Si⁺, Al⁺, Fe⁺, Ca⁺, C⁻, O⁻, N⁻, Si⁻, Al⁻, Fe⁻, C⁺⁺, O⁺⁺, N⁺⁺, Si⁺⁺, Al⁺⁺, Fe⁺⁺, Ca⁺⁺, C⁺⁺⁺, O⁺⁺⁺, N⁺⁺⁺, Si⁺⁺⁺, Al⁺⁺⁺, Fe⁺⁺⁺, Ca⁺⁺⁺) and electrons; 31 diatomic species (C₂, O₂, N₂, Si₂, Al₂, Fe₂, Ca₂ CO, CN, SiC, AlC, NO, SiO, AlO, SiN, FeO, AlN, CaO, C₂⁺, O₂⁺, N₂⁺, CO⁺, CN⁺, NO⁺, AlO⁺, CaO⁺, N₂⁻, C₂⁻, O₂⁻, CN⁻, and AlO⁻); and 40 polyatomic species (CO₂, C₃, CCN, CNC, CNN, C₂O, O₃, N₃, NCO, NO₂, N₂O, NCN, SiC₂, SiO₂, Si₂C, Si₂N, Si₃, AlC₂, AlO₂, Al₂O, OCCN, C₂N₂, CNCOCN, C₃O₂, C₄, NO₃, N₂O₃, N₂O₄, N₂O₅, Al₂C₂, Al₂O₂, Al₂O₃, Fe(CO)₅, N₂O⁺, CO₂⁺, Al₂O⁺, Al₂O₂⁺, AlO₂⁻, NO₃⁻, and NO₂⁻).

In this study, the temperature range for the equilibrium composition was 2000 - 30000 K, assuming only the gaseous phase. Metals and their components are in solid form at low temperatures. At temperatures lower than approximately 2000 K, the plasma component can be established in air. The experiments are conducted at atmospheric pressure (1 bar), and therefore, all results of the thermodynamic properties are presented in this range.

This study examined the effect of aerosols on the thermodynamic properties of air plasma in a state of local thermodynamic equilibrium. Findings indicate that plasma density increases, whereas enthalpy, specific heat, and sound velocity all decrease, as the aerosol concentration in the mixture rises. Specific heat increased at temperatures below 7000 K and decreased at higher temperatures, with notable peaks at approximately 3500 K, 7000 K, and 15,000 K. Mass specific enthalpy is a key indicator of the arc plasma’s energy state. Rapid reduction of this value is the fundamental goal of all arc extinction mechanisms in a circuit breaker to ensure reliable and safe current interruption. However, the presence of metal oxides can reduce the mass enthalpy and heat capacity at a given temperature. A potentially lower specific enthalpy and heat capacity would make arc quenching more difficult. The plasma remains conductive for longer, which extends the duration of the electric arc. A longer arc duration and sustained high temperatures increase the erosion of the circuit breaker’s contacts, reducing their lifespan and potentially their reliability. In extreme cases, excessive oxide accumulation and unfavorable changes in plasma properties can lead to a failure to interrupt the circuit, with serious consequences for the electrical grid and equipment.

This theoretical analysis demonstrates that aerosols significantly impact plasma characteristics, including density, mass enthalpy, heat capacity, and sound velocity. Moreover, the plasma must be a good electrical insulator at low temperatures to facilitate the transient recovery voltage and to avoid restrikes.

While thermodynamic properties alone are insufficient to determine a circuit breaker’s interruption capacity, they provide insight into the impact of aerosols on such capacity. Future studies should investigate the transport proprieties the dielectric strength properties, density variations, energy fluxes, and radiation effects, conducting experiments to comprehensively characterise plasma properties, including the influence of solid-phase aerosols.