## 2 Materials and Methods  
The essential air quality parameters, each playing a unique role in shaping the quality of the air, are summarized in Table 1. Understanding these air quality parameters lays the foundation for developing effective monitoring strategies. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Air Quality Parameters** & **Sources** & **Health Effects** & **Regulatory Limits** \\ \hline Ammonia (NH3) & \(\bullet\) & Agricultural Practices: Fertilizer application, livestock waste. & \(\bullet\) & Respiratory Irritation: High concentrations can lead to respiratory discomfort. \\  & \(\bullet\) & Industrial Activities: Emissions from certain manufacturing processes. & \(\bullet\) & Eye Irritation: Exposure may cause eye irritation and damage. & Set by regulatory bodies to limit ammonia concentrations in ambient air. \\  & \(\bullet\) & Transportation: Vehicle emissions, albeit to a lesser extent. & \(\bullet\) & Systemic Effects: Prolonged exposure may impact respiratory and cardiovascular systems. \\ \hline Hydrogen Sulfide (H2S) & \(\bullet\) & Industrial Processes: Oil refining, natural gas processing. & \(\bullet\) & Respiratory Issues: Low concentrations may cause irritation; higher levels can lead to more severe respiratory effects. & Limits are established to protect public health and the environment. \\  & \(\bullet\) & Volcanic Activity: Natural sources contribute in some regions. & \(\bullet\) & Neurological Effects: Prolonged exposure may impact the central nervous system. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Identifying air quality parameters. The SOLID principles of system architecture design [32] were followed when creating the multi-sensor system. This guarantees the stability and maintenance of the system over time. Following the SOLID acronym, the principles of system architecture design are as follows:  
* S--The Single Responsibility Principle (SRP)
* O--The Open-Closed Principle (OCP)
* L--The Liskov Substitution Principle (LSP)
* I--The Interface Segregation Principle (ISP)
* D--The Dependency Inversion Principle (DIP)  
The five principles are implemented and improved as follows:  
SRP: Each system component, such as sensors, data acquisition modules, and communication interfaces, is designed to perform a single function. For example, the NodeMCU is solely responsible for data collection from various sensors. OCP: The system is designed to be open for expansion but closed for modification. New sensors or modules can be added without changing the existing system structure, improving scalability. LSP: Sensor modules and communication protocols are designed to be interchangeable without affecting system functionality. This ensures that each type of sensor can be replaced or upgraded independently. The modularity of the system makes it flexible and open to work with other compatible devices and sensors. It also makes maintenance easier if one of the modules fails. ISP: The interfaces between the different system components, such as between the NodeMCU and the sensors or between the NodeMCU and the SAL server, are designed to be specific to their needs. This minimizes dependencies and improves modularity. It also provides higher reliability by saving data in two separate and independent locations--in the cloud and on-premises.