0 Introduction Air quality monitoring is an important component of environmental monitoring, and it has evolved from traditional manual sampling and laboratory analysis to automatic monitoring. The monitored items have gradually expanded from the original SO2, NOx, and TSP to include new items such as CO, O3, and toxic and harmful organic compounds in the air. Due to the improvement of people's living standards and the increasing environmental awareness of the whole society, the requirements for environmental information provision are becoming increasingly higher. However, the design of air quality monitoring stations should consider design costs, ease of maintenance, real-time data, and expansion of the monitoring range. Wireless sensor nodes are low-cost and low-power, suitable for multi-point detection in multiple areas, but the wireless transmission distance is short, while monitoring points are generally far from the monitoring center. Existing GPRS (General Packet Radio Service) networks have wide coverage, making long-distance data transmission easy. This paper uses existing wireless sensor networks and GPRS networks to design an air quality monitoring station, introducing the basic structure of the monitoring station, the hardware design of sensor nodes and gateway nodes, and the basic workflow. 1 Basic Structure of the Monitoring Station The monitoring station mainly consists of a wireless sensor network and a GPRS network, as shown in Figure 1. An air quality monitoring station consists of wireless sensor nodes, air quality sensors (detecting SO2, NOx, etc.), A/D converters, gateway nodes, GPRS modules, microprocessors, etc. [1]. The sensor nodes deployed in the monitoring area monitor the data of each point, and then the data is transmitted step by step along other sensor nodes. During the transmission process, the monitored data may be processed by multiple nodes, and after multiple hops, it is gathered to the gateway node, and then the gateway node transmits the data to the control center. 2 Design of sensor nodes As shown in Figure 2, the sensor node consists of four parts: sensor module, processor module, wireless communication module and power supply module [1]. The sensor module is responsible for the collection and data conversion of information in the monitoring area; the processor module is responsible for controlling the operation of the entire sensor node, storing and processing the data collected by itself and the data sent by other nodes; the wireless communication module is responsible for wireless communication with other sensor nodes, exchanging control messages and sending and receiving collected data; the power supply module provides the energy required for the operation of the sensor, processor and wireless module, and manages the energy to achieve the maximum utilization efficiency. 2.1 Design of sensor node hardware circuit The hardware circuit design is based on the system cost, development level, resource processing capability, basic communication protocol capability, secondary development capability and communication reliability of the sensor node. a) Microprocessor module: The ATMEGA1288-bit AVR microprocessor of ATEML is adopted. Its 128 kB in-system programmable Flash memory, 4 kB EEPROM and 4 kB SRAM basically meet the system's storage space requirements[2], and there is no need to expand the storage space, which reduces the system's energy consumption; the 8-channel 10-bit A/D converter basically meets the requirements of sensor data conversion and accuracy; 6 power saving modes that can be selected by software can save most of the power supply of the system; the advanced instruction set and single-cycle instruction execution time, the data throughput of ATmega128 is as high as 1 MIPS/MHz, which can alleviate the contradiction between power consumption and processing speed of the system. b) Wireless Module: Utilizing the low-power, short-range wireless communication module CC2420 manufactured by Chipcon. The CC2420 is a highly integrated industrial RF transceiver compliant with ZigBee technology. Its MAC and PHY layer protocols conform to the 802.1 5.4 standard and operate in the 2.4 GHz band. This chip requires minimal external components, ensuring effective and reliable short-range communication. Data transmission supports rates up to 250 kbit/s, enabling rapid multi-point to multi-point networking. The system is small, low-cost, and low-power, suitable for long-term battery operation, and features hardware encryption, security, reliability, flexible networking, and strong resilience. c) Sensor Module: Employing low-power sensors such as temperature sensors, humidity sensors, and various air quality sensors (e.g., TGS2602). Each node can connect to different sensors to meet the requirements of each monitoring point. The temperature and humidity sensors are used to calibrate the air quality sensor data. d) Power module: Primarily provides power to the sensors, microprocessor, and wireless transceiver, and manages the power supply to improve energy efficiency. It is generally powered by two 1.5V dry cell batteries. 2.2 Data Transmission between CC2420 and ATMEGA128 The connection between CC2420 and ATMEGA128 is very simple, as shown in Figure 3. Four pins, SFD, FIFO, FIFOP, and CCA, represent the data transmission and reception status [1,3]. The processor exchanges data and sends commands with CC2420 through the SPI interface (MISO, MOSI, SCK). At this time, CC2420 is controlled; the processor operates in master mode, acting as the controller for data transmission, while CC2420 is set to slave mode. When SPI is set to master mode for communication, the SS register should be pulled low by the program during SPI initialization. Afterwards, when data is written to the master's SPI data register, the master will start the clock generator. Under the control of the hardware circuit, the data is shifted out via MOSI, and simultaneously shifted in from CC2420 via MISO. When all 8 bits of data are shifted out, the two registers have completed one data exchange. Figure 4 shows the data transmission method between ATMEGA128 and CC2420. 2.3 Basic Workflow of Sensor Nodes Figure 5 shows the basic workflow of the node, mainly including system power-on self-test, data acquisition module, data reception and transmission, power management, and other modules. After the system powers on, the program starts, configures each port, and executes the corresponding modules using interrupts. 3 Design of Gateway Nodes The main functions of the gateway node are: to receive data from other nodes, perform data correction and fusion processing, and then send it to the monitoring center; and to process the instructions issued by the monitoring center accordingly. For the data receiving section, the CC2420 wireless transceiver chip is still used, which unifies the transmission protocol and ensures transmission reliability. Since data processing is also required, no additional sensors are added to the gateway node to improve the processor's data processing capabilities; the ATMEGA128 chip is also used. Monitoring centers are generally far from monitoring points, and using dedicated lines would be costly in practical applications. Utilizing the existing GPRS network can achieve remote data transmission, as its cost-per-use billing is relatively low. The Siemens MC55 GPRS module is used to implement remote data transmission. The MC55 GPRS module's embedded TCP/IP protocol greatly reduces design complexity and also improves the microprocessor's ability to process other data. Connecting the MC55 to the microcontroller is very simple; the standard serial port can be directly connected to the microcontroller's serial port, and no expansion interface is needed if not connected to a PC. The MC55 supports standard AT commands, and communication can be achieved by calling the corresponding AT commands. The main functions of the gateway node are: to receive data from each sensor node, process the data accordingly, and send it to the monitoring center; and to receive instructions from the monitoring center to determine the node's working status. Its basic workflow is shown in Figure 6. 4. Node Protocol In order to build the network to achieve effective data transmission and ensure the maximum efficiency of node power, the sensor nodes adopt the Zigbee protocol. The IEEE 802.15.4 standard is an economical, efficient, low data rate [4] short-range wireless technology that operates at 2.4 GHz and 868/928 MHz. It is the basis of the Zigbee application layer and network layer protocol. Based on this standard, many tiny sensor nodes coordinate with each other to achieve communication. These sensor nodes only need a small amount of energy to relay data from one sensor node to another via radio waves, thereby achieving high communication efficiency. The gateway node communicates with the sensor nodes using the same protocol, while the other part communicates with the monitoring center using the TCP/IP protocol. Communication with the monitoring center utilizes the existing GPRS network, which does not require separate networking and can be easily connected to the Internet. The MC55 module has the TCP/IP protocol embedded, which reduces the design difficulty. 5. Conclusion This system uses a CC2420 wireless module, an ATMEGA128 microprocessor, an air quality sensor, and an MC55GPRS module to construct a wireless sensor network for air quality monitoring. It offers a wide monitoring range, convenient data transmission and management, and low development and operating costs, making it particularly suitable for air quality monitoring in factories, small towns, and residential areas. It is also easy to port and expand its functionality; replacing or adding different sensors allows for the construction of other monitoring networks.