# A Review of Air Pollution Monitoring Technology for Ports
## 3 Air Pollutant Monitoring Systems for Ports
### Port Air Pollutant Monitoring
#### 3.1.1 Continuous Monitoring
In addition to the monitoring systems developed by individual companies, many researchers have investigated methods for the simultaneous monitoring of pollutants such as SO\({}_{2}\), NO\({}_{\chi}\), CO, and PM. [PERSON] et al. [43] innovated from the monitoring principle, improved and innovated the traditional DOAS, and developed a new measurement method, called GP-DOAS (Gas & Particle Differential Optical Absorption Spectroscopy) for simultaneous measurement of gas and particle pollutant concentrations, which can be used for simultaneous measurement of SO\({}_{2}\), NO\({}_{2}\), and PM concentrations. The principle of conventional DOAS is that a light beam transmitted in a medium interacts with the medium. When the medium is some kind of gas and the beam is transmitted in the optical path, the variation in its radiation intensity can be expressed by the Lambert-Beer law. Additionally, according to the Lambert-Bier law, the concentration of the gas can be measured, and the basic principle is shown in Figure 2.  
Figure 1: TH-2000. Figure 2: Lambert–Beer law for measuring gas concentration. After the gas concentration is obtained from the traditional DOAS method, the gas absorption spectrum and air Rayleigh scattering spectrum are then deducted from the absorption spectrum to analyze the attenuation spectrum of the particles, and the attenuation spectrum of the particles is compared with the light source spectrum to analyze the attenuation coefficient of the particles, and finally, the particle concentration is obtained from the attenuation coefficient. The advantages of using this method for monitoring are that the same principle of equipment can be used to monitor several major air pollutants in the port online at the same time, which can save costs and reduce the size of the equipment to a certain extent, but in the case of complex working conditions there will be a certain degree of measurement error, and at present, the technology needs further improvement, but it can be used as an auxiliary tool to make a general judgment on the environmental quality of the port. Unlike devices with improvements to the monitoring principle, there are many different gas detection technologies available today, each with its advantages and disadvantages. To date, five types of low-cost portable gas sensors are most suitable and widely available, namely electrochemical sensors, catalytic sensors, solid-state (semiconductor) sensors, NDIR, and PID (photo-ionization detector) sensors. However, no single sensor is capable of measuring all hazardous gases [42]. In the face of various air pollutants in the port, to meet the monitoring of the main air pollutants, a combination system of multiple sensors can be used. [PERSON] [44] applies emerging technologies and designs a ZigBee-based air pollution monitoring system, which uses ZigBee-based wireless sensor network technology for data transmission, and the data come from the corresponding air pollutant sensors. Most of the sensors used in these monitoring systems are electrochemical sensors and absorption gas sensors, which have been developed to have the appropriate accuracy and immunity to interference. In this mode, the monitoring system can distribute a large number of sensors to obtain a large amount of monitoring data, and the management center can remote control the monitoring system. Thanks to ZigBee technology and today's air pollutant sensor technology, the system not only monitors the accuracy of the data obtained and effectively reduces the cost of the system, but also with only a large number of wireless sensor monitoring nodes needed in each place to monitor the terminal, you can achieve real-time online monitoring of port air pollutants. Similarly, the LoRa-based port air pollutant monitoring system designed by [PERSON] et al. [45] has the same monitoring mode as the ZigBee-based monitoring system, which uses sensors as data collection terminals and transmits data through the network.