# A Survey of Wireless Sensor Network Based Air Pollution Monitoring Systems
## 4 State-of-The-Art WSN Based Air Pollution Monitoring Systems
### Vehicle Sensor Network (VSN)  
In VSN systems, the sensor nodes are typically carried by the public transportations like buses or taxis (see Figure 10). By utilizing the low-cost portable ambient sensors and the mobility of vehicles, one sensor node is able to achieve sufficient large geographic coverage [99, 115]. Authorized air pollution information is available to the public through web pages, Web Apps, mobile Apps, _etc._  
Figure 10: Example of the Vehicle Sensor Network (VSN) system architecture and prototype. Figures are adapted from [98]. _Carrier:_ The sensor nodes are carried by the public transportations (buses, trains and taxis) or specially equipped cars. _Related Works:_  
In [11], the Mobile Air Quality Monitoring Network (MAQUMON) was presented. This system is composed by a number of car-mounted sensor nodes measuring the concentrations of O\({}_{3}\), CO and NO\({}_{2}\). Each sensor node utilizes a GPS module for acquiring time and location information and a Bluetooth link for communicating with the laptop inside car. Collected data are then uploaded to the server through the laptop's Wi-Fi link. Authorized air pollution information is accessible through the sensor node's LCD display or the SensorMap Web App. In [19], a distributed infrastructure based on the WSN and Grid computing for real-time comprehensive air pollution monitoring and mining was presented. In this system, two types of sensor nodes are utilized, namely the Mobile Sensor Node (MS node) and the Static Sensor Node (SS node). The sensor node consists of a Generic Ultra Violet Sensor Technologies and Observation (GUSTO) sensor (able to detect SO\({}_{2}\), NO\({}_{X}\), O\({}_{3}\) and VOCs) and a wireless link (ZigBee or Wi-Fi or others). The MS nodes are mounted on the public transportations and transmitting data to the SS nodes. The SS nodes are able to perform data acquisition and fusion, and further forward the data (from MS nodes and SS nodes) to the central server. Currently, the air pollution information is only available to the researchers. A distributed data mining-algorithm for identifying the relationships between the urban transport and the environment was also proposed. In [98, 116], a vehicular wireless sensor network architecture was proposed and implemented to achieve the micro-climate monitoring. A CO\({}_{2}\) sensor is mounted outside the car to monitor the concentration of CO\({}_{2}\). A ZigBee intra-vehicle wireless network is utilized to communicate with the inside-car processing unit, which equipped with a micro-controller, a GSM short message module and a GPS module. Data are then sent to the GSM base stations and further forwarded to the monitoring server. Authorized air pollution information is available to the public through a Web App. In order to balance the accuracy of sensed data and the cost of communication, an on-demand approach that adjusts vehicles' reporting rates was proposed. In [99], a mobile sensor node prototype that can be mounted on vehicle was introduced and tested. Each sensor node consists of a set of sensors (CO, PM, NO, NO\({}_{2}\) and VOCs) for detecting the pollutants' concentrations, a GPS module for collecting the location information, and a GPRS or Wi-Fi module for communicating with the server. Analyzed data are available to the public through a Web App. This paper claimed that the proposed system demonstrated higher spatial coverage at the expense of lower temporal resolution compared with the SSN systems. In [115], a low-cost air pollution monitoring system using vehicular sensor network was proposed to complement the conventional air pollution monitoring networks. Each sensor node consists of a set of sensors (temperature, relative humidity, NO\({}_{2}\), CO\({}_{2}\), CO and O\({}_{3}\)), a GPS module, and a ZigBee wireless link. Data acquired by the sensor nodes are transmitted back to the central computer for further analysis through the static ZigBee accessing points. These sensor nodes are mounted on the public transportations, like buses.