# 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)
By utilizing the mobility of the public transportations, even with a few sensor nodes, the urban air pollution information with fine-grained (high spatial resolution) level was achieved. In this paper, the air pollution information is only available to the researchers. In [117], a fine-grained vehicular-based mobile air pollution measuring approach was presented. The proposed schema can utilize multiple types of mobile sensor nodes including the proposed Mobile Sensing Box (MSB) and other personal sensing devices. The MSB consists of two ambient sensors (CO and PM) for data collection, a GPS module for location and time information acquisition and a cellular module for data transmission. The car mounted with a MSB travels around the city. Real-time data are received and analyzed by the Cloud Server. Authorized air pollution information is available to the public through the customized Web and mobile Apps. In [118], a GPRS Sensor Array for outdoor air pollution monitoring was proposed, implemented and tested. The system consists of a mobile sensing unit, which was mounted on the public transportation, and an Internet enabled server. Each sensing unit integrated with a set of sensors (CO, NO\({}_{2}\) and SO\({}_{2}\)), a GPS and GPRS modules. Data with location information are sent to the server through the cellular network (GPRS) for further processing and analysis. Authorized air pollution information is available to the public through the customized Web App. _Adavantages:_  
1. Loose constraint on energy consumption (The sensor nodes are powered by the vehicles' batteries.) 2. Loose limitations on weight and size (The carrier of the sensor node is able to carry sufficient enough weight.) 3. Multiple sensors per node (One sensor node can equip with several types of sensors because of the loose limitations on weight and size.) 4. Accurate and reliable data (Sensor node can integrate with assisting tools because of the loose limitations on weight and size.) 5. High mobility of sensor nodes (The highly mobile vehicles significantly enlarge the sensor node's geographic coverage.) 6. Feasibility in maintenance (The vehicles mounted with sensor nodes can be driven to a specific location. Professionals can perform maintenance on large amount of sensor nodes simultaneously.) 7. Well calibrated and maintained sensors (This is because of the feasibility in maintenance of the VSN systems.) 8. Automatic gathering property (The sensor nodes are densely distributed at locations with gathering public transportations automatically. Data with higher spatial resolution and accuracy are achievable in such case.) _Disadvantages or Challenges:_  
1. Uncontrolled or semi-controlled mobility (The routes of the sensor nodes or public transportations are pre-determined. The sensor nodes may squeeze into a small place with crowded transportations and cause redundant sampling. Some locations may never be visited.) 2. Redundant sampling issues (The vehicles may be trapped into traffic jams or parked in parking lots that cause redundant sampling. This issue compromises the spatial and temporal resolutions.) 3. Cost inefficiency on carriers (The specially equipped cars may cost a huge amount of money.) 4. Locating and communication devices requirement (The system requires GPS modules, and wireless modules or cellular modules.) 5. Customized network requirement (A customized wireless network is required when the cellular network is not utilized. The network connectivity may not be guaranteed due to the mobility of vehicles.) 6. 2-Dimensional data acquisition (Only the air quality of urban surface is monitored.) 7. Spatial-to-Temporal resolution trade-off (Higher spatial coverage at the expense of lower temporal resolution [99].) In this section, 20 state-of-the-art TNGAPMSs are discussed and classified into three categories, namely the SSN, CSN and VSN.