# A Survey of Wireless Sensor Network Based Air Pollution Monitoring Systems
## 4 State-of-The-Art WSN Based Air Pollution Monitoring Systems
### Static Sensor Network (SSN)
Authorized air pollution information is available to the public through Email, SMS and customized Web App. This framework is claimed to be simple and reusable in other applications. Also the failure sensor nodes can be identified efficiently and the energy consumption of each sensor node is minimized. Moreover, a simple Clustering Protocol of Air Sensor (CPAS) network was proposed, which proved to be efficient (in simulation) in terms of network energy consumption, network lifetime, and the data communication rate. The QoSs of the network such as delay, accuracy and reliability (fault tolerance) were also considered. In [107], a WSN based indoor air pollution monitoring system was presented. The focuses were the power consumption on sensor, sensor node and network levels. Several methodologies that greatly improved the lifetime (up to 3 years) of the monitoring system have been proposed and simulated. The sensor node equips with several sensors (accelerometer, temperature and relative humidity sensors, CO, VOCs and motion sensors), a ZigBee communication link and a battery. In the simulation, 36 sensor nodes were place in the first floor of a 4-story building. Data acquired by the sensor nodes were available to the researchers only. In [108], an indoor and an outdoor air pollution monitoring architectures based on Wi-Fi were proposed. In this paper, only the indoor one was implemented and tested. Each sensor node consists of several sensors (temperature and relative humidity sensors, CO, methane and solvent vapors sensors) and a Wi-Fi communication link. In order to mitigate the influence factors (temperature and relative humidity) of the gas sensors, a neural network was implemented to obtain the temperature and relative humidity correcting values for the pollutants' concentrations. Sensed data were processed by a PC and published to a customized web page. _Adavantages:_  
1. Loose constraint on energy consumption (The sensor nodes are typically powered by batteries with large capacity or energy harvest devices or power line.) 2. No locating device (The location of a sensor node is known once it was deployed since the sensor node is stationary.) 3. Loose limitations on weight and size (The carrier of the sensor node is able to carry sufficient enough weight. )Multiple sensors per node (One sensor node can equip with several types of sensors because of the loose limitations on weight and size.) * Accurate and reliable data (Sensor node can integrate with assisting tools because of the loose limitations on weight and size.) * Guaranteed network connectivity (Once the stationary sensor node joined the network, the topology is fixed and the connectivity is guaranteed.) * Well calibrated and maintained sensors (The sensor nodes can be well calibrated and maintained by the professionals periodically.) _Disadvantages or Challenges:_
* Careful placement of sensor nodes requirement (This is because of the location dependence of air pollutants.) * Large number of sensor nodes requirement (Data with sufficient geographic coverage and spatial resolution are only achievable by increasing the number of the stationary sensor nodes.) * Resource wasting in certain level (The stationary sensor nodes are in sleep mode most of the time because continuously updating data at one location is pointless[13].) * Inconveniences of calibration and maintenance (The professionals need to visit all stationary sensor nodes, which is a time and manpower consuming task, to perform operations.) * 2-Dimensional data acquisition (Only the air quality of urban surface is monitored.) * Customized network requirement (A customized wireless or wired network is required when the cellular network is not utilized.)