# A Survey of Wireless Sensor Network Based Air Pollution Monitoring Systems
## 1 Introduction  
Over the past few years, air pollution has drawn a lot of interest in terms of research and everyday life. According to data from Google Search, about 46 million results are related to \"2014 Air Pollution\", while the number of results related to \"2014 Nobel Prize\" is only about 27 million (accessed on 2014-8-20). The public concern on air pollution increases significantly due to the serious hazards to the public health, as described in [1]. Heart disease, Chronic Obstructive Pulmonary Disease (COPD), stroke and lung cancer are highly related to air pollution. People breathing in air ofpoor quality could suffer from difficulty in breathing, coughing, wheezing and asthma. In addition to the human health, air pollution also has a major effect on the global environment and the worldwide economy. It is well known that acid rain, haze and global climate change are caused by air pollution. In 2010, the European Commission threatened the UK with legal actions for the breaching of PM\({}_{10}\) (PM\({}_{\text{X}}\) stands for particulate matter with diameter of less than or equal to X \(\mu m\)) limit values. The UK could pay PS300 million per year for this [2]. In order to mitigate the impacts of air pollution on human health, global environment and worldwide economy, governments have put tremendous efforts on air pollution monitoring. With detailed information of the air pollution situation, scientists, policy makers and planners are able to make informed decisions on managing and improving the living environment [3]. Countries adopting proper policies on air pollution can reduce the public health expenses as described above [4]. Traditionally, air pollution situation is monitored by conventional air pollution monitoring systems with stationary monitors. These monitoring stations are highly reliable, accurate and able to measure a wide range of pollutants by using the conventional analytical instruments, such as gas chromatograph-mass spectrometers [5]. The drawbacks of the conventional monitoring instruments are their large size, heavy weight and extraordinary expensiveness. These lead to sparse deployment of the monitoring stations (see Figure 1 and Table 1). In order to be effective, the locations of the monitoring stations need careful placement because the air pollution situation in urban areas is highly related to human activities (e.g., construction activities) and location-dependent (e.g., the traffic choke-points have much worse air quality than average) [10, 11, 12]. Changes in urban arrangement, activities or regulation may affect both the species and the concentrations of air pollutants, which require relocating stations or adding new stations. These requirements are typically hard or even impossible to fulfill due to the cost inefficiency in acquisition and maintenance of the monitoring stations. Moreover, the conventional monitoring \begin{table}
\begin{tabular}{c c c c} \hline \hline
**City** & **Number of Stationary Monitors** & **Coverage Area** & **Coverage Per Monitor (Number of Football Fields)** \\ \hline Beijing, China & 35 [7] & 16,000 km\({}^{2}\) & 64,025 \\ \hline Hong Kong, China & 15 [6] & 2700 km\({}^{2}\) & 25,210 \\ \hline New York, USA & 44 [8] & 1200 km\({}^{2}\) & 3820 \\ \hline London, UK & 123 [9] & 1600 km\({}^{2}\) & 1822 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The number of stationary monitors in selected cities. Figure 1: Deployment of stationary monitors in Hong Kong [6]. instruments involve long-term time-consuming average models. The air pollution situation is updated hourly or even daily. Hence, the air pollution maps built by the conventional air pollution monitoring systems are with extremely low spatial and temporal resolutions. Such low spatio-temporal resolution is sufficient for ambient background monitoring but extremely inadequate for the public to be aware of their personal exposure to air pollution and cannot reflect their personal health risks.