# A Review of Air Pollution Monitoring Technology for Ports
## 3 Air Pollutant Monitoring Systems for Ports
### Ship Exhaust Monitoring  
Air pollution from ships also has a large impact on the port environment, so it is especially important to monitor the emissions from ships entering and leaving the port. For ships, given the conditions and the navigation cycle, generally do not use the combination of sampler and laboratory discontinuous monitoring equipment, more commonly used is the continuous online monitoring equipment for ship flue gas. In terms of monitoring equipment, the guidelines issued by the IMO (International Maritime Organization) recommend the use of monitoring equipment that works on the principle of NDIR. At the same time, it is also clear that the competent authorities of various countries can recognize other forms of monitoring equipment based on full experiment and evaluation. There \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sensor Types** & **Detectable Gases** & **Life Expectancy** \\ \hline \multirow{2}{*}{Electrochemical} & Gases which are electrochemically & \multirow{2}{*}{1–2 years} \\  & active, about 20 gases & \\ \hline Catalytic & Combustible gases & Up to 3 years \\ \hline Solid-state & About 150 different gases & 10 years or more \\ \hline \multirow{2}{*}{Non-dispersive Infrared} & Hydrocarbon gases & \multirow{2}{*}{3–5 years} \\  & and carbon dioxide & \\ \hline \multirow{2}{*}{Photo-ionization} & \multirow{2}{*}{Volatile organic compounds (VOCs)} & Depend on the Ultraviolet \\  & & lamp, normally 6000 h \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Five commonly used gas sensors and their average life expectancy (Adapted from [42]). are many manufacturers on the market whose products have been approved by different classification societies, such as SICK, VIMEX, SIMENS, ABB, EMSYS, etc. Among them, the technology of EMSYS is a new quantum cascade laser infrared spectrometry, which can detect eight kinds of air pollutants at the same time, providing great convenience for the monitoring of ship exhaust gas. In addition to monitoring equipment from various manufacturers, there are many other promising designs for monitoring systems. [PERSON] et al. [49] designed a ship exhaust gas online monitoring system, which combines embedded technology to design the data acquisition terminal of the monitoring system and uses the currently well-developed sensor module for data acquisition, which has some practical application value for improving the current ship exhaust gas monitoring technology. [PERSON] et al. [50], on the other hand, developed a four-wavelength laser system that allowed measuring the laser radiation attenuation signal using differential extinction at three wavelengths and the Mie scattering signal on aerosol particles at a fourth wavelength, and calculated the average volumetric-surface diameter of the aerosol particles based on the measured attenuation signal. The wavelength of laser detection is also selected according to the particle size range, and by applying the differential laser attenuation method to soot particles of different wavelengths in ship exhaust, the concentration and size distribution can be evaluated simultaneously. At the same time, monitoring systems for ship exhaust are not only installed on ships but also on the shipping lanes and nearby terminals where ships sail. [PERSON] et al. [51] monitored SO2 and NO2 concentrations in ship exhaust gases using Multi-axis DOAS (MAX-DOAS).