Although the ground air quality monitoring network can tell us the characteristics of air close to the ground or in a region, it cannot explain how suspended particulates (also know as aerosol) are distributed vertically. Therefore, optical remote sensing technology is applied to monitor optical characteristics of aerosol vertically on a long-term and continuous basis. It provides a strong and powerful vertical profile of pollutants as they change over time.
Light detection and ranging (Lidar) is one of the most widely applied optical remote sensing technology for aerosol now. Developed by Stanford University, Lidar was inspired by the concept of meteorological radar. Emulating radar, Lidar emits directional electromagnetic wave into the atmosphere from a fixed point and receives echo signals, by which we can calculate the position, height, and optical characteristics of the object. The first Lidar was born in the lab in 1953, applying ruby crystal as its excitation source and was capable of emitting a 50-kilowatt giant-pulse laser at one time, with optical band at 1064 nano-meter. It was applied to atmospheric observation for the first time in 1964, and in 1967, its application further expanded to atmospheric pollutant monitoring, and has since embarked on a long journey of technical upgrade and evolution. Today, Lidar, very different from what it was, is now applied extensively to all kinds of environmental monitoring tasks, including vertical monitoring and study of aerosol, cloud, O3 and trace gas. It can even be applied to analysis of all kinds of weather phenomenons, such as large-scale front systems, hurricanes, mountain-valley breezes, and the boundary layer, as well as upper atmosphere such as ozone and ice-crystal cloud in the stratosphere.