There are also ground based techniques that use backscattered radiation to remotely measure properties of the atmosphere. Lidars (light detection and ranging) are active remote sensing instruments which infer temperature, density, and trace constituent concentration profiles from measurements of backscattered laser light. Lidars operate in a variety of modes.
Measurements of ozone profiles use the differential absorption lidar (DIAL) technique. This technique consists of transmitting an intense beam of light (almost always a laser) into the atmosphere, where it is scattered by interactions with aerosol particles and air molecules. Some fraction of the light is scattered in the backward direction and can be collected by a telescope located near the transmitting laser. The signal is then collected by a detector. It is stored as a function of time of the transmitted laser pulse. The time duration between transmission of the laser pulse and its detection can be converted directly into a geometric altitude, provided the beam is transmitted vertically. Figure 7.02 shows a trailer mounted lidar instrument operating to measure vertical profiles of aerosols and temperature. A trailer provides this technique with mobility to take measurements anywhere the trailer can go.
In a DIAL measurement, two or more wavelengths are transmitted into the atmosphere. As in the case of BUV pair measurements and limb scattering pair measurements, one wavelength is selected that is strongly absorbed by the trace species of interest (e.g., ozone), while the other wavelength is selected that is weakly absorbed by the same species. Common wavelengths for the measurement of stratospheric ozone are 308 nm (absorbed wavelength), and 351 nm (not absorbed wavelength). As the two beams are transmitted through the atmosphere and scattered back towards the telescope, the 308 nm beam is attenuated by ozone absorption, while the 351 nm radiation is not.
Wavelength selection is important in the construction of a DIAL system. The use of 308 nm, while nearly optimum for the stratosphere, is unsuitable for measurements in the troposphere. A lidar measuring stratospheric ozone makes two passes through the ozone maximum. For this, a wavelength is required in which ozone does not absorb too strongly, otherwise the returning beam signal will be too weak or even nonexistent. In the troposphere, the concentration of ozone is far less and therefore a wavelength that is strongly absorbed by ozone is necessary. Hence, wavelengths deeper into the UV region are more appropriate for tropospheric ozone lidar instruments. Lidars are capable of making measurements from roughly 15 to 50 km in the atmosphere at a 1 km resolution.