DIAL ozone technique and design
The Differential Absorption Lidar (DIAL) technique uses the absorption properties of a given atmospheric constituent to deduce its atmospheric concentration. Laser beams at two different wavelengths are sent into the atmosphere. The wavelengths are chosen so that one of them is sensitively more absorbed (wavelength "ON") than the other (wavelength "OFF"). The difference in the absorption along the beam path causes the returned lidar signals to yield a different altitude dependence. Knowing from laboratory work the absorption cross-sections of the constituent at both wavelengths, the atmospheric number density of this constituent can be deduced from the ratio of the slope of the logarithm of the signals at the two wavelengths. This technique does not require any calibration. Today, the most common DIAL systems are dedicated to the measurement of ozone, but some systems were also designed for the measurement of water vapor, methane, and carbon dioxide.
There are fourteen DIAL systems included in NDACC. Three systems are dedicated to the measurement of tropospheric ozone (0-10 to 20 km), and the others are dedicated to the measurements of stratospheric ozone (10-45 to 50 km). Thirteen systems are at fixed locations, one system is a mobile system. For detection in the stratosphere where ozone is very abundant, typical wavelengths used are 308 nm ("ON") and 355 nm ("OFF"). For detection in the troposphere where ozone is less abundant, stronger absorbed wavelengths are required, typically 289 nm ("ON") and 299 nm ("OFF").
The twelve fixed NDACC stratospheric ozone DIAL systems are located at Observatoire de Haute-Provence (France), Hohenpeissenberg (Germany), Table Mountain Facility (California), Mauna Loa Observatory (Hawaii), Lauder (New-Zealand), Ny-Alesund (Spitzbergen), Andoya (Norway), Dumont-d'Urville (Antarctica), and Reunion Island (Indian Ocean), Tsukuba (Japan), Eureka (Canada), and Rio Gallegos (Argentina). The mobile system?s home base is at the NASA Goddard Space Flight Center (USA). These systems typically have an overall error ranging from 3% near the ozone concentration peak to 10 and 30% in the lowermost and uppermost stratosphere (limited by ozone abundance and/or signal-to-noise ratio). Use the following link to see the stratospheric ozone data availability chart at NDACC.
The three NDACC tropospheric ozone DIAL systems are located at Observatoire de Haute-Provence (France), Huntsville (Alabama) and Table Mountain Facility (California). These systems typically have an overall error ranging from 7% in the mid-troposphere to 30% in the uppermost troposphere (limited by signal-to-noise ratio). Use the following link to see the tropospheric ozone data availability chart at NDACC.