DIAL Ozone Technique and Design

The Differential Absorption Lidar (DIAL) technique uses the absorption properties of ozone 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 ozone number density can be deduced from the ratio of the slope of the logarithm of the signals measured at the two wavelengths. This technique does not require any calibration. 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").

Data from a total of 16 ozone DIAL systems have contributed to NDACC since the network's inception in 1990. As of 2020, there are still 12 operational ozone lidars contributing to NDACC. Four systems are dedicated to the measurement of tropospheric ozone (0-15 km), and the others are dedicated to the measurements of stratospheric ozone (10-50 km). Ten systems are at fixed locations, while two systems are mobile facilities.

The 8 NDACC stratospheric ozone systems are located at Observatoire de Haute-Provence, France, Hohenpeissenberg, Germany, Table Mountain Facility, California, Mauna Loa Observatory, Hawaii, Lauder, New-Zealand, Eureka (Canada), Reunion Island, Indian Ocean, and Greenbelt, Maryland (mobile system). 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).

The 4 NDACC tropospheric ozone DIAL systems are located at Observatoire de Haute-Provence, France, Huntsville, Alabama, Table Mountain Facility, California, and Greenbelt, Maryland (mobile system). 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).

Today, whether part of NDACC or not, the most common DIAL systems are dedicated to the measurement of ozone, but some DIAL systems were also designed for the measurement of water vapor, methane, and carbon dioxide.

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